The Dioscorea Genus (Yam)-An Appraisal of Nutritional and Therapeutic Potentials

Abstract

The quest for a food secure and safe world has led to continuous effort toward improvements of global food and health systems. While the developed countries seem to have these systems stabilized, some parts of the world still face enormous challenges. Yam (Dioscorea species) is an orphan crop, widely distributed globally; and has contributed enormously to food security especially in sub-Saharan Africa because of its role in providing nutritional benefits and income. Additionally, yam has non-nutritional components called bioactive compounds, which offer numerous health benefits ranging from prevention to treatment of degenerative diseases. Pharmaceutical application of diosgenin and dioscorin, among other compounds isolated from yam, has shown more prospects recently. Despite the benefits embedded in yam, reports on the nutritional and therapeutic potentials of yam have been fragmented and the diversity within the genus has led to much confusion. An overview of the nutritional and health importance of yam will harness the crop to meet its potential towards combating hunger and malnutrition, while improving global health. This review makes a conscious attempt to provide an overview regarding the nutritional, bioactive compositions and therapeutic potentials of yam diversity. Insights on how to increase its utilization for a greater impact are elucidated.

Full text

foods
Review
The Dioscorea Genus (Yam)—An Appraisal of
Nutritional and Therapeutic Potentials
Jude E. Obidiegwu 1, * , Jessica B. Lyons 2and Cynthia A. Chilaka 3
1National Root Crops Research Institute, Umudike, Km 8 Umuahia-Ikot Ekpene Road,
P.M.B 7006 Umuahia, Abia State, Nigeria
2Department of Molecular and Cell Biology and Innovative Genomics Institute, University of California,
Berkeley, 142 Weill Hall #3200, Berkeley, CA 94720-3200, USA; jblyons@berkeley.edu
3
Institute of Pharmacology and Toxicology, Julius Maximilian University of Würzburg, Versbacher Stra
β
e 9,
97078 Würzburg, Germany; adaku80@yahoo.com or cynthia-adaku.chilaka@uni-wuerzburg.de
*Correspondence: ejikeobi@yahoo.com
Received: 8 August 2020; Accepted: 12 September 2020; Published: 16 September 2020


Abstract:
The quest for a food secure and safe world has led to continuous effort toward improvements
of global food and health systems. While the developed countries seem to have these systems
stabilized, some parts of the world still face enormous challenges. Yam (Dioscorea species) is an
orphan crop, widely distributed globally; and has contributed enormously to food security especially
in sub-Saharan Africa because of its role in providing nutritional benefits and income. Additionally,
yam has non-nutritional components called bioactive compounds, which offer numerous health
benefits ranging from prevention to treatment of degenerative diseases. Pharmaceutical application
of diosgenin and dioscorin, among other compounds isolated from yam, has shown more prospects
recently. Despite the benefits embedded in yam, reports on the nutritional and therapeutic potentials
of yam have been fragmented and the diversity within the genus has led to much confusion.
An overview of the nutritional and health importance of yam will harness the crop to meet its
potential towards combating hunger and malnutrition, while improving global health. This review
makes a conscious attempt to provide an overview regarding the nutritional, bioactive compositions
and therapeutic potentials of yam diversity. Insights on how to increase its utilization for a greater
impact are elucidated.
Keywords: yam; Dioscorea; nutritional composition; bioactive compounds; therapeutic potential
1. Introduction
The nomenclature “Yam” applies to members of the Dioscorea genus of the Dioscoreaceae family
within the order Dioscoreales [
1
]. The yam crop was initially referred to as Inhame by New Guinea
users who predominantly used them as a starchy food source [
2
]. In the course of the 16th century,
French sailors erroneously changed the name from Inhame to Igname. Within this period, English
seamen called the crop “yam.” Yam was a source of food for enslaved people during their East to
West historic migration [
2
]. The roots, tubers and rhizomes of yams have been used since pre-historic
times by aboriginal peoples as a food, as well as for traditional medicine [
3
]. Dioscorea comprises over
600 species, with varying global distribution across Africa, Asia, Latin America, the Caribbean and
Oceania (Figure 1A) [
4
]. Among the wide species reported, only about 10 species are estimated to have
been domesticated across Africa, Asia and Latin America for food and income generation [
5
]. Yam
plants have unique climbing and twining vines that sprout from their characteristic rhizomes or tubers.
These rhizomes and tubers most often serve as photosynthetic sinks for starch and other secondary
metabolites [6].
Foods 2020,9, 1304; doi:10.3390/foods9091304 www.mdpi.com/journal/foods

Foods 2020,9, 1304 2 of 45
Across different ethnic communities and geographic regions, diverse species of Dioscorea have
been adopted within different habitation as a food source due to the high nutritional benefits and
therapeutic values toward treatment and cure of certain health problems [
7
,
8
]. Whilst yam is one of the
most important staple root and tuber crops worldwide, it is still classified as an orphan crop because it is
highly underutilized and receives little investment or/and research attention toward crop improvement.
Yam plays a significant role in food security, medicine and economy in the developing countries. Its
importance places it as the fourth most essential and utilized root and tuber crop globally after potatoes
(Solanum spp.), cassava (Manihot esculenta) and sweet potatoes (Ipomoea spp.) and the second in West
Africa after cassava [
9
,
10
]. This is evident in annual global production, especially in West Africa
(Figure 1B). In 2018, the Food and Agricultural Organization (FAO) of the United Nations reported a
worldwide production of approximately 72.6 million tons over 8.7 million hectares of harvested area
at a yield rate of 83515 hg/ha, with Africa contributing 97.1% of global production [
11
].Remarkably,
among the African nations, three countries (Nigeria—67.4%, Ghana—11.1%, C
ô
te d’Ivoire—10.3%)
in the west recorded the highest proportion of production, although the production increase (85.1%
between 2000–2018) in Africa is attributed mainly to the increase in the area of yam field into marginal
lands and non-traditional yam growing areas [
12
]. While D. alata originated in Asia and is the most
globally cultivated yam species, D. rotundata represents a great significance in respect to production
volume in the West of Africa, followed by D. alata and D. cayenensis [
13
]. Statistics have shown
evidence of an annual production increase of yam between 2011 and 2018 in several countries on
the African continent including Cameroon, Central African Republic, C
ô
te d’Ivoire, Gabon, Ghana,
South Sudan and United Republic of Tanzania [
11
]. This could be ascribed to its serving as a major
food source and cash crop, thus, combating malnutrition, food insecurity and poverty. In addition,
the significance of yam in the cultural, social and religious environment of West Africa cannot be
overemphasized [
4
,
14
,
15
]. Its symbolism as king of crops is manifested in its use in ceremonies such as
those for fertility and marriages, as well as an annual festival held to celebrate its harvest. Importantly,
the cultural and linguistic diversity that cuts across West Africa has no influence on the beliefs, social
values and religious practices attached to the yam crop.
Foods 2020, 9, x FOR PEER REVIEW 2 of 46
Across different ethnic communities and geographic regions, diverse species of Dioscorea have
been adopted within different habitation as a food source due to the high nutritional benefits and
therapeutic values toward treatment and cure of certain health problems [7,8]. Whilst yam is one of
the most important staple root and tuber crops worldwide, it is still classified as an orphan crop
because it is highly underutilized and receives little investment or/and research attention toward crop
improvement. Yam plays a significant role in food security, medicine and economy in the developing
countries. Its importance places it as the fourth most essential and utilized root and tuber crop
globally after potatoes (Solanum spp.), cassava (Manihot esculenta) and sweet potatoes (Ipomoea spp.)
and the second in West Africa after cassava [9,10]. This is evident in annual global production,
especially in West Africa (Figure 1B). In 2018, the Food and Agricultural Organization (FAO) of the
United Nations reported a worldwide production of approximately 72.6 million tons over 8.7 million
hectares of harvested area at a yield rate of 83515 hg/ha, with Africa contributing 97.1% of global
production [11].Remarkably, among the African nations, three countries (Nigeria—67.4%, Ghana—
11.1%, Côte d’Ivoire—10.3%) in the west recorded the highest proportion of production, although the
production increase (85.1% between 2000–2018) in Africa is attributed mainly to the increase in the
area of yam field into marginal lands and non-traditional yam growing areas [12]. While D. alata
originated in Asia and is the most globally cultivated yam species, D. rotundata represents a great
significance in respect to production volume in the West of Africa, followed by D. alata and D.
cayenensis [13]. Statistics have shown evidence of an annual production increase of yam between 2011
and 2018 in several countries on the African continent including Cameroon, Central African Republic,
Côte d’Ivoire, Gabon, Ghana, South Sudan and United Republic of Tanzania [11]. This could be
ascribed to its serving as a major food source and cash crop, thus, combating malnutrition, food
insecurity and poverty. In addition, the significance of yam in the cultural, social and religious
environment of West Africa cannot be overemphasized [4,14,15]. Its symbolism as king of crops is
manifested in its use in ceremonies such as those for fertility and marriages, as well as an annual
festival held to celebrate its harvest. Importantly, the cultural and linguistic diversity that cuts across
West Africa has no influence on the beliefs, social values and religious practices attached to the yam
crop.
Figure 1.(A) Global distribution of yam production in 2018 (Africa 96.2%, America 2.0%, Caribbean
1.0%, Oceania 0.6%, Asia 0.2%, Europe 0%), (B) Top yam producing countries in 2018 (Nigeria—
65.9%, Ghana–10.7%, Côte d’Ivoire—9.9%, other countries—14.5%) [11].
Yam’s potential as a source of food is attributed to its high levels of carbohydrates including
fiber, starch and sugar, contributing about 200 dietary calories per person per day to 300 million
people in the tropics [12]. It also provides other nutritional benefits such as proteins, lipids, vitamins
and minerals [16]. According to International Institute of Tropical Agriculture (IITA), the global
annual consumption of yam is placed at 18 million tons, with 15 million tons only in West Africa
amounting to about 61 kg per capita in the region [17]. In West Africa, yam tuber may be eaten boiled,
fried, baked or roasted in combination with tomato stew, sauces and in some cases, typically in poor
Figure 1.
(
A
) Global distribution of yam production in 2018 (Africa 96.2%, America 2.0%, Caribbean
1.0%, Oceania 0.6%, Asia 0.2%, Europe 0%), (
B
) Top yam producing countries in 2018 (Nigeria—65.9%,
Ghana–10.7%, Côte d’Ivoire—9.9%, other countries—14.5%) [11].

Foods 2020,9, 1304 3 of 45
Yam’s potential as a source of food is attributed to its high levels of carbohydrates including fiber,
starch and sugar, contributing about 200 dietary calories per person per day to 300 million people
in the tropics [
12
]. It also provides other nutritional benefits such as proteins, lipids, vitamins and
minerals [
16
]. According to International Institute of Tropical Agriculture (IITA), the global annual
consumption of yam is placed at 18 million tons, with 15 million tons only in West Africa amounting
to about 61 kg per capita in the region [
17
]. In West Africa, yam tuber may be eaten boiled, fried,
baked or roasted in combination with tomato stew, sauces and in some cases, typically in poor rural
communities, with traditional palm oil. The tuber may also be pounded into moldable dough which is
consumed with traditional African soups. The consumption of raw yam tubers of species D. soso,D.
nako and D. fandra in Madagascar has also been reported [
12
]. In Asia, especially Japan and China, D.
japonica and D. polystachya, usually eaten raw, can also be grated and used as an ingredient in tororo
udon/soba noodles [18,19].
In the quest to identify other benefits of Dioscorea species, studies have revealed yam therapeutic
potentials as a result of its bioactive compound content. A bioactive compound is defined as a
substance that can exert biological effect, thus, causing a reaction or triggering a response in a living
tissue [
20
]. A study on seven different varieties of yams (Dioscorea spp.) reported reasonable quantities
of these compounds including flavonoids, phenols, saponins, tannins and alkaloids [
21
]. Another
study described the pharmacological activities of yam peptides and proteins such as antioxidant,
immunomodulatory, estrogenic, angiotensin I-converting enzyme inhibiting, carbonic anhydrase and
trypsin inhibiting, chitinase, anti-insect, anti-dust mite, lectin and anti-proliferative activities [
22
].
These authors reported the therapeutic potentials of peptides and proteins isolated from several species
of yams including D. alata,D. cayenensis,D. japonica,D. pseudojaponica and D. polystachya (formerly
known as D. opposita or D. batatas), as well as the possible clinical applications for the treatment of
inflammatory diseases, cardiovascular diseases, aging disorders, menopause, cancers and osteoporosis.
Furthermore, the use of different species of Dioscorea for birth control and skin infections has been
reported [
23
,
24
]. Nashriyah et al. [
25
] reported the use of D. hispida in cosmetics for pigmentation
remedy. This is not surprising as since time immemorial, the utilization of natural products with
therapeutic potentials including mineral, plant and animal substances as main sources of drugs have
existed [
26
]. This long standing historic use of plants as therapeutic resources serves as a proof of their
efficacy. The diversity in yams has the potential to enrich the human body with starch and energy [
27
],
as well as supplemental metabolites while serving as a source for medicinal use at level of traditional
therapeutics and industrial medical pharmacy. The therapeutic potential of yam is of interest especially
in developing countries where a majority of the population lacks access to standard health care, which
even when available is far beyond the reach of many locals due to the financial burden, thus, yam may
contribute to providing health benefits beyond its nutritive values.
While there are pharmacological prospects of yam, its antinutritional components cannot be
overlooked. The utilization of some species of yam, such as D. bulbifera and D. hispida, have been
hindered due to the bitter taste caused by the presence of furanoid norditerpenes (diosbulbin) and
dioscorine, respectively [
28
,
29
]. However, in the context of extreme food scarcity, processing such as
soaking, boiling and roasting are used to reduce or eliminate the bitterness. In addition, diosbulbin
and dioscorine have been reported to trigger fatal paralysis of the nervous system [
30
]. This is evident
in the utilization of these yam extracts in the preparation of arrow poison or sedative drugs often used
for hunting in different countries including Malaysia, Indonesia, South Africa and Bangladesh [
4
,
25
].
In addition, other toxic compounds and allergens such as oxalate, saponin, phytic acid, tannin and
histamine have been reported [
31
], with some species such as D. hispida having cyanide [
32
]. Shim
and Oh [
33
] described histamine as one of the major compounds that induce allergic reactions such
as an itch on the skin. Although histamine may be the principal allergen in yam, studies have also
reported the potential of dioscorin from D. batatas (presently known as D. polystachya) to cause allergic
reaction [34].

