A report prepared for
Horticulture New Zealand
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New Zealand Institute for Crop & Food Research Limited
Private Bag 4704, Christchurch, New Zealand
Crop & Food Research Confidential Report No. 1928
Nutritional attributes of spinach, silver
beet and eggplant
LJ Hedges & CE Lister
© 2007 New Zealand Institute for Crop & Food Research Limited
1 Executive summary 1
1.1 Introduction 1
1.2 Spinach 1
1.3 Silver beet 1
1.4 Eggplant 1
1.5 Conclusions 2
2 Spinach (Spinacia oleracea) 2
2.1 Introduction 2
2.2 Composition 2
2.2.1 Core nutrients 2
2.2.2 Phytochemicals 3
2.2.3 Phenolic compounds 4
2.3 Health benefits 6
2.3.1 Studies involving spinach or spinach extracts 6
2.3.2 Antioxidant activity 6
2.3.3 β-carotene 8
2.3.4 Lutein and zeaxanthin 8
2.3.5 Phenolic compounds 9
2.3.6 Other phytochemicals in spinach 9
2.4 Anti-nutritive compounds 10
2.5 Factors affecting nutrient levels 11
2.6 Quotes and trivia 11
3 Silver beet /Swiss chard (Beta vulgaris var. cylca / cicla) 12
3.1 Introduction 12
3.2 Composition 12
3.2.1 Core nutrients 12
3.2.2 Phytochemicals 13
3.2.3 Phenolics 14
3.3 Health benefits 15
3.3.1 Antioxidant activity 15
3.3.2 Betalains 16
3.3.3 Chlorophyll 16
3.4 Factors affecting health benefits 16
3.5 Quotes and trivia 16
4 Eggplant/aubergine (Solanum melongena) 16
4.1 Introduction 16
4.2 Composition 17
4.2.1 Core nutrients 17
4.2.2 Phytochemicals 18
4.3 Health benefits 19
4.3.1 Antioxidant activity 19
4.3.2 Cardiovascular disease 19
4.3.3 Cancer 19
4.4 Factors affecting health benefits 20
4.5 Quotes and trivia 20
5 Conclusions 21
6 References 21
Appendix l Nutritional information on boiled spinach, raw silver beet and
eggplant (per 100 g edible portion) from Athar et al.(2004) 26
Appendix ll Major functions of main micronutrients contained in spinach,
silver beet and eggplant 28
Nutritional attributes of spinach, silver beet and eggplant
LJ Hedges & CE Lister, June 2007
Crop & Food Research Confidential Report No. 1928
New Zealand Institute for Crop & Food Research Limited
1 Executive summary
This report covers spinach, silver beet and eggplant, important vegetables in
the New Zealand market that have not been covered in previous reports. It is
hoped to cover other exotic vegetables in a future report.
Spinach is an extremely nutritious vegetable, rich both in core nutrients and
phytochemicals. The major micronutrients in spinach are vitamins A (from
β-carotene), C, K and folate, and the minerals, calcium, iron and potassium.
Spinach also provides fibre and is low in calories. The phytochemicals of
most importance are the carotenoids, β-carotene, lutein and zeaxanthin and
A number of studies have shown spinach to have strong antioxidant activity
and high levels of antioxidant compounds such as phenolics and carotenoids.
Antioxidant activity is important as many chronic diseases and health issues
associated with ageing are believed to result from excessive oxidative stress.
One of the major health benefits attributed to two major compounds in
spinach, lutein and zeaxanthin, is that of protecting against eye diseases
such as macular degeneration (gradual loss of central vision, associated with
old age). Epidemiological and laboratory studies have also shown that
spinach, spinach extracts, and spinach compounds may delay or retard age-
related loss of brain function, reduce the extent of post-ischaemic stroke
damage to the brain, and protect against cancer through various different
1.3 Silver beet
Similar in composition to spinach, silver beet (or Swiss chard) is rich in core
nutrients. Less is known about its phytochemical content, probably because it
is less popular. However, like spinach it contains high levels of lutein and
zeaxanthin. Multi-coloured stalks also contain betalains, which have strong
antioxidant activity. Other phenolics in silver beet, such the flavonoid
kaempferol, are also important antioxidants.
Compared with spinach and silver beet, eggplant is low in core nutrients.
Studies have shown mixed results, ranging from low to high, for its
antioxidant activity. Little is known of its phytochemicals, although the
pigment that gives the purplish colour of some cultivars, nasunin, has
received some research interest in its possible antioxidant and anti-cancer
Spinach is a highly nutritious vegetable, with a wide range and high levels of
phytochemicals. Silver beet has a similar profile of phytochemicals, but has
been less specifically studied. Coloured silver beet cultivars may offer
considerable potential because of the more unusual pigments they contain
and this could be investigated further. Similarly, although eggplant appears to
contain relatively small amounts of nutrients, research in future may discover
new features of their anthocyanins or equally may identify more nutritious
cultivars from the many that are grown worldwide.
2 Spinach (Spinacia oleracea)
Spinach deserves its reputation as an extremely nutritious vegetable. Its
nutrients include a range of vitamins and minerals (micronutrients), which can
prevent deficiency diseases and are essential for normal physiological
function, as well as phytochemicals (also known as non-nutrients, bioactives
or phytonutrients) thought to help prevent chronic health problems such as
cancer and heart disease, as well as other health problems associated with
2.2.1 Core nutrients
Spinach is best known for being a rich source of iron, and although it is a
good vegetable source, some other vegetables such as watercress are more
richly endowed and red meat is a much better source. But equally, there is a
lot more to the nutritional value of spinach than just iron.
Spinach contains an array of micronutrients and phytochemicals. The major
micronutrients in spinach are vitamins A (from β-carotene), C, K and folate
and the minerals, calcium, iron and potassium. Other nutrients present in
smaller quantities include vitamin E, some B vitamins – thiamine (B1),
riboflavin (B2) and B6, and the minerals magnesium, manganese and zinc
(Table 1). Spinach also provides fibre and has the additional advantage of
being low in calories.
0 10203040506070 8090100
Total vitamin A equivalents
Total niac in equivalents
Thi amin B1
% RDI or AI for Mal es % RDI or AI for Females
Figure 1: Contributions to RDI or AI by the major micronutrients in 100 g boiled, drained
spinach leaves. Adapted from Athar et al. (2004) and NHMRC (2006).
See Appendix I for full data from the New Zealand FOODFiles database.
The phytochemicals of most importance are the carotenoids, β-carotene,
lutein and zeaxanthin, along with phenolic compounds. Other phytochemicals
include chlorophyll, glutathione, α-lipoic acid and betaine (Joseph et al.
Carotenoids are a group of yellow-orange-red pigments, found in a variety of
fruits and vegetables. Often the colours of the carotenoids present in plants
are masked by chlorophyll, to the extent that some of the largest amounts of
carotenoids are found in dark green leafy vegetables such as spinach and
kale. Besides conferring colour, carotenoids also have antioxidant properties.
These compounds are especially effective in quenching singlet oxygen and
peroxyl radicals. They appear to act synergistically with other carotenoids
and other antioxidants.
There are two general classes of carotenoids – the carotenes, and their
oxygenated derivatives, the xanthophylls. The body can convert α-carotene,
β-carotene and β-cryptoxanthin into retinol, or vitamin A, and so are referred
to as having provitamin A activity. Lycopene, lutein and zeaxanthin are
converted to vitamin A, but have other health benefits. Because of their
similarity, lutein and zeaxanthin are often reported as a combined total.
Carotenoids are fat-soluble compounds and thus are best absorbed in the
body if accompanied by a small amount of some form of oil or fat in a meal.
Processing such as chopping and cooking assists in releasing carotenoids
from the food matrix, which also increases their bioavailability. As with iron,
New Zealand data give lower values for β-carotene than US data, but this
may be the result of analysing different cultivars or using different analytical
Table 1: Major carotenoids in spinach and other assorted vegetables (per
100 g) (Athar et al. 2004; USDA 2006).
Broccoli, raw 361 1403
Kale, raw 9226 39550
Lettuce (iceberg) 299 277
Lettuce (romaine) 3484 2312
Silver beet 3652 11015
Spinach, raw 5626 12198
Spinach, boiled 6288 11308
Spinach, frozen 7035 12651
Spinach, boiled (NZ data) 3840 NA
2.2.3 Phenolic compounds
Phenolic compounds are a large group of secondary plant products, present
in most if not all plants. They differ in chemical structure and reactivity, but all
have at least one benzene ring with a hydroxyl group bound to a carbon
atom. Chemical structures range from quite simple compounds such as
caffeic acid to highly polymerised substances such as tannins. There are
numerous different groups of phenolics but the most common phenolics
found in foods are generally phenolic acids, flavonoids, lignans, stilbenes,
coumarins and tannins (Harbourne 1993). Flavonoids and phenolic acids are
the most widely studied classes of compounds in this group. Major spinach
flavonoids reported in the USDA flavonoid database include good amounts of
luteolin and quercetin and small amounts of kaempferol and myricetin
(Table 2) (USDA 2003).
Table 2: Levels of selected flavonoids in raw
spinach (mg/100 g FW).
A very recent prospective study showed a significant 40% decrease in
epithelial ovarian cancer incidence for the highest versus lowest quintile of
kaempferol intake and a significant 34% decrease in incidence for the highest
versus lowest quintile of luteolin intake. Interestingly, however, this study
found that there was no effect of several flavonoids together (Gates et al.
2007). According to Pandjaitan et al. (2005), spinach contains some unusual
flavonoid compounds, including glucuronides and acylated di- and
triglycosides of methylated and methylenedioxy derivatives of 6-oxygenated
In vitro phenolic compounds have very strong antioxidant activity. However,
their uptake and bioavailability, which are major factors in their in vivo effects,
are not fully understood. It appears that they are not well absorbed and are
taken into the bloodstream as metabolites, which tend to have lower
antioxidant activity than the parent compound, so there is some conjecture as
to the magnitude of their effects as antioxidants (Higdon 2005). However, it
has also been shown that the antioxidant activity of plasma increases after
ingestion of polyphenols (Scalbert et al. 2005). Polyphenols also have other
important biological effects such as anti-inflammatory, anti-proliferative, anti-
viral and anti-allergenic properties.