Foods 2020,9, 1304 4 of 45
To utilize the full potential of Dioscorea spp. given their contributory role in food security as a
staple crop to a large number of the world’s population and their beneficial health effects, the need
for harmonization of indigenous and scientific knowledge of this crop becomes imperative. Such
knowledge will help to promote its utilization, thus contributing to the attainment of United Nations
(UN) sustainable development goals (SDGs 1, 2 and 3) especially in the developing countries. In
this framework, the present review aims to reveal the global importance of yam by providing a
comprehensive report on the nutritional and bioactive composition of Dioscorea species. In addition,
the review will highlight the therapeutic benefits and impact on human health associated with the
consumption of yam, as well as future perspectives of yam production, utilization and research.
2. Yam Nutritional Value
Over the years, several scientific studies have evaluated and reported the nutritional qualities
of different Dioscorea species, as shown in Tables 1and 2. The nutritional abundance of yam varies
depending on the species and variety, as well as the environmental conditions and agricultural
practices engaged during planting [
35
,
36
]. The analytical method used for estimation also plays a
significant role in nutritional levels recorded in yam. The major component of yam is water, which
contributes up to 93% of fresh/wet weight of the tuber especially in D. bulbifera,D. delicata and D.
pentaphylla [
37
–
39
]. While the moisture content of other Dioscorea species ranges between 51% to
90% (Table 1), it is noteworthy to highlight that D. hispida varieties were reported to have the lowest
moisture content, ranging from 15.8%–37.8% of fresh weight [
32
]. High values of moisture have also
been reported in other root and tuber crops, with values ranging from 60%–79% in cassava, potato
and sweet potatoes [
40
,
41
]. The moisture content of roots and tubers plays a very important role
in determining the susceptibility of the crops to microbial spoilage and maintaining the shelf life
of produce. Thus, species and varieties with low moisture content have longer shelf life and are
more suitable for prolonged storage [
42
,
43
]. According to FAO, an estimate of about 22% and 39%
postharvest losses of yam occur in the major and minor seasons, respectively, due to high moisture
content [
44
], hence contributing significantly to income loss for both farmers and traders. In addition
to spoilage caused by high moisture content, it is important to highlight the importance of moisture as
it relates to nutritional content of yam. A study on the effect of storage on nutritional content of yam
revealed an increase in protein content, total sugar and reducing sugar from 13.0%–14.6%, 6.5%–9.8%
and 1.7%–2.3%, respectively, as moisture decreased by 67.8% to 56.5% [45].

Foods 2020,9, 1304 5 of 45
Table 1. Proximate compositions of some species of Dioscorea.
Species Proximate Composition (Percentage, %)
Moisture Crude Protein Crude Fat Crude Fiber Ash Starch Reference
D. alata 64.9–87.8 0.6–18.7 0.23–5.28 0.75–11.0 0.69–8.81 15.6–84.3 [35–37,39,42,46–59]
D. abyssinica NR 3.13–5.37 0.31–1.22 1.94–4.91 2.31–3.58 NR [36]
D. bulbifera 61.6–92.5 0.89–15.8 0.30–8.13 0.61–18.2 0.05–8.15 12.5–62.7 [36,37,39,42,43,47,51,58,60–66]
D. cayenensis 62.2–89.4 2.62–6.63 0.27–7.86 0.17–3.26 0.63–5.48 80.75 [36,37,42,48,58,60,67]
D. delicata 92.7 0.41 NR 4.87 NR 0.54 [37]
D. deltoidea 80.2 1.6 0.2 1.5 0.6 NR [65]
D. dodecaneura 68.4 1.50 NR NR NR 18.46 [37]
D. dumetorum 64.3–90.2 0.19–10.3 0.37–3.65 0.82–5.65 2.17–7.79 17.0–63.34 [37,42,49,50,60,61,68–71]
D. esculenta 50.65–86.67 5.60 –10.50 0.08–2.58 1.23–7.82 0.25–8.50 17.25 [37,39,42,48,51,72]
D. fordii NR 9.8–10.2 NR 0.92–1.14 NR 75.7–77.1 [35]
D. hamiltonii 78.73 4.37 10.2 4.15 8.70 NR [38]
D. hispida 15.8–37.8 1.13–6.20 1.99–9.36 NR 0.29–1.24 11.5 [32]
D. laxiflora 82.0 0.26 NR 2.34 NR 8.92 [37]
D. nipponica NR NR NR NR NR 35.4 [47]
D. olfersiana 84.6 0.42 NR 9.53 NR 0.54 [37]
D. oppositifolia 78.5–92.1 7.00–13.54 4.40–7.42 4.92–8.47 2.60–6.38 NR [38,39,51]
D. pentaphylla 90.1–93.1 6.48–9.18 4.01–6.24 5.14–7.24 3.36–4.64 NR [38,39,51]
D. persimilis NR 7.70–8.20 NR 0.88–0.92 NR 68.2–72.2 [35]
D. piperifolia 55.4–74.8 2.27–4.38 NR NR NR 18.2–26.1 [37]
D. polystachya NR 6.30–12.2 NR 0.99–1.50 NR 60.7–72.5 [35,47]
D. praehensilis 64.1 3.64–5.38 0.26–7.83 1.41–3.21 2.13–4.90 NR [36,42]
D. pyrifolia NR 1.34 NR NR 0.88 NR [73]
D. remotiflora 78.18 1.91 0.47 1.22 0.85 NR [74]
D. rotundata 54.5–75.2 0.09–8.28 0.09–3.39 0.41–4.33 1.03–4.92 22.0–80.8 [36,37,42,48,49,52,58,72,75–79]
D. sanpaulesis 69.2 0.77 NR 10.3 NR 2.62 [37]
D. sinuata 75.6 2.32 NR NR NR 8.00 [37]
D. spicata 81.5–89.3 6.38–8.20 3.26–4.78 4.67–6.31 5.18–5.20 NR [38,51]
D. steriscus 72.5 0.83 NR 16.8 2.06 9.02 [80]
D. subhastata 89.0 0.59 NR 0.95 NR 3.69 [37]
D. tomentosa 84.5–93.7 5.25–9.54 2.86–6.84 3.21–4.38 2.48–6.53 NR [38,39,51]
D. trifida 69.4–81.3 0.38–6.79 0.03–0.30 NR 0.2–3.37 7.94–64.0 [37,48,81,82]

Foods 2020,9, 1304 6 of 45
Table 1. Cont.
Species Proximate Composition (Percentage, %)
Moisture Crude Protein Crude Fat Crude Fiber Ash Starch Reference
D. triphylla 76.9 2.3 0.2 0.6 0.6 NR [65]
D. versicolor 80.1 1.7 0.2 1.1 0.5 NR [65]
D. villosa 76.4 2.21 6.01 3.50 3.13 NR [83]
D. wallichi 71.1–76.4 10.5–10.8 1.18–3.34 7.48–9.23 6.36–8.42 NR [39,51]
D. polystachya: Chinese yam formerly known as D. opposita and D. batatas, NR: not reported.
Table 2. Mineral composition of yam (Dioscorea species).
Species No of
Varieties
Minerals (mg/100g) Reference
K Na P Ca Mg Cu Fe Mn Zn
D. abyssinica 113 NR NR 5.1–56.5 31.02–118.8 NR NR 20.3–69.7 NR 0.48–0.77 [36]
D. alata 17 1157–2016 52–82.7 117–194 62.5–78.0 64.0–74.6 6.4–6.9 9.9–10.9 3.1–4.3 3.4–4.3 [54]
D. alata 17 240–400 190–380 100–340 20.04–80.2 24.31–97.2 NR NR NR NR [53]
D. alata 19 NR NR NR 31.64–45.3 32.68–47.8 0.42–0.48 0.83–2.2 NR 0.82–2.6 [35]
D. alata 120 1055-2010 8.30–13.1 NR 26.0–53.5 39.0–59.5 NR NR NR 1.01–1.8 [57]
D. alata 31 476.8 ±0.1 68.9 ±0.02 163.7 ±0.10 285.8 ±0.02 116.3 ±0.69 NR 2.48 ±0.02 NR 2.12 ±0.00 [58]
D. alata 32 622.5–742.5 62.5–95.0 219.0–239 6.50–16.50 40.0–41.5 0.10–0.15 1.50–2.00 2.15–2.20 6.65–6.80 [42]
D. alata 116 1055–2010 8.4–13.1 87.8–190.0 26.0–41.0 39.0–58.0 1.23–1.57 NR 0.48–2.21 1.01–1.41 [55]
D. alata 14 NR NR 26.59–49.12 11.24–120.0 NR NR 17.75–51.1 NR 0.38–1.18 [36]
D. alata 11 3932.9 ±0.16 75.4 ±0.02 NR 3032.1 ±0.25 120.7 ±0.005 1.216 ±0.001 124.3 ±0.004 1.33 ±0.001 5.7 ±0.001 [56]
D. alata 11 5.25 ±2.12 0.35 ±0.0 NR 0.22 ±0.99 0.65 ±0.71 NR 0.75 ±0.73 NR NR [50]
D. alata 11 786.3 ±0.14 44.56 ±0.3 140.14 ±0.14 448.36 ±0.11 656.31 ±0.07 11.20 ±0.14 24.30 ±0.19 6.36 ±0.21 2.26 ±0.01 [51]
D. buibifera 112 NR NR 8.72–55.26 15.74–121.3 NR NR 20.26–90.9 NR 0.4–8.33 [36]
D. buibifera 11 NR NR 0.521 1410.0 250 NR NR NR NR [66]
D. buibifera 11 1554.4 ±0.36 78.24 ±0.07 154.42 ±0.53 338.15 ±0.09 396.20 ±1.07 2.14 ±0.04 19.20 ±0.20 9.40 ±0.14 1.48 ±0.03 [51]
D. buibifera 31 525.8 ±1.41 87.8 ±0.10 159.5 ±0.04 378.5 ±0.10 128.7 ±0.04 NR 3.14 ±0.02 NR 2.79 ±0.01 [58]
D. buibifera 21 560 ±49 17.8 ±9.8 61.61 ±0.8 29.3 ±4.8 25.9 ±9.2 0.21 ±0.03 2.92 ±0.3 0.35 ±0.03 0.53 ±0.06 [65]
D. buibifera 32 1250–1475 92.5–102.5 223.5–224.5 103–116.5 76.5–83.5 0.20 6.00–6.50 1.30–1.35 6.10–6.35 [42]
D. cayenensis 12 NR NR 19.15–26.12 6.3–27.6 NR NR 17.2–27.95 NR 0.74–0.75 [36]
D. cayenensis 21 262.3 ±0.25 8.53 ±0.05 19.5 ±0.10 22.53 ±0.13 61.53 ±0.25 NR 0.79 ±0.02 NR 0.39 ±0.01 [67]
D. cayenensis 31 523.8 ±0.04 76.8 ±0.03 167.8 ±0.02 345.8 ±0.01 120.2 ±0.55 NR 2.50 ±0.08 NR 2.18 ±0.02 [58]
D. cayenensis 32 700–825 62.5–70.0 164.5–190.5 74.5–80.0 57.5–38.0 0.20 5.0–5.5 1.2–1.25 5.45–5.85 [42]

Foods 2020,9, 1304 7 of 45
Table 2. Cont.
Species No of
Varieties
Minerals (mg/100g) Reference
K Na P Ca Mg Cu Fe Mn Zn
D. deltoidea 21 340 ±51 9.12 ±1.6 33.1 ±0.6 46.9 ±6.2 22.8 ±7.1 0.10 ±0.0 1.85 ±1.0 0.31 ±0.02 0.22 ±0.04 [65]
D. dumetorum
11 7.03 ±0.78 0.41 ±0.14 NR 0.81 ±0.21 0.95 ±0.71 NR 0.07 ±0.14 NR NR [50]
D. dumetorum
32 670–772 72.5–77.5 269–286 27.5–29.5 61.5 0.10 2.0–2.50 2.50–2.65 5.80 [42]
D. dumetorum
32 0.03 0.02 NR 0.19–0.21 0.65–0.72 1.36–1.48 0.13–0.16 0.34–0.38 0.03–0.18 [71]
D. dumetorum
3NR NR 151 57.8 NR NR 8.89 NR NR [68]
D. esculenta 32 765–795 87.5–92.5 273.5–294.5 20.5–27.0 67.5–73.0 0.10 2.0 2.70–2.95 6.20–7.80 [42]
D. esculenta 11 1594.3 ±1.34 86.40 ±0.14 138.10 ±0.14 314.01 ±0.33 436.06 ±0.54 3.40 ±0.01 11.48 ±0.11 5.46 ±0.11 1.76 ±0.04 [51]
D. fordii 13 NR NR NR 28.56–30.05 34.58–35.63 0.45–0.51 1.82–2.02 NR 1.79–1.85 [35]
D. oppositifolia
11 1431 ±1.56 102.2 ±0.54 78.2 ±0.08 680.6 ±0.82 432.5 ±1.11 2.74 ±0.03 22.0 ±0.08 6.34 ±0.01 3.24 ±0.08 [38]
D. oppositifolia
12 1534–1624 124–168.2 114.1–124.1 294.2–646.2 540.1–634.1 7.62–14.56 32.16–40.76 7.42–9.04 1.56–6.26 [51]
D. pentaphylla
11 1322 ±2.40 95.2 ±0.12 96.1 ±0.06 632.1 ±0.22 380.0 ±0.74 12.60 ±0.14 103.48 ±0.94 1.32 ±0.01 3.10 ±0.01 [38]
D. pentaphylla
11 1441.0 ±0.98 96.20 ±0.63 158.18 ±0.21 444.24 ±0.09 532.12 ±0.56 13.26 ±0.05 66.32 ±0.14 3.46 ±0.21 3.42 ±0.01 [51]
D. persimilis 13 NR NR NR 46.55–47.64 46.70–47.42 0.382–0.423 1.73–1.93 NR 1.32–1.45 [35]
D. polystachya
110 NR NR NR 39.73–55.82 33.26–54.47 0.35–0.54 1.43–2.58 NR 0.99–2.27 [35]
D. praehensilis
15 NR NR 20.9–39.0 13.1–118.2 NR NR 18.36–76.4 NR 0.4–1.09 [36]
D. praehensilis
31 1000 ±21.2 80.0 ±7.07 200.5 ±0.71 79.5 ±3.54 43.5 ±0.71 0.40 ±0.14 9.0 ±0.0 0.95 ±0.07 5.4 ±0.57 [42]
D. remotiflora 21 4891 ±25 79 ±6 720 ±20 242 ±14 250 ±10 3.3 ±0.2 12.4 ±0.5 4.1 ±0.2 7.1 ±0.3 [74]
D. rotundata 32 475–900 70.0–87.5 158–211.5 91.5–103.3 35.5–53.0 0.20–0.25 5.0–6.75 1.15–1.80 6.30–6.80 [42]
D. rotundata 16 NR NR 26.96–40.21 22.77–114.4 NR NR 17.75–78.3 NR 0.35–1.02 [36]
D. rotundata 11 1591 10.4 NR 31.0 51.0 NR NR NR 1.23 [57]
D. rotundata 33 9.00–71.00 NR 22.00–35.00 2.00–4.00 11.00 NR 1.00 NR 1.00 [76]
D. rotundata 31 530.7 ±0.10 80.75 ±0.14 168.7 ±0.01 278.8 ±0.15 125.7 ±0.08 NR 2.88 ±0.02 NR 2.34 ±0.00 [58]
D. rotundata 31 209.13 ±0.03 185.2 ±0.05 54.00 ±0.04 132.02 ±0.04 45.90 ±0.02 10.06 ±0.05 81.85 ±0.01 NR 5.46 ±0.02 [78]
D. spicata 11 1255 ±0.48 52.2 ±0.11 86.1 ±0.11 172.0 ±0.21 112.4 ±0.32 0.78 ±0.21 22.36 ±0.38 0.98 ±0.14 4.18 ±0.13 [38]
D. spicata 11 1136 ±0.74 66.34 ±0.54 166.30 ±0.27 234.10 ±0.58 324.16 ±0.24 7.41 ±0.11 24.10 ±0.26 6.70 ±0.14 2.56 ±0.04 [51]
D. tomentosa 11 1354 ±1.34 32.2 ±0.18 96.1 ±0.04 272.1 ±1.01 120.4 ±0.08 1.34 ±0.01 24.56 ±0.04 1.32 ±0.04 5.20 ±0.03 [38]
D. tomentosa 11 1245.6 ±1.14 46.14 ±0.30 104.06 ±0.09 266.36 ±0.16 321.04 ±0.14 2.46 ±0.14 28.50 ±0.07 2.10 ±0.11 5.40 ±0.02 [51]
D. trifida 13 830–1350 NR 50.0–120.0 40.0 40.0–50.0 0.67–1.19 NR NR 0.62–1.79 [81]
D. triphylla 21 317 ±32 4.15 ±0.7 56.6 ±0.1 39.7 ±8.1 27.3 ±5.6 0.18 ±0.05 1.00 ±0.05 0.25 ±0.07 0.39 ±0.1 [65]
D. versicolor 21 250 ±4 4.91 ±2.5 40.8 ±0.2 14.3 ±1.8 18.3 ±3.8 0.18 ±0.02 0.39 ±0.1 0.14 ±0.0 0.3 ±0.06 [65]
D. villosa 31 145.33 ±1.15 5.40 ±0.10 43.82 ±0.49 28.06 ±4.01 9.47 ±0.23 NR NR 0.032 ±0.0 0.26 ±0.0 [83]
D. wallichi 11 1361.7 ±1.01 63.01 ±0.27 106.40 ±0.11 748.31 ±0.32 578.06 ±0.19 2.46 ±0.08 20.14 ±0.04 3.31 ±0.05 6.66 ±0.01 [51]
K: potassium, Na: sodium, P: phosphorus, Ca: calcium, Mg: magnesium, Cu: cupper, Fe: iron, Mn: manganese, Zn: zinc, NR: not reported,
1
dry weight,
2
fresh weight,
3
Sample type: not
specified, D. polystachya: Chinese yam formerly known as D. opposita and D. batatas.

Foods 2020,9, 1304 8 of 45
2.1. Yam as a Source of Dietary Energy
Yam is classified as an energy food source to consumers especially in sub-Saharan Africa (SSA)
because of its high starch content which amounts up to 80% in dry weight basis (Table 1) [
84
]. Among
the Dioscorea spp., D. alata has been reported to contain a relatively high starch content when compared
to others, up to 84.3% [
54
]. A study by Afoakwa et al. [
85
] evaluated the starch composition of seven of
the cultivated yam species (D. cayenensis,D. rotundata,D. alata,D. bulbifera,D. esculenta,D. praehensalis,
D. dumentorum) in SSA and reported a range between 63.2% and 65.7%. The variation in the starch
content of D. alata recorded by these authors may be dependent on several environmental factors and
agronomic practices, as well as the degree of maturity. The degree of maturity of yam tuber has a
great influence on the physicochemical quality of food [86]. In addition, van Eck [87] highlighted the
importance of maturity as it influences starch and tuber yield of potatoes when compared to other
genetic variations. Similar values of starch have been recorded in other root and tuber crops including
potatoes, cassava and cocoyam, as well as cereal grains. While studies have shown high starch content
in yams, low (below 1%) starch content were reported in D. delicata and D. olfersiana (Table 1) [
37
].
Wang et al. [
88
] reported a starch content ranging from 20% to 30% for Chinese yam. Yam starch
granules consist of a mixture of branched (amylopectin) and un-branched (amylose) chain polymers
of D-glucose usually occurring at a percentage ratio of 78:22 [
89
]; nevertheless the values may vary
depending on species as well as genotype. Using the iodo-colorimetric method, Otegbayo et al. [
90
]
reported a wide variability of amylose content between 15.1% and 27.0% of 43 genotypes in 5 species
(D. alata,D. rotundata,D. cayenensis,D. dumetorum and D.bulbifera). Amylose content as high as 39.3
g amylase/100 g starch was reported in Thai yam (D. hispida) using a simplified amylose assay [
91
],
comparable to the value reported in Taiwanese D. alata (39 g amylose/100 g starch) [
92
]. A much lower
amylose content ranging from 1.4% to 8.7% of starch was recorded for six D. trifida of the Venezuelan
Amazon [
81
]. It is important to point out that the discrepancies observed by the latter authors, who
reported a wide variability in amylose content of 1.4% to 8.7%, 1.4% to 3.6% and 2.2% to 5.9% was
as a result of analytical methods including colorimetric (iodine binding with amylose), differential
scanning calorimetry and amperometric, respectively. The ratio of amylose to amylopectin content of
yam starches is very crucial as it affects the starch properties and functional characteristics such as
crystallinity and digestibility. While high amylopectin content of starch granules results to low levels
of retrogradation susceptibility and high peak viscosity, starch granules with high amylose content
demonstrates high retrogradation and absorbs limited water content during cooking [93].
In addition, yam contains dietary fiber, which plays a vital role in the digestive system of humans
as well as animals. Adequate intake of fiber increases water holding capacity, aids in regular bowel
movement, fecal bulkiness and less intestinal transit. It also promotes beneficial physiological effects
such as reduction of blood sugar and cholesterol level, trapping of toxic substances and encourages
the growth of natural microbial flora in the gut [
94
–
97
]. The crude fiber reported in different species
of yam ranged between 0.17% and 18.2% with the minimum and maximum concentrations being
recorded in D. cayenensis and D. bulbifera, respectively. Several dietary fiber constituents such as
hemicelluloses, cellulose, lignin and pectins have been reported in yam. Abara et al. [
98
] examined the
dietary fiber components of four raw and cooked Dioscorea species (D. alata,D. bulbifera,D. cayenensis
and D. rotundata) using detergent system analysis and reported low levels of fiber components ranging
from 0.08%–0.27% (lignin), 0.80%–1.13% (cellulose) and 0.15%–0.28% (hemicelluloses). Interestingly, no
significant difference in dietary fiber was observed between the raw and cooked yams, irrespective of
their species. Among the species investigated, D. bulbifera had the highest cellulose and hemicelluloses
while lignin was higher in D. alata. A recent study investigated cell wall carbohydrates of 43 genotypes
from five yam species (D. rotundata,D. alata,D. bulbifera,D. cayenensis and D. dumetorum) using
detergent system analysis and recorded the highest cell wall carbohydrate in D. bulbifera at 2.1%, 3.2%
and 1.1% for hemicelluloses, cellulose and lignin, respectively [
99
]. The discrepancies of the values in
the two studies may be attributed to genotypic variations as highlighted by Otegbayo et al. [
99
]. In
line with this fact, Shajeela et al. [
51
] observed a higher crude lipid content in D. oppositifolia when

Foods 2020,9, 1304 9 of 45
compared to the other nine Dioscorea species investigated. Among the two varieties of D. oppositifolia
tubers studied by these authors, D. oppositifolia var dukhumensis (7.42 g/100 g) was reported to contain
higher crude lipid than the variety oppositifolia (4.40 g/100 g). This trend was also observed for crude
protein, with D. oppositifolia var dukhumensis having higher crude protein content of 13.42 g/100 g
compared to 8.44 g/100 g recorded for D. oppositifolia var oppositifolia [
51
]. This is in agreement with an
earlier study by Arinathan et al. [100].
Protein is an essential nutrient required for growth and organ development in humans and
animals. It helps in the repair of body tissue, synthesis of enzymes and hormones and also contributes
to energy supply. Although roots and tubers are known for their low protein content when compared to
pulses/legumes (beans 15%–38%, pea 14%–36%, cowpea 20%–34%, soya bean 29%–50% and groundnut
17%–31%) and cereals such as maize (9.4%), sorghum (11.6%), rice (7.1%), wheat (12.6%) [
40
,
101
],
yam is reported to have higher dietary protein compared to other root and tuber crops including
cassava [
102
,
103
]. Yam species like D. alata have been reported to have comparable higher protein levels
(18.7%) than grains [
56
]. Contrary to the high protein reported in D. alata, Alinnor and Akelezi [
78
]
recorded very low protein content (0.09%) in D. rotundata as against 8.28% reported in the same
species by another study [
77
]. Yam tubers are a considerably good source of essential amino acids
including phenylalanine and threonine but are limited in tryptophan and sulphur amino acids [
102
].
In addition, a more recent study on amino acid profiling of different yam species including D. alata,D.
bulbifera,D. esculenta,D. oppositifolia,D. pentaphylla,D. spicosa,D. tomentosa and D. wallichi revealed
the prevalence of aspartic acid (5.21–9.36 g/100 g) and glutamic acid (3.20–8.12 g/100 g) in all the yam
species investigated [
104
]. In Africa, consumption of starchy staples, primarily yam and cassava,
contributes a great proportion of protein intake in the region ranging from 5.9% in the Southern and
Eastern Africa to 15.9% in West Africa [102].
Other minor components such as lipids have also been reported in yam. Although these
components are present at a very small fraction [
103
], they have a great impact on the functionality
of starch [
84
]. A wide range of concentrations of lipid between 0.03% and 10.2% have been reported.
The highest lipid level (10.2%) was recorded in D. hamiltonii [
38
]. It is important to note that lipid
content is highly influenced by the extraction solvent used, as this determines the lipid fraction (bound
or unbound) extracted [
105
]. While lipids supply energy to humans and animals and act as building
blocks for cell membranes, they may also serve as pharmacological agents in the body [
106
]. Mondy
and Mueller [
107
] highlighted the possibility of tuber lipid being of limited nutritional importance;
however, it enhances the cellular integrity of the cell membrane, proffers resistance to bruising and
reduces enzymatic browning of the tuber.In addition, the ash content of yams is reported to range from
0.1% to 8.8% (Table 1) which is comparable to the values reported in other roots and tubers including
potatoes, cassava and cocoyam [
108
–
110
]. Ash refers to the inorganic residue in any food material and
it directly signifies the total amount of minerals present within the food. However, recent studies have
shown that ash content measurement of yam starch can be influenced by inefficient starch purification
methods, thus leading to higher values.
2.2. Yam as a Source of Minerals
Yams also contain inorganic components such as minerals which play very important roles in the
body metabolism (Table 2). These components can be divided into two groups based on their body
requirement. They include the macrominerals (potassium, sodium, calcium, phosphorus, magnesium,
chloride and sulfur), which are required in larger amounts; and the microminerals or trace minerals
including copper, iron, manganese, zinc, iodine, cobalt, fluoride and selenium, needed in the body in
small amounts. A study on the mineral profiling of 43 genotypes from five yam species revealed the
intra- and inter-species variation in the mineral content of yam [
99
] as observed for other nutritional
components. Potassium, sodium and chloride play a crucial role in the maintenance of total body fluid
volume and charge gradients across cell walls [
111
] and are also responsible for nerve transmission
and muscle contraction. The recommended daily allowance (RDA) of potassium in adults and children