Spinach has been included in a number of comparative studies examining
levels of phytochemicals, but again findings are not consistent, with amounts
of phenolics varying considerably (Table 3). However, as a generalisation,
spinach ranks just within the top third of vegetables and has moderate levels
Table 3: Total phenolics in fresh, raw spinach as reported in and adapted
from scientific literature (where necessary, values converted to amounts
per 100 g fresh weight according to USDA food composition data).
Total phenolics (mg/100 g) Author
32.5 mg GAE Chun et al.( 2005)
90.0 mg GAE Chu et al. (2002)
188.3 mg GAE (immature) Pandjaitan et al. (2005)
256.3 mg GAE (mid-mature)
196.9 mg GAE (mature)
270 mg GAE Wu et al. (2004)
109.6 mg GAE Turkmen et al. (2005)
89.4 mg CAE Ninfali et al. (2005)
GAE = gallic acid equivalents. CAE = caffeic acid equivalents.
2.3 Health benefits
The roles of core nutrients are outlined in Appendix II Table 1.
2.3.1 Studies involving spinach or spinach extracts
An early case-control study found that women who consumed spinach or
carrots more than twice a week had a lower risk of developing breast cancer
than those who consumed none, though the authors acknowledged that the
data did not allow them to distinguish among several potential explanations
(Longnecker et al. 1997). Another human study demonstrated that increased
intake of spinach carotenoids increased serum carotenoid concentrations as
well as macular pigment optical density, which is considered a possible
predictor of macular degeneration (Kopsell et al. 2006).
An animal study involved feeding young rats a freeze-dried aqueous spinach
extract and observing changes related to mental function as they aged. It was
found that the rats who received the spinach extracts had overall the greatest
retardation of age-related declines in neuronal and cognitive function,
compared with those in other groups. Although two groups had been fed
other diets high in antioxidants (strawberry extracts or vitamin E
supplements), the effects were most pronounced overall in the spinach-fed
group (Joseph et al. 1998). A subsequent study found that supplementing the
diet with various antioxidants, including a spinach extract, resulted in
reversing age-related declines in a number of neuronal and behavioural
parameters (Joseph et al. 1999). It was hypothesised that the protective
effect could be related to the antioxidant activity of the supplemented diets.
Wang et al. (2005) found that supplementing the diet with spinach reduced
post-ischaemic stroke brain damage in rats, possibly via anti-apoptosis
activity. Those authors also postulated that besides antioxidant activity, anti-
inflammatory mechanisms might be involved. A lowering of blood pressure
was noticed in spontaneously hypertensive rats fed peptides isolated from
spinach leaf Rubisco, which were shown to have anti-cholinesterase (ACE)
inhibitory properties (Yang et al. 2003).
There have also been studies using spinach or spinach extracts in cancer.
Topical and oral administration of a water-soluble spinach extract reduced
the number of induced papillomas in a cancer model using mice (Nyska et al.
2001); neoxanthin from spinach inhibited the proliferation of prostate cancer
cells in a laboratory study (Asai et al. 2004), and 13 flavonoids from spinach
showed anti-mutagenic activity in a bacteria-based model (Edenharder et al.
2.3.2 Antioxidant activity
Epidemiological studies have shown that large intakes of fruit and vegetables
protect against a range of chronic diseases and problems associated with
ageing, and this is generally attributed to their phytochemical content.
Phytochemicals may have antioxidant, anti-inflammatory, vasodilatory, anti-
cancer or anti-bacterial activities. One of the most important ways in which
fruit and vegetables are believed to exert their protective effects is through
their antioxidant activity.
Antioxidants are compounds that deactivate free radicals and other oxidants,
rendering them harmless. Free radicals are highly unstable molecules,
present in the body both from external sources (e.g. pollution, smoking,
carcinogens in the environment) and internal sources as the result of normal
physiological processes. If uncontrolled, free radicals can damage cell
components, interfering with major life processes. For example, they may
damage DNA, leading to cancer, or may oxidise fats in the blood, contributing
to atherosclerosis and ultimately to heart disease. Although the body
produces antioxidants and has other defence mechanisms, it is thought that
antioxidants from the diet also have an important role.
The major antioxidants in spinach are vitamin C, the carotenoids and various
phenolic compounds. It should be noted that vitamin A does not exert
antioxidant activity, but carotenoids (some of which may be converted to
vitamin A) do.
Antioxidant activity of spinach
Several studies have examined the antioxidant activity of spinach and other
vegetables. Cao et al. (1996) gave spinach an antioxidant score of 17 (based
on ORAC scores using three different radicals), rating it third of the 22
vegetables examined. Ranked ahead of spinach were garlic and kale, but the
former is only consumed in small amounts and the latter is rarely eaten.