Foods 2020,9, 1304 10 of 45
is 4700 mg/day and 3000 mg/day, respectively. Yams are better sources of potassium than other root
and tuber crops (cassava, potatoes and sweet potatoes) as well as cereals (maize, rice, wheat) [
112
].
Otegbayo et al. [
99
] reported a range of 775 to 1850 mg/kg of potassium in yam, which correlates with
the values previously reported [
42
,
55
]. High levels of 1157–2016 mg/100 g dry matter of potassium in
different cultivars of D. alata was also recorded by a much earlier study [
54
], thus suggesting that yam
contributes immensely to the RDA of potassium for the consumers.
In contrast to the high potassium concentration recorded in different yam species (Table 2), sodium
was detected at lower concentrations (0.35–380 mg/100 g dry weight). Another important macro
mineral present in yam is calcium. Calcium is the most abundant mineral in the body with 99% found in
bone and teeth, while 1% is found in serum. It plays a vital role in muscle functions, nerve transmission,
vascular contraction, intracellular signaling, vasodilation and hormonal secretion [
113
]. The RDA
of calcium (1000–1300 mg) in individuals varies depending on the age, with younger individuals
requiring more calcium for the development of bone and teeth [
114
]. As with other nutrients, calcium
content in yam varies with yam species and/or variety. So far, available studies revealed that D.
bulbifera has a higher calcium content of up to 1410 mg/100 g (Table 2) [
66
]. It is important to note
that metabolism of calcium involves other nutrients such as amino acids and vitamin D, as well as
phosphorus. Phosphorus is important in maintaining healthy bones and teeth, acid-base balance
of the body and DNA and RNA structure [
115
]. Phosphorus content was below the RDA (700 mg)
recommended for healthy adults in all yam species studied except for D. remotiflora (720 mg/100 g) [
74
].
In addition to the macrominerals mentioned above, yam is reported to be a good source of magnesium.
Magnesium plays a vital role in the body metabolic processes, nerve transmission, appropriate muscle
tasks and cardiac tempo, as well as synthesis and stability of DNA [
116
,
117
]. Shajeela et al. [
51
]
reported magnesium range of 540–634 mg/100 g in two varieties of D. oppositifolia, while the authors
recorded 532 mg/100 g and 578 mg/100 g of magnesium in D. pentaphylla and D. wallichi, respectively.
In addition, microminerals such as iron, manganese, zinc and copper have been reported in
different yam species, hence, contributing toward the RDA of these nutrients in the body of consumers.
Iron is crucial for the formation of hemoglobin in red blood cells that bind and transport oxygen in
the body. Bashiri et al. [
118
] reported the importance of iron in respiration and energy metabolism
processes. It also plays a very important role in the immune system and has been implicated in the
amalgamation of collagen and neurotransmitters. Otegbayo et al. [
99
] reported a range between 1.1
and 3.9 mg per 100 g of iron content in different species of yam, with D. dumetorum >D. bulbifera >
D. alata>D. cayenensis >D. rotundata. Although the iron content of different species of yam has been
reported to be low as compared to cereals (maize, rice and wheat), Mohan and Kalidass [
38
] observed
very high values of iron (103 mg/100 g) in D. pentaphylla. Importantly, the range at which iron is found
in a majority of the yam species meets the RDA (11–18 mg/day) of iron. Copper, zinc and manganese
are components of numerous enzymes and have also been reported in different species of yam tubers,
with values in wild yam being as high as 13.3 mg/100 g (D. pentaphylla), 7.1 mg/100 g (D. remotiflora) and
9.4 mg/100 g (D. bulbifera), respectively [
51
,
74
]. While yam contributes immensely to the nutritional
requirement of consumers, the non-nutritional components and benefits will be discussed herein.
3. Bioactive Compounds in Yam
In addition to the nutritional constituents of Dioscorea species, few studies have explored the
pharmaceutical potentials of different species of Dioscorea. They contain substantial amounts of
secondary metabolites referred to as bioactive compounds. Bioactive compounds are produced
within the plants besides the primary biosynthetic and metabolic routes associated with plant growth
and development. These compounds are not needed for their daily functioning but may provide
various functions such as protection, attraction or signaling to the plant [
119
]. Bioactive compounds
can be described as phytochemicals found in plants/food that have the capacity to influence the
cellular or physiological activities in humans as well as in animals. They modulate metabolic
processes by exhibiting numerous beneficial health effects such as anti-oxidative, anti-hypertensive,

Foods 2020,9, 1304 11 of 45
anti-inflammatory and anti-diabetic activities, inhibition of receptor activities, inhibition or induction
of enzymes and induction and inhibition of gene expression, thus resulting in the promotion of better
health [
120
]. However, it is worth noting that bioactive compounds can also have antinutritional
properties, thus eliciting toxicological effects in humans and animals. A wide list of bioactive
compounds such as phenolics, flavonoids, allantoin, dioscin, dioscorin, diosgenin, polyphenols,
tannins, hydrogen cyanide, oxalate, saponin and alkaloids have been reported in yam by several
studies as listed in Table 3. Their content in yam varies within and between species as reported by Wu
et al. [
35
]. These authors reported a varied range of 0.032%–0.092% dry weight and 0.62%–1.49% dry
weight of dioscin and allantoin detected in 25 yam landraces from four species (D. alata,D. polystachya,
D. persimilis and D. fordii), respectively. Inter- and intra-species diversity as it relates to bioactive
compounds content has also been highlighted by Price et al. [
121
]. Using high performance liquid
chromatography (HPLC), dioscin ranging between 0.086 and 0.945
µ
g/mLwas reported in yams of
African origin including D. cayenensis,D. mangenotiana and D. rotundata. In addition, dioscin has been
reported in other parts of yam plant such as rhizomes and roots as reviewed by Yang et al. [
122
]. Hence,
this section will elucidate the bioactive constituents of yams.

Foods 2020,9, 1304 12 of 45
Table 3. Bioactive compound profile of yam (Dioscorea species).
Species Phytochemicals Reference
D. alata Phenolics, phenol, flavonoid, flavonol, phytates/phytic acid, saponin, oxalates, alkaloid, tannins, allantoin, dioscin,
diosgenin, dioscorin, hydrogen cyanide [35,42,46,48,51,54,58,59,99,123]
D. bulbifera
Carotenoid, phenolics, phenol, polyphenol, flavonoid, terpenoid, saponin, steroid, alkaloid, tannins, phytates/phytic
acid, oxalates, hydrogen cyanide [29,42,51,56,58,62–64,99,123,124]
D. belophylla Saponins, alkaloids, flavonoids, tannins and phenols [125]
D. cayenensis Phenolics, phenol, saponin, alkaloid, tannins, phytates/phytic acid, oxalates, dioscin [29,42,48,58,67,99,126]
D. deltoida Polyphenol [29]
D. dumetorum Phenols, flavonoid, alkaloid, tannins, phytates/phytic acid, oxalates, dioscorine [42,70,99,127]
D. esculenta Phenolics, tannins, phytates/phytic acid, oxalates, hydrogen cyanide [42,48,51]
D. fordii Allantoin, dioscin [35]
D. glabra Phenol, flavonoid [123]
D. hamiltonii Phenol, flavonoid [123]
D. hirtiflora Phenol, flavonoid [127,128]
D. hirsute Dioscorine [128]
D. hispida Dioscorine, phenol, flavonoid [123]
D. japonica Phenols, flavonoilds, glycans [129,130]
D. mangenotiana dioscin [126]
D. oppositifolia Phenolics, phenol, flavonoid, tannins, oxalates, hydrogen cyanide [38,51,123]
D. panthaica Saponins [131]
D. persimilis Allantoin, dioscin [35]
D. pentaphylla Phenolics, tannins, oxalates, hydrogen cyanide, phenol, flavonoid [38,51,123]
D. polystachya Flavones, polyphenols, allantoin, dioscin [35,132]
D. praehensalis Tannins, phytates/phytic acid, oxalates, [42]
D. preussii Saponins [133]
D. pubera Phenol, flavonoid [123]
D. rotundata Phenolics, phenol, tannins, phytates/phytic acid, oxalates, saponin, alkaloid, hydrocyanatem dioscin [42,48,58,75,77,79,99,126]

Foods 2020,9, 1304 13 of 45
Table 3. Cont.
Species Phytochemicals Reference
D. sansibarensis Dioscorine [128]
D. spicata Phenolics, tannins, oxalates, hydrogen cyanide [38,51]
D. tomentosa Phenolics, tannins, oxalates, hydrogen cyanide [38,51]
D. trifida Phenolics [48]
D. triphylla Polyphenol [29]
D. versicolor Polyphenol [29]
D. villosa Flavonoid, phenol, saponin, alkaloid, tannins, Phytates/Phytic acid, oxalates [84]
D. wallichi Phenolics, tannins, oxalates, hydrogen cyanide, phenol, flavonoid [51,123]
D. polystachya: Chinese yam formerly known as D. opposita and D. batatas, NR: not reported.

Foods 2020,9, 1304 14 of 45
3.1. Steroidal Saponin
Saponins are a diverse group of glycosidic compounds containing triterpenoid and steroidal
aglycone that occur naturally in plants and in lower marine organisms. While triterpenoid saponins
are mostly found in dicotyledonous angiosperms, steroidal saponins are mainly present in monocot
species such as Dioscoreaceae [
134
]. Steroidal saponins vary in their structural constitutive frameworks,
sugars and aglycones, leading to a broad range of biological activities exerted by these compounds.
Depending on the aglycone moiety, steroid saponins can be classified into spirostane, stigmastane,
furostane, cholestane, ergostane and pregnane families [
135
]. Members of the Dioscorea genus mostly
contain spirostane and furostane steroid glycosides; however, studies have reported the possible
presence of other steroidal saponins [
136
,
137
]. Predominantly, yams contain a spirostane steroidal
sapogenin known as diosgenin with a structure of 25R-spirost-5-en-3b-ol consisting of a hydrophilic
sugar moiety linked to a hydrophobic steroid aglycone with a molecular formula C
27
H
42
O
3
[
138
,
139
].
The presence of diosgenin, an aglycon of dioscin has also been reported in several species of
yams, making yam one of the leading sources of steroidal sapogenin diosgenin, a precursor used for
the synthesis of the steroidal drugs estrogen and progesterone in the pharmaceutical industry [
140
].
The synthesis of cortisone and hormonal drugs such as sex hormone, progestational hormone
and other steroids with diosgenin extracted from D. zingiberensis,D. villosa and D. composite have
been reported [
141
,
142
]. The utilization of diosgenin is linked to its pharmacological activities and
medicinal properties including decreasing oxidative stress, inducing apoptosis, suppressing malignant
transformation, preventing inflammatory events, promoting cellular differentiation/proliferation
and regulating the T-cell immune response, thus, resulting in antidiabetes, anticancer, neuro- and
cardiovascular protective, immunomodulatory, estrogenic and skin protective effects [
134
,
138
,
143
,
144
].
A study by Tada et al. [
145
] examined the efficacy of diosgenin extracted from D. composita or D. villosa
against skin aging. Their findings revealed potential of diosgenin to enhance DNA synthesis of skin
using a human 3D skin equivalent model anda restoration of keratinocyte proliferation in aged skin. In
addition, spirostanes possess better antimicrobial activity when compared to other steroid glycosides;
however, the activity is dependent on the type and sequence of the sugars [135].
The underlying mechanism of action of diosgenin may vary depending on the disease and has
been reviewed by Cai et al. [
139
], Chen et al. [
143
] and Raju and Rao [
138
]. Although diosgenin has
been reported in several yam species, its content varies considerably within the Dioscorea genus with
D. barbasco (Mexican wild yam) and D. zingiberensis (Chinese yam) being very important sources for
diosgenin [
146
,
147
]. Perhaps this explains the reason why China and Mexico account for more than
half (about 67%) of world diosgenin production [
148
] and the utilization of this compound takes
preeminence in these countries. The presence of diosgenin has also been recorded in D. alata [
144
].
Contreras-Pacheco et al. [
149
] use gas chromatography-mass spectrometry (GC-MS) to quantify and
characterize diosgenin in sixty accessions of two Dioscorea species (D. sparsiflora and D. remotiflora)
collected from the city of Jalisco, Mexico. Their findings showed diosgenin at a range between 0.02 and
0.16 mg/kg in dry basis. In the same vein, Yi et al. [
146
] recorded a range of 0.78 mg/g to 19.52 mg/g in
three Dioscorea species including D. zingiberensis,D. septemloba and D. colletti; however, diosgenin was
not detected in D. polystachya. Huai et al. [
150
] revealed that the intra-species diversity with respect
to significant differences in the amount of disogenin in different yam varieties may be attributed to
climatic factors and environmental conditions such as growing and storage conditions.
In addition to diosgenin, other steroidal saponins have been reported in several Dioscorea species.
An extensive review by Sautour et al. [
151
] revealed over 50 saponins in 13 Dioscorea species, including D.
cayenensis,D. bulbifera,D. colletii,D. futschauensis,D. deltoidea,D. panthaica,D. nipponica,D. pseudojaponica,
D. parviflora,D. spongiosa,D. polygonoides,D. zingiberensis and D. villosa. The authors also highlighted
the pharmacological properties of the saponins as regards cytotoxic and antifungal properties.