Similarly, Pellegrini et al. (2003) ranked spinach first, eighth and first
respectively out of 31 popular vegetables as measured by the FRAP, TRAP
and TEAC antioxidant assays. A similar result was found by Wu et al. (2004)
and Chu et al. (2002). The latter not only ranked spinach extremely highly for
antioxidant activity but also in terms of antiproliferative activity. By contrast,
however, spinach was placed towards the middle of the field of 20 common
vegetables in another recent study (Chun et al. 2005). Table 4 lists absolute
values from some of these studies. It is apparent that considerable variation
exists, even when the same assay is used. Various factors can account for
such disparity, including the use of different assays, different methods of
sample preparation, different cultivars etc. The Panjaitan study also illustrates
another source of variation, that of maturity.
Table 4: Antioxidant activity of fresh raw spinach as reported in and
adapted from scientific literature (where necessary values are converted
to per 100 g fresh weight according to USDA food composition data).
Antioxidant activity (per 100 g FW) Author Assay
1445.7 µmol TE (immature)
2416.7 µmol TE (mid-mature)
1448.2 µmol TE (mature)
Pandjaitan et al. (2005) ORAC
2732 µmol TE Ninfali et al. (2005) ORAC
2640 µmol TE Wu et al. (2004) ORAC
1307 µmol TE Ou et al. (2002) ORAC
849 µmol TE Pellegrini et al. (2003) TEAC (ABTS)
35.2 mg VCE Chun et al. (2005) VCEAC
362.9 µmol VCE Chu et al. (2002) TOSC
ORAC = Oxygen radical absorbance capacity; TEAC (ABTS) = Trolox equivalent antioxidant capacity
using ABTS radical; VCEAC = Vitamin C equivalent antioxidant capacity; TOSC = Total oxyradical
scavenging capacity; TE = Trolox equivalents; VCE = Vitamin C equivalents.
The health benefits associated with carotenoid-rich foods can in part be
attributed to the β-carotene they contain. β-Carotene may help prevent the
formation of lesions that lead to cancer, and in vitro cell experiments have
indicated that carotenoids also have other properties consistent with anti-
cancer activity. For instance, they may play an important role in the cell
communication that leads to the removal of pre-cancerous cells. However,
results have been somewhat inconsistent.
There have also been mixed results on the effect of dietary β-carotene on
cardiovascular disease. It has been established that the development of
cardiovascular disease involves the oxidation of low-density lipoprotein (LDL)
and its subsequent uptake by foam cells in the vascular endothelium, where it
can lead to the development of atherosclerotic lesions. It has been thought
that β-carotene, which itself is carried in LDL, might help prevent this
oxidation because several in vitro studies had shown that it could scavenge
potentially damaging radicals. However, whilst some research has shown
higher plasma levels of carotenoids to be associated with better vascular
health and lower cardiovascular disease risk, other studies have shown no
effect (Higdon 2005; Cooper et al. 1999). Further, some recent studies have
produced contradictory results on the ability of β-carotene to stabilise LDL
against oxidation (Cooper et al. 1999).
2.3.4 Lutein and zeaxanthin
Amounts of lutein and zeaxanthin in some common vegetables are shown in
Table 4. Spinach contains some of the largest amounts of lutein and
zeaxanthin from vegetable sources. Although kale contains more, it is not
commonly eaten and is therefore not a major food source.
Studies have shown that these compounds are selectively accumulated in
different parts of the eye, where they are by far the most abundant of the
major carotenoids present. This has led to the suggestion that they may be
important in protecting against age-related eye problems, particularly macular
degeneration and the formation of cataracts. There is some epidemiological
evidence to support this (Sies & Stahl 2003; Mares-Perlman et al. 2002).
However, data are scarce and study findings not always consistent (Granado
et al. 2003; Mares-Perlman et al. 2002).
The fact of their antioxidant activity has led to speculation that these
carotenoids, particularly lutein, which is more widely dispersed in the body,
could protect against diseases such as cancer and cardiovascular disease as
well as positively affecting immune function. Epidemiological research on the
influence of these particular carotenoids on site-specific cancers is relatively
new and sparse. The most promising areas of research would appear to be in
relation to skin cancer (in combination with other carotenoids) (Slattery et al.
2000; Stahl et al. 2000), and breast cancer (Mares-Perlman et al. 2002).
Again, however, results are not clear, with some studies finding no
associations and others reporting only inconsistent results.
In terms of cardiovascular disease (CVD), studies have found high serum
levels of lutein and zeaxanthin to be associated with a reduced risk of
coronary heart disease (Dwyer et al 2001; Irribaren et al. 1997). Additionally,
the consumption of green leafy vegetables (which also contain lutein and
zeaxanthin) was associated with a reduced incidence of stroke in the Nurse’s
Health and Health Professionals Follow-up Study (Joshipura et al. 1999).
2.3.5 Phenolic compounds
Phenolic compounds are particularly important antioxidant compounds.
Because of their structure, they are very efficient scavengers of free radicals
and they also serve as metal chelators (Shahidi & Naczk 1995). They
comprise two main groups – flavonoids and phenolic acids, both of which are
present in spinach (Pandjaitan et al. 2005). Phenolic acids have been studied
largely in relation to their antioxidant activity, but flavonoids, in addition to
antioxidant properties, have other potential health-promoting activities
including anti-allergic, anti-inflammatory, anti-microbial and anti-cancer
properties (Cody et al. 1986; Harbourne 1993). There are many ways in
which flavonoids may act to prevent cancer, including inducing detoxification
enzymes, inhibiting cancer cell proliferation and promoting cell differentiation
(Kalt 2001). Some flavonoids further help to prevent heart disease through
inhibiting blood platelet aggregation and providing antioxidant protection for
low density lipoprotein (Frankel et al. 1993).
2.3.6 Other phytochemicals in spinach
Relatively little is known of the health effects of chlorophyll, the pigment that
causes the green colour in plants and is the primary photosynthetic
compound. Some research suggests that it may be important in protecting
against some forms of cancer, as it is thought that the chlorophyll binds to the
mutant DNA and prevents it proliferating. A recent study found that
chlorophyll had phase 2 enzyme inducing potential and although its activity
was relatively weak, because it is present at high concentrations in so many
edible plants, it may have some of the protective effects observed with diets
rich in green vegetables (Fahey et al. 2005).
An extremely important endogenous antioxidant (synthesised within the
body), glutathione is relatively rare in foods. One of its major functions is to
protect DNA from oxidation, but it also detoxifies carcinogens, boosts the
immune system, supports liver health and reduces inflammation (Joseph et
Like glutathione, α-lipoic acid is a vital antioxidant largely synthesised in the
body, but also present in some foods. It is important for energy metabolism
and its antioxidant activity may protect against chronic diseases. In addition,
it may assist memory (Joseph et al. 2002).
It is believed that D-glucaric acid may lower blood cholesterol (Joseph et al.
Another endogenous compound, coenzyme Q10 is a critical component in
energy metabolism, but also acts as an antioxidant in cell membranes and
lipoproteins. The best food sources are meat, fish and oils, but spinach is one
of the best vegetable sources (Joseph et al. 2002).
This is a lesser known compound and does not appear to have been
extensively researched. It is thought that it may prevent cardiovascular
disease by lowering levels of homocysteine, a compound associated with the
development of heart disease (Joseph et al. 2002).
2.4 Anti-nutritive compounds
Plants also contain compounds which can potentially compromise health.
These are termed ‘anti-nutritive’ compounds and include oxalates, which are
present in spinach. In plant tissues these compounds are present as end-
products of metabolism, but in humans they compromise nutrient absorption
(especially of minerals such as calcium and possibly iron) as well as
contributing to the formation of kidney stones and gout. Along with rhubarb
and beet, spinach is high in oxalates, though amounts can vary according to
a number of factors including cultivar, growing conditions and cooking
method (Noonan & Savage 1999). The large amount of oxalates in spinach
compromises the absorption of the calcium and iron that the food also
contains, but there is no evidence of it causing toxicity problems. Blanching
spinach lowers oxalate amounts. Soaking and cooking, particularly boiling,
reduces oxalates in other foods such as soybean and yams (Noonan &
Savage 1999), and may have a similar effect with spinach. Neither Joseph et
al. (2002) nor Pratt & Matthews (2004) consider the oxalates in spinach a
major issue and the consensus appears to be that a moderate intake as part
of mixed diet should not pose any health problems.
2.5 Factors affecting nutrient levels
Several factors combine to determine the amounts of core nutrients and other
phytochemicals in a food. These include the variety/cultivar of the plant,
agronomic issues such as soil type, cultivation protocols (irrigation, pest
control, use of fertiliser), the degree of maturity at harvest, and processing
practices (harvesting, storage, method of processing).
The fact of inter-cultivar variation in general is well established and spinach is
no exception to this. A study of 11 commercial lines and 15 breeding lines
showed large variations in antioxidant activity and phenolic content.
Variations were also notable by growing season, with significantly higher
levels of antioxidant activity and phenolic content in over-winter spinach
(sown in late autumn and harvested in early spring) than that sown in early
autumn and harvested in late autumn (Howard et al. 2002). A subsequent
study also showed phenolic content variation according to genotype, as well
as level of maturity, with amounts significantly higher at the mid-maturity
stage (Pandjaitan et al. 2005).
Processing such as freezing also affects the nutrient profile of spinach (Table
3). The amounts of some nutrients such as vitamin C and folate are reduced
with freezing, but others, such as β-carotene, lutein and zeaxanthin, are
improved. Similar effects have been observed with other forms of processing
in other vegetables, such as cooked tomatoes. It is believed that that this
occurs in part because freezing disrupts the food matrix, making these
compounds more easily available for absorption in the body, and for
extraction and measurement in the laboratory.
Cooking has both beneficial and deleterious effects upon nutrients. Whilst it
degrades vitamin C and folate, it can make carotenoids like β-carotene and
lutein and zeaxanthin more bioavailable. Light cooking or steaming is often
recommended to enhance carotenoid bioavailability while minimising the loss
of other nutrients (Joseph et al. 2002).
2.6 Quotes and trivia
Spinach was the first vegetable to be sold in a frozen form
According to a number of internet websites, the cartoon character
Popeye is credited with increasing the consumption of spinach in the US
by 33% in the 1930s.
In the US, 56% of readers surveyed by the food magazine, Bon Appetit,
in 2005 ranked spinach as their favourite vegetable, ahead of more
predictable choices such as asparagus and broccoli. The survey asked
10 000 readers to rank a dozen vegetables in terms of preference. The
availability of washed, bagged, baby spinach has added considerably to
its popularity (Sagon 2005).