Foods 2020,9, 1304 15 of 45
3.2. Dioscorin
Dioscorin is the main storage protein of the Dioscorea tuber, accounting for approximately 90%
of extractable water-soluble proteins [
152
,
153
]. Yam tubers contain two dioscorin proteins, dioscorin
A and dioscorin B, encoded by genes that share about 69% sequence similarity [
154
,
155
]. Using
Raman spectroscopy, Liao et al. [
155
] showed that the secondary structure of dioscorin A (molecular
weight [MW] ~ 33 kDa) of D. alata L. is mostly of alpha-helix whereas that of dioscorin B (MW ~
31 kDa) belongs to anti-parallel
β
-sheet. The authors also highlighted that the major amino acids
(phenylalanine, tyrosine, methionine, tryptophan and cysteine) microenvironment exhibited a clear
difference between dioscorin A and B [
155
]. An earlier study by this group of authors on the secondary
structure of dioscorin of three yam species (D. alata L., D. alata L. var. purpurea and D. japonica) reported
the similarity in molecular mass across the three species. However, they observed dissimilarity in
the amino acid composition and secondary structure of dioscorin between D. alata L., D. alata L. var.
purpurea and D. japonica [156].
In contrast to other storage proteins, dioscorin also exhibits enzymatic activities, such as carbonic
anhydrase, trypsin inhibitor, dehydroascorbate reductase, monodehydroascorbate reductase and
lectin activities [
152
,
157
–
159
]. The antioxidant potential of dioscorin purified from yam tuber has
also been reported [
160
,
161
]. Liu et al. [
161
] examined the antioxidant activities of dioscorin in the
tubers of two Dioscorea species (D. alata L. and D. batatas [presently known as D. polystachya]) using
(2,2-diphenyl-1-picryl-hydrazyl-hydrate) and hydroxyl radicals scavenging activity assay, reducing
power test and anti-lipid peroxidation test. Their findings revealed that dioscorins from the two yam
species exhibited different scavenging activities against DPPH (1, 1-diphenyl-2-picrylhydrazyl) and
hydroxyl radicals, with D. alata dioscorin showing higher antioxidant and scavenging activities as
compared to that of D. polystachya. This variation is ascribed to the variation in amino acid composition
and protein conformations [
161
]. Dioscorin has been reported to inhibit the activities of angiotensin
converting enzymes, thus suggesting its potential for control of hypertension [
162
]. In addition, Fu et
al. [
163
] demonstrated the potential of dioscorin isolated from D. alata as a Toll-like receptor 4 (TLR4)
activator as well as an inducer of the cytokine expression in macrophage through TLR4-signalling
pathways, thus, resulting in the activation of innate and adaptive immune system.
3.3. Alkaloids
Alkaloids are a large and structurally diverse group of amino acid-derived heterocyclic nitrogen
compounds of low molecular weight, widely distributed across plant kingdoms, microorganisms and
animals and deriving their name from their alkaline chemical nature [
164
]. Due to the complexity
of alkaloids, no single taxonomic principle could completely classify them [
165
]. Alkaloids can
be grouped into classes based on their natural and biochemical origin, as well as by chemical
structures (heterocyclic and non-heterocyclic alkanoids). Structurally, they can be divided into classes
such as quinolines, isoquinolines, indoles, pyrrolidines, pyridines, pyrrolizidines, tropanes and
terpenoids and steroids [
166
]. Presently, over 18,000 alkaloids have been reported in different plant
species [
167
] including those of Dioscorea. While alkaloids have been utilized pharmaceutically because
of their therapeutic activities such as anti-microbial, anti-hypertensive, anti-cancer, anti-inflammatory,
anti-human immunodeficiency virus (HIV) and many others, some alkaloids are highly toxic to
humans and animals [
164
,
165
,
168
]. They may contribute to undesirable sensory qualities such as
bitterness in food crops such as yams [
21
]. Alkaloids have been reported in several species of yams
(D. alata,D. oppositifolia,D. hamiltonii,D. bulbifera,D. pubera,D. pentaphylla,D. wallichii,D. glabra
and D. hispida) at values between 7.2 and 16 mg per 100 g dry weight [
123
] (Table 3). A study from
West Africa characterized the antinutritional factors in flour samples from four Dioscorea species and
reported alkaloid levels ranging between 0.02 and 0.11 mg/100 g [
169
]. Similarly, Senanyake et al. [
170
]
recorded alkaloid levels of 0.94, 1.64 and 1.89 mg/100 g in D. alata (Rajala), D. alata (Hingurala) and D.
esculenta (Kukulala), respectively. Alkaloid has also been reported in D. belophylla (Prain) Haines at a
concentration of 0.68 mg/100 g [125].

Foods 2020,9, 1304 16 of 45
One of the major alkaloids in yam is dioscorine, a toxic isoquinuclidine alkaloid with molecular
formula C
13
H
19
O
2
N [
171
,
172
]. Dioscorine has been reported in several yam species including D.
hispida,D. hirsute,D. dumetorum and D. sansibarensis (Table 3) [
128
]. The presence of dioscorine in
yam is associated with bitter taste and has been shown to induce nausea, dizziness and vomiting.
Dioscorine has exhibited the potency to trigger fatal paralysis of the central nervous system when
ingested [
4
], a reason that explains the use of dioscorine in the production of poisons for hunting
purposes. Due to the water solubility of this toxin, it is easily removed by the traditional processing
methods used for yam processing such as washing, boiling and soaking.
3.4. Flavonoids
Flavonoids, ubiquitous in photosynthesizing cells, naturally occur as aglycons, glycosides and
methylated derivatives [
173
]. Structurally, flavonoids (C
6
-C
3
-C
6
) contain a 2-phenyl-benzo(
α
)pyrane
or flavane nucleus, which comprises two benzene rings (A and B) linked through a heterocyclic pyrane
ring (C) [
174
]. Based on the position of the carbon of the C ring (on which B ring is attached), the degree
of unsaturation as well as oxidation of the C ring, flavonoids can be classified into subgroups [
175
]. For
isoflavones, the B ring is linked in position 3 of the C ring, while the B ring of neoflavonoids is linked
to position 4 of the C ring. Other subgroups of flavonoids in which the B ring are linked to position
2 include chalcones, flavones, flavonols, catechins, flavanonols, flavanones and anthocyanins. The
pharmacological potential of these compounds cannot be overemphasized. Flavonoids have shown
antioxidant, anti-inflammatory, antihypertensive, antidiabetic, antimicrobial, anticonvulsant, sedative,
antidepressant, anti-proliferative, anticancer, cardioprotective, antiulcerogenic and hepatoprotective
activity [176].
The presence of flavonoids has been reported in wide varieties of yams (Table 3). A recent study
by Padhan et al. [
177
] investigated the flavonoid content of nine Dioscorea species including D. alata,D.
oppositifolia,D. hamiltonii,D. bulbifera,D. pubera,D. pentaphylla,D. wallichii,D. glabra and D. hispida.
Their findings revealed flavonoid content ranging from 0.62 to 0.85 mg/g dry weight, of which levels
detected in D. alata and D. hispida were significantly lower compared to other Dioscorea species. In
addition, the authors reported potential antioxidant activities of the yam tuber extracts to range from
1.63 to 5.59%. D. bulbifera and D. pubera with significantly higher amount of bioactive compounds
such as flavonoids exhibited higher radical scavenging activity compared to other Dioscorea species
irrespective of the screening method (DPPH, ABTS, nitric oxide and superoxide radical scavenging
assay) used [
177
]. Flavonoids have also be quantified in D. belophylla (Prain) Haines (8.8 mg/100 g),
D. alata (Rajala) (5.2 mg/100 g), D. alata (Hingurala) (9.8 mg/100 g) and D. esculenta (Kukulala) (12.4
mg/100 g) [
125
,
170
]. Another Nigerian study also reported flavonoid content as well as the associated
antioxidant activity of three yam species (D. cayenensis,D. dumetorum and D. bulbifera) [60].
3.5. Phenols and Phenolic Acids
Phenols and phenolic acids are a group of abundant secondary metabolites found in plants.
Simple phenol is characterized by one or more hydroxyl groups (-OH) attached directly to the aromatic
system and comprising of resorcinol, phenol, phloroglucinol and catechol [
178
,
179
]. On the other
hand, phenolic acids are used to describe phenolic compounds having a benzene ring, a carboxylic
group and one or more hydroxyl and/or methoxyl groups in the molecule [
180
]. Phenolic acids
are rarely present in free form, occurring in bound form such as esters, amides or glycosides [
181
].
They comprise two parent structures, the hydroxybenzoic acid and hydroxycinnamic acid. While the
hydroxybenzoic acid (vanillic, gallic, protocatechuic and syringic acid) are the simplest phenolic acids
found in nature consisting of seven carbon atoms (C
6
-C
1
), hydroxylcinnamic acids (ferulic, caffeic,
sinapic and p-coumaric acid) are the most common in fruits and vegetables and have nine carbon
atoms (C6-C3) [182].
Dioscorea species have been identified as a possible source of phenols as well as phenolic acids
(Table 3). Zhao et al. [
183
] evaluated the total phenolic acids of two yam species (D. oppositifolia and

Foods 2020,9, 1304 17 of 45
D. hamiltonii) using an HPLC system. Their findings reported the presence of total phenolic acid
in both yams; however, the content in D. oppositifolia (297.3 mg/mL) was almost double that of D.
hamiltonii (158.2 mg/mL) which contributed to the significantly better antioxidant, anti-inflammatory
and immune regulation effects of D. oppositifolia compared to D. hamiltonii. Among the phenolic acids
detected in the two yam species, syringic acid was recorded the highest in both yams [
183
]. Similarly, a
study profiled the phenolic compounds in D. alata andreported the presence of ferulic, sinapic, caffeic
and p-coumaric acid and vanillic acid [
184
]. An earlier study reported phenolic constituents in 10
yam cultivars from five species highlighting the prevalence of these compounds in D. alata and D.
bulbifera when compared to other species (D. cayenensis,D. dumetorum and D. rotundata) irrespective
of the cultivar [
185
]. The phenolic concentration of D. rotundata (12–69 mg catechin/100g) was the
lowest among the five species and Graham-Acquaah et al. [
186
] reported a similar range (20–37 mg
catechol/100 g) in two cultivars of D. rotundata. The latter authors observed a significant variation
across tuber sections. In one D. rotundata cultivar (Puna), the order of concentration of phenol in the
sections were head >mid-section >tail, whereas the head and mid-section of Bayere fitaa cultivar had
a similar phenol concentration but significantly higher than that of the tail section [
186
]. Similarly,
Padhan et al. [
177
] found significant variation in the phenol content (2.1–9.62 mg/g dry weight) of
various yam species (D. alata,D. bulbifera,D. oppositifolia,D. pubera,D. hamiltonii,D. pentaphylla,D.
glabra,D. hispida and D. wallichii), with a significantly higher concentration in D. bulbifera compared to
other species.
3.6. Other Bioactive Compounds
In addition to the bioactive compounds described above, tannins, phytates and oxalates have been
reported in different species of yam (Table 3) [
42
,
54
,
99
,
187
]. Their content in yams varies depending
on species, variety, soil type and other environmental factors. The presence of tannin, phytate and
oxalate ranging from 56–1970 mg/kg, 270.7–379.4 mg/kg and 487–671 mg/kg on a dry matter basis,
respectively, were recorded in 43 genotypes from five yam species (D. alata,D. rotundata,D. dumetorum,
D. bulbifera and D. cayenensis) of major landraces in Nigeria [
99
]. These compounds are referred to as
anti-nutritional compounds because of the toxic effects associated with their consumption. Tannins are
water-soluble polyphenols known for their astringent taste and ability to bind to and precipitate various
organic compounds including proteins, amino acids and alkanoids, thus decreasing digestibility and
tastiness [
188
]. Structurally, tannins are classified into two groups, the hydrolysable and the condensed
tannins. Studies on experimental animals showed possible effects of tannins on feed intake and
efficiency, net metabolizable energy, growth rate and protein digestibility [
189
]. Their relationship
with reduced sensory quality of food cannot be neglected. Other adverse effects of tannins such as
increase in excretion of protein and essential amino acids and damage to the mucosal lining of the
gastrointestinal tract have also been reported [
189
]. On the other hand, studies have also shown
the pharmacological potential of tannins including antioxidant and free radical scavenging activity;
anticarcinogenic, antimutagenic, cardio-protective properties; and antimicrobial activities [190].
Phytate (myo-inositol hexakisphosphate), a salt form of phytic acid, is the major storage form of
phosphate and inositol found in a wide range of plants [
191
]. Its classification as an antinutrient is
associated with its capacity to form complexes with nutrients especially dietary minerals including
zinc, calcium and iron, thus reducing their availability in the body and causing mineral related
deficiency in humans. In addition, the formation of insoluble complexes by phytate with other food
components such as protein, lipids and carbohydrate have been reported thereby negatively impacting
the utilization of these nutrients [
191
,
192
]. Notwithstanding these negative effects, dietary phytate
exerts numerous positive health effects on humans including anticancer and antidiabetes activities and
protection against renal lithiasis, dental caries, HIV and heart related diseases as extensively reviewed
by Kumar et al. [
191
]. On the other hand, oxalate, salt of oxalic acid, occurs as an end product of
metabolic processes in plant tissues. Oxalates may occur as insoluble calcium oxalate, soluble oxalate
or in combination of the two forms as reported for yam tubers [
99
]. They bind to minerals especially

Foods 2020,9, 1304 18 of 45
calcium, magnesium and iron, resulting in unavailability of these minerals to human and animal
consumers [
193
]. Other detrimental effects such as intense skin irritation as a result of contact with
Dioscorea mucilage has been linked to the presence of calcium oxalate crystals.
Furthermore, hydrogen cyanide which is formed as a result of hydrolysis of glycosides by enzymes
in plants and is a neurotoxin found in cassava (Manihot esculenta), has been reported in yam though at
lower concentrations. Shajeela et al. [
51
] reported hydrogen cyanide ranging from 0.16 to 0.34 mg per
100 g in nine Dioscorea species with the highest level recorded in D. tomentosa and D. oppositifolia var
oppositifolia. Using spectrophotometry methods, cyanide was also reported in D. alata and D. hispida
Dennst sampled from Sleman, Yogyakarta [
194
]. Albeit of the antinutritional properties of yam tubers,
steps and methods of processing before consumption have proven to efficiently destroy these toxic
compounds [99,195].
4. Therapeutic Potentials of Yams
Pharmaceutical and phytomedical products derived from plants have a long history of use by
natives as traditional medicine and a proven evidence of efficacy. Gurib-Fakim [
196
] highlighted that
tribal people in the tropics use plants for medicine as direct therapeutic agents and starting points for the
elaboration of semi-synthetic compounds. A majority of secondary plant compounds used in modern
medicine were identified through ethnobotanical investigations. Ethnobotany is an interdisciplinary
field of research with specific focus on the empirical knowledge of indigenous people with respect to
natural plant substances that influence health and wellbeing and their associated risk [
196
]. Natives
of different ethnic communities that either cultivate or have wild Dioscorea spp. have utilized them
for medicinal purposes (Table 4). Unfortunately, documentation of the importance and utilization on
Dioscorea is still limited. Research has shown that yam bioactive compounds and its supplementations
play vital roles in weight changes, activities of carbohydrate digestive and transport enzymes, changes
in the morphology of intestines, alterations in blood lipids, lipid peroxidation reduction and liver
damage prevention [
197
]. A recent study by Pinzon-Rico and Raz [
198
] highlighted the high demand
and robust market of four wild yam species including D. coriacea,D. lehmannii,D. meridensis and
D. polygonoides in Bogota, Colombia. The four species have been implicated in blood purification
probably because of their effect on reducing blood cholesterol, triglycerides, uric acid and glucose.
The general acceptability and long history of local consumption of yams among various communities
across the continents may be attributed to its safety and portends high regulatory acceptability [
199
].
Current research has shown that yams contain substantial amounts of secondary metabolites referred
to as bioactive compounds that have pharmaceutical potentials as discussed and the health benefits
associated with yam consumption is discussed hereafter.

Foods 2020,9, 1304 19 of 45
Table 4. Medicinal uses of yam (Dioscorea species).
Species Source of Extract Biological Properties/Administration Reference
D. alata Tuber/bulb Cure piles, gonorrhea and leprosy, anti-inflammatory, purgative, diuretic, anti-rheumatic
properties; prevent cancer, reduce blood sugar, diabetes [200,201]2, [202,203]1
Tuber Antihelminthic properties [204,205]2
Leaf Fever [206]2
D. bartletti Rhizome Stagnation of blood, anemia [207]2
D. belophylla Tuber Treatment of fever, dysentery, headache and malaria [208]2
D. bulbifera Tuber Treatment of dementia [209]1
Treatment of diabetes [210]1
Leprosy and tumors [211,212]1
Microbial infections and pig cysticercosis [213]1
Antispasmodic, analgesic, aphrodisiac, diuretic and rejuvenative tonic [214]1
Effects on liver and heart, reduces carbuncles, lung abscesses, breast lumps, goiter [212]1
Abdominal pain [215,216]2
Cough [217]2
Oral contraceptive. [218,219]2
Raw tuber consumed as an appetizer [220]2
Rheumatism [221]2
Aphrodisiac [222]2
Aerial bulb Oxidative stress induced pathological disorders [210,223]1
Anthelmintic treatment [224]1
Leaf Treatment of Elephantiasis [225]2
Leaf paste fights dermatological diseases [226]2
Stem Fresh stem shoots are used on hair to fight dandruff[7]2
D. bellophylla Tuber Lowers blood cholesterol and reduces heart attack [7]2
D. cayenensis Tuber Anti-diarrheal [206]2
D. collettii Rhizome Cervical carcinoma, urinary bladder carcinoma, renal tumor [227]1
D. deltoidea Tuber Digestive disorders, sore throat, diarrhea, abdominal pains, wounds, burns, anemia [228–230]2
Anti-rheumatic and treatment of ophthalmic conditions [231]2
Antihelmintic treatment [229]2
Birth control, oral contraceptive [217]2
Antihelminthic [204,232]2
D. dumetorum Tuber Treatment of diabetes [233]1
Control hyperlipidemia, hypercholesterolemia and hyperketonemia [234]2
Jaundice treatment [235]2

Foods 2020,9, 1304 20 of 45
Table 4. Cont.
Species Source of Extract Biological Properties/Administration Reference
D. esculenta Tuber Inflammations, nervous disorders and respiratory infections [235]2
Dysentery and pain relief [7]2
D. hamittonii Tuber Stomach ache and appetizer [235]2
Management of diarrhea [236]2
Piles [220]2
D. hirtiflora Tuber Gonorrhea treatment [127]1
D. hispida Leaf/root/tuber Treatment of mole, insect bites and insomnia [225]2
Tuber Treatment of vomiting, indigestion and serves as a purgative when consumed fresh [7]2
Treatment of wounds and injuries [219]2
Ophthalmic ointment [237]2
D. japonica Rhizome Diarrhea and dysentery due to spleen deficiency, fatigue, wasting and thirsting, seminal
emission, vaginal discharge and frequent urination [238]1
Inflammation, asthma, rheumatoid arthritis [239]1
coughing and wheezing [239]1
D. membranacea Rhizome Cancer [240]1
D. nipponica Rhizome
Dissipation of lumps and goiter, clears heat, relieves toxicity, cools the blood, stops bleeding and
coughing, calms sneezing, poisonous snake bites, bleeding due to blood-heat and whooping
cough
[241]2
Anti-rheumatic, analgesic, aids blood circulation, anti-diuretic, aids digestion [242,243]1
D. oppositifolia Rhizomes/tuber Relief of menopausal syndromes, rejuvenation of early mothers [220,244]2
Leaf/flower/tuber Antiseptic for ulcer; the roots are chewed to cure toothache and aphtha [245,246]2
Tuber Increasing fertility in men [222,236,247]2
Constituent in epileptic and nasal relief formula [219]2
D. panthaica Rhizome Gastric diseases, bone injuries, rheumatic arthritis [122]2
Cardiovascular diseases [248]1
D. pentaphylla Leaf/vine Treatment of paralysis [225]2
Tuber/flower/young
shoot Rheumatism [7,249]2
Tuber Pain relief and reduce swelling [219]2
Stomach disorders [7,250]2
D. polystachya Rhizome Consumptive cough and dysentery, aid for digestion and gastric motility and for restraining
nocturnal emissions [251]1
D. prazeri Tuber Antihelminthic [204]2

Foods 2020,9, 1304 21 of 45
Table 4. Cont.
Species Source of Extract Biological Properties/Administration Reference
D. pubera Tuberous
rhizome/bulb Cure colic pain [246,252]2
Tuber Weakness [253]2
D. septembola Rhizome Rheumatism, urethra, renal infection [254]1
D. spongiosa Rhizome Rheumatism, urethral, renal infections [255]2
D. sylvatica Tuber Decoction used to treat cuts, wounds and sores [256]2
D. trinervia Tuber Chronic diarrhea, asthma and diabetes [7]2
D. vexans Tuber Anti-fertility [257]2
D. villosa Rhizome Rheumatism [135]1, [258]2
Menstrual complaints, perimenopausal symptoms [259,260]2
D. wallichii Tuber Flatulence and stomach pain [7,204,261]2
De-appetizer [222]2
D. zingiberensis Rhizome Cough, anthrax, rheumatic heart disease, rheum, arthritis, tumefaction, sprain [262]2
1Medical study; 2Ethnobotanical/review study; D. polystachya: Chinese yam formerly known as D. opposita and D. batatas.

Foods 2020,9, 1304 22 of 45
4.1. Antimicrobial Potential of Yam
Over the years human medicine has improved greatly; but infections caused by microbes such
as bacteria, viruses, fungi and parasites remain a lingering hurdle to overcome, especially with the
emergence of widespread drug resistant forms of these microbes and adverse side effects to certain
antibiotics [
263
]. Research into plant sourced antibiotics has intensified and the antimicrobial potentials
of certain yam species have been investigated and reported. Using crude extracts and compounds
isolated from the bulbils of the African medicinal plant D. bulbifera, Kuete et al. [
264
] showed that these
extracts and compounds can be effective drugs against a wide range of resistant gram negative bacteria.
The inhibitory effect of the extracts was dependent on the concentration but still less effective compared
to standard antibiotics. Likewise tuber mucilage extract of D. esculenta have exhibited antibacterial
properties against three human bacterial strains including Escherichia coli,Pseudomonas aeruginosa
and Staphylococcus aureus [
263
]. The inhibitory potential of D. alata tuber extracts against Salmonella
typhimurium,Vibrio cholerae,Shiegella flexneri,Streptococcus mutans and Streptococcus pyogenes have also
been reported [
31
]. In addition, endophytic fungi isolated from rhizome extract of D. zingiberensis, a
Chinese medicinal plant, has shown its antibacterial potential for use for the production of antibacterial
natural products [
265
]. The same trend was observed by Sonibare and Abegunde [
127
]. Using the agar
well diffusion and pour plate method, the authors reported extracts of D. dumetorum and D. hirtiflora
tubers as possible sources of antimicrobial agents with their antimicrobial efficacy directly linked to
the phenolic contents of the plants and DPPH scavenging activity. Kumar et al. [
24
] compared the
antibacterial activity of D. pentaphylla tuber extracts and antibiotics (penicillin and kanamycin) on
five selected bacterial strains (Vibrio cholera,Shigella flexneri,Salmonella typhi,Streptococcus mutans and
Streptococcus pyogenes). Their findings revealed a significant inhibitory activity of D. pentaphylla tuber
extracts against the tested bacteria. This activity was attributed to diosgenin content in the tubers.
4.2. Antioxidant Activities of Yam
Antioxidant activities have been reported in different species of Dioscorea, including D. alata,D.
bulbifera,D. esculenta,D. oppositifolia and D. hispida (Table 4) [
266
–
270
]. Using a DPPH assay, Murugan
and Mohan [
268
] reported radical scavenging activity of 79.3% for 1000
µ
g/mL D. esculenta extract
with IC
50
value of 38.33
µ
g/mL, whereas IC
50
value of 18.25
µ
g/mL was recorded for the reference
standard (ascorbic acid). The same trend was observed by the author when the ABTS assay was used,
with radical cation scavenging activity range of 46.1% to 64.1% at concentration between 125 and
1000
µ
g/mL and IC
50
value of 40.50
µ
g/mLwhile IC
50
value was 20.67
µ
g/mLfor trolox. The author
attributed the antioxidant and free radical scavenging activity to high content of total phenolic and
flavonoid compounds. Similarly, Padhan et al. [
177
] examined the antioxidant activity of nine different
yams (D. alata,D. bulbifera,D. pentaphylla,D. pubera,D. glabra,D. oppositifolia,D. wallichii,D. hispida
and D. hamiltonii) cultivated in Koraput, India. Their findings revealed antioxidant capacity ranging
from 1.63% to 5.59%, with IC
50
values of 101–1032, 77.9–1164, 47–690 and 27–1023
µ
g/mLfor ABTS,
DPPH, nitric oxide and superoxide scavenging activity, respectively. Among the yam species evaluated,
antioxidant capacities of D. pubera,D. pentaphylla and D. bulbifera were significantly higher with lower
IC
50
values than the standards when compared to the other species. The variation in scavenging
activities observed in the different yam species is attributed to the disparity in the content of the
bioactive compounds in the yam species [177].
4.3. Anti-Inflammatory Activity of Yam
Several animal studies have reported the anti-inflammatory activity of Dioscorea species. Olayemi
and Ajaiyeoba [
271
] investigated the anti-inflammatory potential of defatted methanol extract of
D. esculenta tuber on Wistar rats. Their finding showed a significant dose-dependent inhibition
of the carrageenan at doses of 100 mg/kg and 150 mg/kg which was comparable to that of 150
mg/kg acetylsalicylic acid (reference standard). Chiu et al. [
130
] confirmed that D. japonica ethanol