The myth about spinach and its high iron content may have first been
propagated by Dr E von Wolf in 1870, because a misplaced decimal
point in his publication led to an iron-content figure that was 10 times too
high. In 1937, German chemists reinvestigated this "miracle vegetable"
and corrected the mistake. It was described by TJ Hamblin in the British
Medical Journal, December 1981 (Wikipedia 2007).
“One man's poison ivy is another man's spinach.” George Ade
(1866-1944) American humorist (www.foodreference.com/html/qspinach.
3 Silver beet /Swiss chard (Beta
vulgaris var. cylca / cicla)
A staple in many domestic gardens, silver beet is both decorative and
nutritious. It is the same species as beetroot, but is a variety which does not
develop a swollen taproot. Both the stalk and leaves are edible, with differing
cuisines preferring one over the other. These days, multi-coloured cultivars
are available, which provide visual interest as well as additional kinds of
Nutritionally silver beet deserves to be highly valued, as it is an excellent
source of many core nutrients as well as phytochemicals. Its slightly bitter
taste is perhaps to blame for its lack of popularity, but if served mixed with
other sweeter vegetables, it can provide an interesting contrast in flavour.
Its nutritional profile is very similar to that of its close relative, spinach.
Besides its many nutrients, it too contains anti-nutritive oxalates (Section
2.4), but there is similarly no evidence of this being a major health issue.
3.2.1 Core nutrients
Silver beet is particularly rich in vitamins A (through β-carotene), C and,
unusually for stalky/leafy material, vitamin E. In addition, it provides useful
amounts of a wide range of minerals (Figure 2).
See Appendix I for full data from the New Zealand FOODFiles database.
Total vitamin A equivalents
Total niac in equivalents
% RDI or AI for Males % RDI or AI for Females
Figure 2: Contributions to RDI or AI by the major micronutrients in 100 g raw silver beet,
adapted from Athar et al. (2004) and NHMRC (2006).
Compared with spinach, there is relatively little research on silver beet,
although it has some unusual and interesting phytochemicals, such as
vitexin, an apigenin derivative (Gil et al. 1998). Besides the green pigment,
chlorophyll, the multi-coloured cultivars also contain betalains, which include
the red betacyanins and yellow betaxanthins.
Silver beet shares many of the same major phytochemicals with spinach. Of
particular interest, because they are less common, are the high levels of the
carotenoids lutein and zeaxanthin (Table 1).
Pyo et al. (2004) found that the major phenolic acid in silver beet was syringic
acid and the major flavonoid was kaempferol. Phenolic composition as well
as content differed between leaves and stems, with leaves containing more
than stems and a red cultivar containing more than a white cultivar. Levels of
phenolics correlated well with antioxidant activity, which was in the order red
leaf > white leaf > red stem > white stem (Table 5).
Table 5 Total phenolics in differently
coloured cultivars of Swiss chard
(GAE/100 g fresh weight) (Pyo et al. 2004).
Red leaf 128.1
Red stem 29.7
White leaf 124.7
White stem 23.2
Table 6 shows flavonoids listed for Swiss chard in the USDA Flavonoid
database. There may, however, be other flavonoids present, as this database
only lists certain more common flavonoids. For example, three less common
flavonoids were also identified in an Italian study, vitexin-2”O-rhamnoside, its
demethylated form 2"-xylosylvitexin, isorhamnetin 3-gentiobioside, and rutin
(Ninfali et al. 2007).
Table 6: Levels of selected flavonoids in
Swiss chard (mg/100 g FW) (USDA 2007).
Betalains are relatively uncommon phytochemicals, occurring in only 13
families of the Caryophyllales order and in some genera of the
Basidiomycetes (Kugler et al. 2004). Although visually similar to
anthocyanins, they are chemically different. These two classes of compounds
are also mutually exclusive, never occurring together in the same plant. It has
been assumed that they perform similar functions within the plant, attracting
pollinators and seed dispersers as well as having physiological roles. Thus
they may protect the plant against oxidative damage, and act as transport
vehicles for monosaccharides, and as osmotic regulators. Pigments can also
be the result of stress in the plant, such as the stresses of drought, low
temperatures or wounding (Stintzing & Carle 2004).
Nineteen betaxanthins and nine betacyanins were identified in the stalks of
multicoloured silver beet cultivars (Kugler et al. 2004). A very recent study of
members of the Amaranthaceae, to which both silver beet and spinach
belong, investigated structure-activity relationships of various betaxanthins
and betacyanins in terms of their free-radical scavenging capacities (Cai et
al., 2003). Antioxidant potential was found to be related to structural features
of the betalains. In betaxanthins, an increasing number of hydroxy and imino
residues increased free radical scavenging. In betacyanins, glycosylation
reduced activity whereas it increased with acylation. Also, 5-O-glycosylated
betacyanins produced lower antioxidant values than their 6-O-glycosylated
3.3 Health benefits
See sections 2.3.1, 2.3.2, and 2.3.3 for the health effects of β-carotene, lutein
and zeaxanthin, and phenolic compounds, respectively.