Foods 2020,9, 1304 23 of 45
extract elicited an in vivo anti-inflammatory effect on mouse paw oedema induced by λ-carrageenan.
Pre-treatment using dried yam (Dioscorea spp.) powder on Sprague-Dawley rats before inducement
of duodenal ulcer by intragastric administration of cysteamine-HCl (500 mg/kg) revealed that dried
yam powder exerted a significant protective effect by reducing the incidence of perforation caused by
cysteamine and preventing duodenal ulcer, which was comparable to the pantoprazole effect [
272
]. The
observed effect of yam powder was attributed to its potential to lower inflammatory cytokines as well as
scavenging free radicals and up-regulating activity of carbonic anhydrase. The hydro-methanol extract
of D. alata tubers which contain different bioactive phytocompound has also shown to significantly
down-regulate the pro-inflammatory signals in a gradual manner compared to a reference control
(
µ
g/mL) [
203
]. Mollica et al. [
273
] reported the anti-inflammatory activity of extract from D. trifida on
food allergy induced by ovalbumin in mice. In addition extracts from leaf, rhizome and bulbil have
exhibited anti-inflammatory activity.
4.4. Anticancer Activity of Yam
Synthetic medications and chemotherapy for cancer management comes with a multitude of
side effects that are often intolerable for most cancer patients; thus, naturally occurring bioactive
compounds in plants are increasingly becoming better alternatives.
In vitro
cytotoxicity screening
provides insights and preliminary data that help select plant extracts with potential anticancer
properties for future work and
in vivo
replication. A study by Itharat et al. [
240
] showed that aqueous
and ethanol extracts of rhizome of D. membranacea and D. birmanica were cytotoxic against three
human cancer cell lines while remaining non cytotoxic to normal cells. The use of active compounds
naphthofuranoxepins (dioscorealide A and B) and dihydrophenanthrene from D. membranacea (locally
known as Hua-Khao-yen) rhizome in Thai medicine is highly potent and has exhibited cytotoxic
activity against five types of human cancer cells [
274
–
276
]. This was supported by a more recent study,
which highlighted the utilization of dioscorealide B as a possible anticancer agent for liver cancer
and cholangiocarcinoma [
277
]. The hepatotoxic compound diosbulbin B has also been reported as a
major antitumor bioactive component of D. bulbifera (air potato) in dose-dependent manner, with no
significant toxicity in vivo at dosage between 2 and 16 mg/kg [278,279].
Plants with steroidal saponins have exhibited anticancer effects [
280
–
282
] and these bioactive
compounds are abundant in different Dioscorea species. According to Zhang et al. [
283
], deltonin exerts
an apoptosis-inducing effect, which may correlate with ROS-mediated mitochondrial dysfunction, as
well as the activation of the ERK/AKT signaling pathways, thereby suggesting deltonin as a potential
cancer preventive and therapeutic agent [
284
]. Cytotoxicity studies using steroidal saponins from
Dioscorea collettii var. hypoglauca showed they were active against human acute myeloid leukemia
under
in vitro
conditions [
285
]. In an anticancer drug screen by the National Cancer Institute (NCI),
USA, protoneodioscin, a furostanol saponin compound isolated from Dioscorea collettii var. hypoglauca,
exhibited cytotoxicity effects against most cell lines including leukemia, central nervous system, colon,
prostate cancer [
227
]. It is interesting to note that no compound in the NCI data base shares a similar
cytotoxicity pattern to those of protoneodioscin, thus indicating a unique anticancer pathway. The
polysaccharide of RDPS-I purified from the water extract of Chinese yam tuber exerted a significant
inhibition on the cancer cell line of melanoma B16 and Lewis lung cancer in mice in-vivo [
286
]. Another
study by Chan and Ng [
279
] investigated the biological activities of lectin purified from D. polystachya
cv. Nagaimo. The authors observed after 24 h treatment the inhibitory role of lectin on the growth of
some cancer cell lines including nasopharyngeal carcinoma CNE2 cells, hepatoma HepG2 cells and
breast cancer MCF7 cells, with IC50 values of 19.79
µ
M, 7.12
µ
M and 3.71
µ
M, respectively. Through
the induction of phosphatidylserine externalization and mitochondrial depolarization, it has been
revealed that D. polystachya lectin can evoke apoptosis in MCF7 cells [
279
]. Furthermore, diosgenin has
been reported to significantly inhibit the growth of sarcoma-180 tumor cells
in vivo
while enhancing
the phagocytic capability of macrophages
in vitro
, thus suggesting that diosgenin has the potential
to improve specific and non- specific cellular immune responses [
287
]. The anticancer mechanism

Foods 2020,9, 1304 24 of 45
of action for diosgenin may be attributed to modulation of multiple cell signaling events including
molecular candidates associated with growth, differentiation, oncogenesis and apoptosis [288].
4.5. Anti-Diabetic Activity of Yam
Notwithstanding the availability of numerous anti-diabetic medicines in the pharmaceutical
industry and market, diabetes and related complications remain a medical burden. Plants’ anti-diabetic
potential stems from their ability to restore the function of the pancreatic tissues which leads to three
possible outcomes: increasing the insulin output, inhibiting the intestinal absorption of glucose and
restoring the facilitation of metabolites in insulin dependent processes [
234
]. There is minimal evidence
on specific action pathways in the treatment of diabetes; however, we can infer that most plants that
contain bioactive substances such as flavonoids, alkaloids and glycosides offer a buffer to patient
management [
289
]. D. dumetorum, commonly known as bitter yam, has long been proven to play
active role in the treatment of diabetes in traditional medicine due to its hypoglycemic effect [
233
].
Literature reveals that aqueous extract of D. dumetorum tuber, known for its alkaloid (dioscoretine)
content, control hypercholesterolemia, hyperlipidemia and hyperketonemia [
234
]. In 2015, a study
which evaluated the anti-diabetic potential and free radical scavenging activity of copper nanoparticles
(CuNPs) synthesize with the aid of D. bulbifera tuber extract revealed a promising antidiabetic and
antioxidant properties [
210
]. In animal studies, extract of D. bulbifera and D. alata tuber showed
significant reduction in blood glucose level as well as increased body weight in rats treated with
streptozotocin and alloxan, respectively [
290
,
291
]. Another study showed, however, consumption of
D. bulbifera by female diabetic rats decreased hyperglycemia and bone fragility [292]. A similar trend
was observed on dexamethasone-induced diabetic rats treated with D. polystachya extract [293].
The quest for novel drugs in the clinical treatment of diabetic complications such as peripheral
neuropathy has led to the discovery of DA-9801, an ethanol extract of D. japonica,D. rhizoma and D.
nipponica, as a potential therapeutic agent [
294
,
295
]. Peripheral neuropathy is a common disorder
among diabetic patients, a result of the malfunctioning of the peripheral nerves. Peripheral neuropathy
is characterized by symptoms such as pain, numbness and chronic aberrant sensations, which often
disrupt sleep and can lead to depression, thus affecting the quality of life [
238
]. An investigation
conducted by Song et al. [
296
] on the inhibitory effects of DA-9801 on transport activities of clinically
important transporters showed that inhibitory effects
in vitro
did not translate into
in vivo
herb drug
interaction in rats. Interestingly, Jin et al. [
297
] and Moon et al. [
238
] further buttressed the potential
therapeutic applications of DA-9801 for the treatment of diabetic peripheral neuropathy. These studies
show that DA-9801 reduced blood glucose levels and increased the response latency to noxious thermal
stimuli. It is anticipated that DA-9801 can be used as a botanical drug for the treatment of diabetic
neuropathy. Transporters are critical in the absorption, distribution and elimination of drugs, thus
modulating efficacy and toxicity [
296
]. This prediction of interaction is vital in clinical studies and
the drug development process. Sato et al. [
298
] demonstrated that the natural product diosgenin
remains a candidate for use in acute improvement of blood glucose level in type I diabetes mellitus.
Also, Omoruyi [
299
] supports the use of D. polygonoides extracts in clinical management of metabolic
disorders such as diabetes.
4.6. Anti-Obesity and -Hypercholesterolemic Activities of Yam
Jeong et al. [
300
] reported the anti-obesity effect of D. oppositifolia extract on diet-induced obese
mice. In their study, a high-fat diet was given to female mice with 100 mg/kg of n-butanol extract
of D. oppositifolia for 8 weeks. The authors observed a significant decrease in total body weight and
parametrial adipose tissue weight; as well as decrease in total cholesterol, triglyceride level and low
density lipoprotein (LDL)-cholesterol in blood serum; female mice associated with the ingestion of
D. oppositifolia n-butanol extract. The observed effect of D. oppositifolia n-butanol extract is mediated
through suppression of feeding efficiency and absorption of dietary fat [
300
]. An earlier study, which
evaluated the anti-obesity effect of methanol extract of D. nipponica Makino powder, reported the

Foods 2020,9, 1304 25 of 45
effectiveness of the extract against body and adipose tissue weight gains in rodents induced by a
high-fat diet [
301
]. The anti-obesity potential of extract of D. steriscus tubers extracted using a solvent
cold percolation method have been reported [
302
]. When compared with a commercially available
anti-obesity medication (herbex), D. steriscus tubers extract showed a significantly higher anti-obesity
activity. The author attributed the result to be associated with the bioactive compounds of D. steriscus
tubers, which can act as lipase and
α
-amylase inhibitors and thus are useful for the development of
anti-obesity therapeuticals [302].
Extracts of Dioscorea species have been used in clinical management of other metabolic disorders
such as abnormal cholesterol level. Several animal studies have shown the antilipemic effects of
sapogenin and diosgenin-rich extract of Dioscorea species like D. polygonoides (Jamaican bitter yam) on
hypercholesterolemic animals such as mice and rat, thus resulting in the reduction in the concentrations
of blood cholesterol [
303
]. Another study which investigated the effect of D. alata L. on the mucosal
enzyme activities in the small intestine and lipid metabolism of adult Balb/c mice showed constant
improvement in the cholesterol profile of the liver and plasma of mice fed with 50% raw lyophilized
yam for a duration of 21 days [
304
]. The authors also observed an increase in fecal excretions of neutral
steroid and bile acids whereas absorption of fat was reduced in mice fed with 50% yam diet. Yeh et
al. [
305
] observed a significant reduction in plasma triglyceride and cholesterol in male Wistar rat as a
result of consumption of a 10% high cholesterol diet supplemented with 40% D. alata.
4.7. Yam as an Agent for Degenerative Disease Management
In an animal study using Swiss albino mice with streptozotocin induced dementia, D. bulbifera
tubers were reported as having the potential to preserve memory while serving as a preserving,
curing and restorative agent [
209
]. The authors further highlighted the possible delay of onset of
neurodegenerative diseases as well as mitigation with the ingestion of dietary polyphenols that
confer protection to oxidative stress and neurodegeneration. Also, the neuroprotective effect of D.
pseudojaponica Yamamoto using senescent mice induced by D-galactose indicated the useful potential
of yam for treatment of cognitive impairment, a process partly mediated via enhancing endogenous
antioxidant enzymatic activities [
306
]. The steroidal saponin—diosgenin—one of the major bioactive
compounds in yam, was found to aid the restoration of axonal atrophy and synaptic degeneration, thus
improving memory dysfunction in transgenic mouse models of Alzheimer’s disease [
307
]. Diosgenin
administration prior to surgery in rat test models reduced significantly the death rate while improving
impaired neurological functions, thus establishing the potential cerebral protection of diosgenin against
transient focal cerebral ischemia- reperfusion (I/R) injury [
254
]. In an
in vivo
study using mice, the same
group of authors reported that diosgenin enhanced neuronal excitation and memory function in normal
mice, which is mediated by 1,25D3-MARRS (membrane associated, rapid response steroid-binding)
triggered axonal growth [
308
]. This seems to support a school of thought that sees diosgenin as a new
category of cognitive enhancers with potential of reinforcing neuronal networks and thus, formed
the basis for the use of humans as test models. Tohda et al. [
309
] conducted a Japanese version of
the Repeatable Battery for Assessment of Neuropsychological Status (RBANS) test on 28 healthy
volunteers (between the ages 20 and 81 years) under diosgenin-rich yam extract administration with
findings confirming a significant improvement in cognitive function. However, the limitation of this
study remains the sample size, non-randomized selection of volunteers (sampled adults were only
well-educated Asians) and daily dietary intake and physical activity levels of the individuals were
not accessed.
Studies on animal studies have reported the anti-osteoporosis potential of yam [
310
–
312
]. Extracts
of D. alata leaves and roots examined on mouse spleen and bone marrow cells showed the ability of
stimulating proliferation on both cells thereby significantly increasing the cell concentrations [
312
].
Another study on ovariectomized female BALB/C mice revealed that 2 weeks feeding with D. alata
powder prevented loss of bone mineral density and improved bone calcium status, however, the
uterine hypertrophy was not stimulated [
313
]. Similarly, Han et al. [
311
] investigated the
in vivo
effect

Foods 2020,9, 1304 26 of 45
of ethanol extract of D. spongiosa on glucocorticoid-induced osteoporosis in rat. Their findings revealed
that D. spongiosa extract inhibited glucocorticoid-induced osteoporosis and improved the bone tissue
metrology, BMC, BMD and biomechanical indicators. In addition, the authors observed a repair of the
microscopic changes of the cancellous and trabecular bones. Based on the changes in the biochemical
indexes, these effects were linked to the ability of the yam extract to inhibit excessive bone transition
and bone resorption [
311
]. Other health effects on degenerative diseasessuch as hypertension and
osteoarthritis by Dioscorea species have been reported [314–316].
4.8. Yam as an Agent for the Management of Menopausal Symptoms
Menopause is associated with a decline in estrogen level produced by the ovaries resulting in
several side effects including mental changes, hot flashes, skin aging, osteoporosis and cardiovascular
problems [
317
]. Hormone replacement therapies (HRT) such as estrogen and progesterone replacement
have been deployed to handle these challenges with side effects [
318
], however, HRT may predispose
users to development of degenerative diseases such as ovarian cancer [
319
]. Rossouw et al. [
320
]
reported an increase in the incidence of coronary heart disease and breast cancer amongst women on
estrogen and progestin therapy, hence necessitating alternative treatment options that are as effective
and less detrimental. Many traditional systems have implemented treatment plans with a number of
plant species for the management of physiological changes associated with menstruation, conception,
pregnancy, birth, lactation and menopause [
321
]. There is reported evidence that Dioscorea species,
while serving as nutritional supplements, proffer medicinal properties and relief of menopausal
symptoms [
322
]. A Taiwanese study examined the efficacy of D. alata in the treatment of menopausal
symptoms on 50 women [
323
]. The authors recorded an evident improvement in the accessed
parameters, including feeling tense/nervous or excitable, insomnia, musculoskeletal pain as well as
the positive effect of the blood hormone profile among women that received D. alata. Similarly, Wu
et al. [
324
] found that replacing two-thirds of staple food with yam for 30 days positively influenced
antioxidant status, lipids and sex hormones of 22 apparently healthy postmenopausal women.
Chinese anti-menopausal medicine formula containing rhizomes of D. oppositifolia L. have shown
the potential to regulate serum levels of estrogen, follicle-stimulating hormone and luteinizing hormone
thereby alleviating some side effects in post-menopausal women [
325
]. This is in line with the study by
Lu et al. [
244
], whose research result supports the use of D. oppositifolia in Chinese medicine for easing
menopausal disorders. Proteins isolated from D. alata,D. zingiberensis and D. oppositifolia showed
potential to upregulate the translational levels of estrogen receptor beta, thus possibly reducing the
risk of ovarian cancer [
244
]. D collettii var. hypoglauca have been implicated in the production of
herbal formula feng bei bi xie used primarily for the treatment of cervical carcinoma which is prevalent
within female aging period [
326
]. In Central America, patients with blood stasis and anemic conditions
are treated with a decoction obtained from the rhizomes of D. bartletti [
207
]; while in Latin American
communities, the use of decoctions to ameliorate pains of childbirth, painful menstruation, ovarian
pains and vaginal cramping have been reported [
327
]. The diosgenin composition of yam has placed
Dioscorea species as major constituents for commercial progesterone production used for treatment
of menopausal hot flashes [
207
]. When administered orally to female Sprague Dawley rats, Higdon
et al. [
328
] reported an increase in uterine weight, vaginal opening, vaginal cell proliferation and
reduced bone loss. This estrogenic influence mechanism is consistent with the findings of Michel et
al. [
207
] who reported mild
in vitro
biding affinity for estrogen alpha and beta receptors in their test
models. Although an
in vitro
bioassay does not necessarily correspond to
in vivo
efficacy, the data seem
to implicate a significant influence of Dioscorea species in management of issues related to women’s
reproductive health.
4.9. Yam as Pharmaceutical Excipient
Although much effort have been shown on the importance of yam starch in relation to food, limited
attention has been given to its other potentials such as an excipient for the pharmaceutical industry.

Foods 2020,9, 1304 27 of 45
Zuluaga et al. [
329
] highlighted that yam starch could be used as a pharmaceutical excipient for
tablet and capsule formulation comparable to potato starch, with further potential as thickening agent.
Nasipuri [
330
] reported yam starch as an efficient binder/disintegrant in tablet formulations containing
both soluble and insoluble organic medicinal substances. Studies have shown that D. dumetorum
and D. oppositifolia starches are highly compressible and form tablets with acceptable crushing force.
Both species possess small granule size, large specific surface, volume-surface mean, surface-number
mean and spherical symmetry. These qualities imply better performance as an excipient especially
with respect to product process and better homogeneity of mixes when compared with starches from
D. alata and D. rotundata, with larger granules and high amylase content [
331
,
332
]. However, under
high compression pressures, D. roundata and D. alata can be used for tablet formulations where faster
disintegration and dissolution is desired [332,333].
5. Conclusions and Future Perspectives
Dioscorea species provide safety nets as foods and conventional and unconventional medicine
during famine and endangered periods. Yam constituents such as flavonoid, diosgenin and dioscorin,
tannin, saponin and total phenols places them as good food source of bioactive compounds to
consumers [
23
]. However, exploitation of the rich diversity within the Dioscorea genus may lead to
extinction if proper steps are not taken in terms of advocacy and conservation. This will directly
result in loss of this potential source of active compounds for the pharmaceutical industry as well
as constituting a huge genetic loss with respect to crop improvement and breeding. Rational and
sustainable use is highly encouraged within the array of wild species. Sensible utilization of this
diversity entails understanding species availability, ease of access, possibility of preservation, replanting
and establishment of priorities in respect to its optimal pharmaceutical use [
334
]. Plant derived drugs
will receive more acceptance in modern medicineand health systems if they can be efficacious,
safe and quality controlled as in the case of synthetic products [
335
]. The understanding of the
pharmacologically active compounds within Dioscorea diversity will assist in standardizations and
analysis of formulations [
334
]. The gaps in knowledge of chemical composition, ecological factors and
geographical spread of diversity and environmental impacts as relates to chemical biodiversity and
plant variability need to be urgently addressed.
Investigation to study the medicinal potential of over 600 wild and domesticated Dioscorea species
requires a multidisciplinary dimension involving indigenous natives who have a thorough grasp
of these plants while adopting a well thought out strategy that puts into context society, health,
conservation and sustainable use of species biodiversity. Numerous synthetic contraceptives and
steroid related hormonal medications are made of dioscin. Unfortunately, the global need of dioscin is
around 8000 tons but present production status puts it at 3000 tons [
336
]. The increasing pressure from
pharmaceutical industries is laying a high demand burden thus making this vital resource a scarce
commodity looking into the future. Another perspective to this problem is the ineffectiveness of methods
for extracting bioactive compounds. It has been reported that the rate of extraction and separation
are generally low, with only the extraction of diosgenin accorded priority [
337
]. It is imperative that
efficient strategies that concentrate diosgenin from its natural sources are optimized [
144
] putting
into consideration other compounds and eliminating waste. It is however important to develop
carrier systems like nanoparticles for targeted delivery of yam bioactive extracts and compounds, thus
improving efficacy while reducing side effects [
144
]. The need to standardize analytical protocols
toward achieving optimal extraction should not be overlooked. Although extraction methods, such
as soxhlet, maceration and hydrodistillation, have been extensively applied in the extraction of
bioactive compounds, with newly developed methods shown to be much cleaner, higher yielding
and efficient, such as thein situ pressurized biphase acid hydrolysis extraction reported by Yang et
al. [
338
] should be explored. The recent technological advancement in chromatographic techniques
such as liquid chromatography-mass spectrometry (LC-MS) and high-resolution mass spectrometry
(HR-MS) should be utilized for identifying and quantifying the various compounds in yam species.

Foods 2020,9, 1304 28 of 45
In addition, sophisticated instrumentation such as HR-MS should be applied to unravel possible
beneficial unknown compounds in yam crop.
Further
in vivo
study is highly encouraged with respect to oxidative stress and antioxidant
activities using purified compounds isolated from yam species. For most developing and poor
countries, it is imperative to diversify into functional foods, including from Dioscorea species. These
can be consumed on a regular basis thus serving both nutritional and medicinal purposes. These
plants, often in the wild, can be targeted for increased production and conservation. The local populace
should be enlightened on the consumption values which directly can lead to reduction in the cost of
health care while leading to improved diet. However, due to the huge demand by pharmaceutical
industries and agencies, most wild Dioscorea species are threatened in their natural habitat. The
indigenous knowledge and therapeutic potential of most of the Dioscorea species is fast eroding as the
situation worsens with increasing urbanization, industrialization and over-exploitation. Efforts toward
developing comprehensive information on the therapeutic use, dosage and chemical compounds
implicated in the treatment of diseases should be accelerated. Most significant is the ascertaining of
the safety level and toxicity profiling of these compounds found in undomesticated yams. This will
help ease burden in the rural communities that solely depend on these traditional medicine as health
remedies. For instance, due to the high cost of steroid based pharmaceuticals in the management of
women’s health disorders, the alternative reliance on herbal remedies is the preferred option to treat
hormonally regulated health situations in most impoverished communities. Thus, there is need to
provide sufficient empirical scientific basis to support the traditional use of the diversity inherent in
the yam crop for hormone therapy related treatment.
Recently, the poisoning cases are occasionally reported in association with the rising popularity of
Dioscorea consumption prescriptions in clinical use. Chronic and excessive exposure to D. bulbifera
tuber has caused liver injury in some patients [
339
]. Also,
in vivo
and
in vitro
experimental studies
have demonstrated that D. bulbifera tuber could induce hepatotoxicity [
340
,
341
], increase relative liver
weight and can cause death [
214
]. It is noteworthy to mention that concentrated herbal preparations
of samples of Dioscorea species abound, with most of them having little or no information about the
exact composition and required dosage, thereby increasing health risks to potential consumers [
214
]. It
thus becomes imperative to establish estimated toxicity values for Dioscorea species towards efficient
utilization in food based clinical management. In view of this, further investigation is required and there
is need for relevant government and donor agencies to invest in initiatives that support this research
direction. A promising option is the application of CRISPR-Cas (clustered regulatory interspaced short
palindromic repeats-CRISPR-associated) mediated genome editing to remove toxic or antinutritive
compounds in yams. This precision breeding technique has the potential to alter one or more pathways
or traits in a given cultivar, more efficiently than conventional breeding and without disturbing the
complement of traits for which it is preferred. The current outlook for non-transgenic genome edited
crops is that they may avoid the heavy regulatory burden placed on transgenic “genetically modified”
plants [
342
]. The optimization of protocols for genome editing in yam is well underway [
343
,
344
]
and the availability of more and better yam genome assemblies is proceeding apace [
344
–
347
]. An
understanding of the genetic regulation of desirable nutritional and pharmacological compounds can
also be leveraged to increase their amounts, especially with increased variety of possible CRISPR-Cas
based manipulations. Genome editing is part of a bright future for scientists working to improve the
nutritional quality of yams, while making consumption or clinical use safer.
Scientific investigations into the clinical use of Dioscorea species with focus on reducing the risk of
ovarian cancer, treatment of menopause complications and female ageing diseases needs to focus on
characterizing the bioactive compounds and proteins isolated from diverse species. This includes amino
acid sequencing,
in vivo
pharmacokinetic study as well as modulating mechanisms. This will help in
the establishment of multi-target based anti-menopausal drug screening, towards developing more
effective drug candidates for future use [
244
]. Furthermore, research to support the use of Dioscorea
as a therapeutic agent against asthma, urinary tract infections and bladder related complications,

Foods 2020,9, 1304 29 of 45
rheumatism, arthritis, pelvic cramps and so forth, need to be promoted. Most of the studies have been
limited to
in vitro
and animal models. It is very important to have further insights into the effects of
yam on degenerative diseases while putting into consideration the feasibility and long term effects
on humans. Limited or no data on safety, toxicity and efficacy as use as contraceptives on human
health, during pregnancy, lactation and childhood suggest an issue of concern. The paucity of data
on the safety of diosgenin and other bioactive compounds suggests that further investigation should
focus on development, toxicity, neurotoxicity and allergenicity. While preclinical and mechanistic
findings tend to support the use of diosgenin as a novel, multitarget-based chemopreventive and
therapeutic agent against different forms of cancer [
288
], research should also focus on developing and
evaluating standards of evidence. On a commercial scale, the introduction of Dioscorea extracts into the
growing international market of natural herbs is highly encouraged. The Mexican experience [
348
]
of biodiversity loss of wild Mexican yams should form the basis for conscious sustainable natural
resource management especially in Africa as mentioned earlier.
Author Contributions:
Conceptualization, J.E.O.; writing—original draft preparation, J.E.O. and C.A.C.;
writing—review and editing, J.E.O., J.B.L. and C.A.C. All authors have read and agreed to the published
version of the manuscript.
Funding:
This Research received no external funding. Publication made possible in part by support from the
Berkeley Research Impact Initiative (BRII) sponsored by the University of California (UC) Berkeley Library.
Acknowledgments: J.E.O. and J.B.L. were supported by National Science Foundation Award No. 1543967.
Conflicts of Interest: The authors declare no conflict of interest.
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