3.3.1 Antioxidant activity
Silver beet ranked in about the top third (n = 33), according to one of the few
studies in which it is included, measured according to three different methods
which reflect different modes of antioxidant activity (Table 7).
Table 7: Ferric reducing-antioxidant power (FRAP), total radical trapping
antioxidant parameter (TRAP) and Trolox equivalent antioxidant capacity
(TEAC) of extracts of eggplant, silver beet and spinach (Pellegrini et al.
FRAP TRAP TEAC
Vegetable Value Rank Value Rank Value Rank
Mmol trolox/kg fresh weight
Eggplant 3.77 24 2.82 15 1.10 25
Silver beet 11.60 10 2.91 13 3.53 10
Spinach 26.94 1 5.79 8 8.49 1
Extracts of a red-stemmed variety showed high antioxidant activity, superior
to both the synthetic antioxidant buylated hydroxytoluene (BHT) and
tocopherol according to the DPPH radical scavenging assay. White-stemmed
extracts were also higher than BHT, though slightly lower than tocopherol in
this assay. However, BHT performed best according to the thiocyanate
method, with the red-stemmed cultivar approximately the same as
tocopherol, and the white-stemmed cultivar lower. Antioxidant activity of the
stem extracts was around one half to two-thirds that of the leaves (Pyo et al.
Studies relating to betalains in silver beet are lacking, but it is likely that
findings for beetroot can be extrapolated to silver beet. Most research on
beetroot and betalains has focused on antioxidant activity. According to a
recent review, findings from various studies ranked beetroot among the 10
most potent vegetables in terms of antioxidant capacity, with other studies
agreeing that betalains were at least in part responsible for this (Stintzing &
Carle 2004). As already discussed, the radical scavenging capacity of
different betalains is related to different structural features (Cai et al. 2005).
A number of studies have ranked the antioxidant capacity of beetroot highly,
with betalains credited at least in part for this (Cao et al. 1996, Halvorsen et
al. 2002, Kähkönen et al. 1999, Ou et al. 2002, Vinson et al. 1998,
Wettasinghe et al. 2002a). As mentioned earlier, betalains have been
identified in the stalks of differently coloured silver beet cultivars (Kugler et al.
See Section 2.3.6.
3.4 Factors affecting health benefits
As with spinach, cooking degrades vitamin C and folate in silver beet, but
makes the carotenoids more bioavailable. An 80% loss of vitamin C was
observed after boiling for 10 minutes. Around 50% of the flavonoids from a
green cultivar leached into cooking water during boiling, although less
leaching was observed with a yellow cultivar (Gil et al. 1998). In that study,
vitamin C also decreased under modified atmosphere packaging, though no
effect upon flavonoid content was observed.
3.5 Quotes and trivia
In Korea, silver beet is used to reduce inflammation and stop bleeding.
According to Wikipedia, Europeans prefer silver beet stalks, whereas
Americans prefer the leaves.
4 Eggplant/aubergine (Solanum
“Eggplant” may seem an odd name for today’s large purplish, pear-shaped
vegetables, but is actually an accurate description of early varieties, which
were indeed egg-shaped and white in colour. In fact, eggplants come in a
variety of colours from dark purplish black, to pale purple, white, orange and
green and can be solid colours, striped or mottled. Equally, they have diverse
shapes from egg-shaped to sausage-shaped and pear-shaped, and can vary
in weight from around 20 to over 400 g (Hanson et al. 2006).
Eggplants originated from Asia and are particularly valuable vegetables in
tropical countries as they are some of the few vegetables that produce high
yields in both hot and wet environments (Hanson et al. 2006). They belong to
the same family (Solanaceae) as tomatoes, potatoes, capsicum and deadly
Eggplants are usually harvested before they are physiologically mature
(Whitaker & Stommel 2003). Bitterness can be a problem with eggplants, and
the consensus appears to be that this is a function of variety and maturity –
the sausage-shaped Asian varieties tend to be less bitter, and the more
mature a fruit is, the more likely it is to have accumulated bitter compounds.
Prolonged storage in the refrigerator is also said to encourage their build-up.
Eggplant is not rich in core nutrients, but it can contain unusual pigments and
large amounts of other phytochemicals, particularly phenolic compounds,
which are thought to confer much of its high antioxidant activity. The different
varieties can contain different antioxidant compounds and/or proportions of
these compounds (Whitaker & Stommel 2003).
Some sources refer to bitterness in eggplant and relate it to the presence of
alkaloids (McGee 2004), but there is little information on alkaloids in eggplant
fruit. Alkaloids in other members of the Solanaceae cause allergic reactions
in some people.
4.2.1 Core nutrients
In comparison with spinach and silver beet, eggplant provides little in the
form of micronutrients (Figure 3). It is, however, an important source of
energy in those less-developed countries where it is more of a staple.
Total niac in equivalents
% RDI or AI for Mal es % RDI or AI for Females
Figure 3: Contributions to RDI or AI by the major micronutrients in 100 g raw eggplant,
adapted from Athar et al. (2004) and NHMRC (2006).
See Appendix I for full data from the New Zealand FOODFiles database.
The eggplant phytochemicals that have received most research attention are
the phenolic compounds. Chlorogenic acid is the major phenolic in eggplant
(Whitaker & Stommel 2003; Kahlon et al. 2007), accounting for 99% of total
phenolics in one study (Kalogeropoulos et al. 2007). Chlorogenic acid is one
of the most abundant phenolic acids in fruit and vegetables, and in vitro and
animal studies have shown its antioxidant and anti-cancer activities (Gonthier
et al. 2003). Chlorogenic acid was present in all cultivars studied by Whitaker
& Stommel (2003), and in tissues from all zones of the eggplant.
Like purple sweet potato, purple-coloured varieties of eggplant contain
anthocyanins that are relatively unusual because they are acylated, and are
considered to be more stable than non-acylated forms (Ichiyanagi et al.
2006). Their stability has made them of interest for use as natural colourants
in the food industry, but they also have particular physiological functions,
such as α-glucosidase inhibitory activity (Matsui et al. 2001). Nasunin
(delphinidin-3-(p-coumaroylrutinoside)-5-glucoside), the major anthocyanin
component of eggplant, has been studied for its antioxidant and anti-cancer
activity (Noda et al. 2000; Matsubara et al. 2005).
Orange and yellow cultivars would be expected to contain carotenoids, which
would account for their colour. Similarly, green cultivars would contain
chlorophyll. However, no detail on the composition of these cultivars has
4.3 Health benefits
4.3.1 Antioxidant activity
Some studies have rated the in vitro antioxidant activity of eggplant as high
(Wu et al. 2004), though others have ranked it average (Table 5, Ninfali et al.
2005), and low (Halvorsen et al. 2002). As already discussed with respect to
spinach, these differences could be the result of a number of different factors,
including different eggplant cultivars, growing conditions or assay
4.3.2 Cardiovascular disease
Eggplant is believed to lower blood cholesterol in South American countries
(Botelho et al. 2004), which has prompted some of the research into its
possible effects on cardiovascular disease. However, an early trial in humans
showed only a modest and transitory effect of consumption of a powdered
eggplant preparation in terms of total cholesterol and its fractions,
triglycerides and apolipoproteins in blood (Guimaraes et al. 2000). Of eight
commonly consumed vegetables, eggplant showed amongst the lowest bile
acid binding capability, several-fold less than okra and beets (Kahlon et al.
2007). (By binding bile acids, their circulation in the body is prevented, and
results in reduced fat absorption, the excretion of toxic metabolites and the
utilisation of cholesterol from the bloodstream to synthesise more bile acids,
thereby lowering levels of cholesterol in the blood.) Also contrary to
expectations, Bothelo et al. (2004) found that rather than decreasing plasma
cholesterol and preventing the development of atherosclerosis, in a mouse
study eggplant appeared to increase oxidative stress.
Other studies have not directly investigated the cardiovascular system, but
have findings that may also be relevant in terms of protecting against
cardiovascular disease. A study comparing the protective effects of various
vegetable extracts upon mouse liver microsome lipid peroxidation showed
that although not the best of the extracts investigated, a water-soluble
eggplant extract showed a high, though somewhat variable, level of
protection. Boiling and freezing slightly reduced the protective effect, whereas
freeze-drying slightly increased it (Gazzani et al. 1998). Anti-inflammatory
activity, which may be relevant to heart disease and cancer as well as other
diseases, was observed in a mouse study in which an extract of white
eggplant ripened to yellow prevented oedema and vascular permeability (Han
et al. 2003).
A number of processes are involved in the progression of events that result in
cancer, including the growth of new blood vessels (angiogenesis) to enable
the tumour to grow and metastasise. Some polyphenols are able to prevent
this process. Nasunin, the major anthocyanin in eggplant skins, has
antioxidant activity (Noda et al. 2000) and it suppressed microvessel growth
in an ex vivo animal study and an in vitro human cell assay (Matsubara et al.
4.4 Factors affecting health benefits
Hanson et al. (2006) tested antioxidant activity in 35 differently coloured and
shaped eggplant cultivars from different parts of the world. Many, though not
all, of the top radical scavengers were from purple cultivars, but the best was
a white variety with green stripes from Indonesia, followed by a white and
purple striped variety from Malaysia. Smaller vegetables tended to have more
radical scavenging power than larger ones, and levels of phenolics correlated
well with radical scavenging ability. Phenolics were twice as concentrated in
the eggplant skins than the pulp, and the types of phenolics were also
different. Different growing seasons also affected levels of phenolics and
ascorbic acid. Similar inter-cultivar differences were found by Whitaker &
Stommel (2003), as well as differences in the composition of tissue from
different parts of the vegetable.
Using an animal model, a very recent study demonstrated that nasunin
isomers were quickly absorbed in their original acylated forms and that their
bioavailability was similar to that of other (non-acylated) anthocyanins
(Ichiyanagi et al. 2006). However, anthocyanins in general appear to among
the least well absorbed polyphenols (Manach et al. 2005).
4.5 Quotes and trivia
Some people claim to be able to identify the "sex" of an eggplant; this is
important apparently because male eggplants are supposed to be less
bitter than female eggplants.
The spongy structure of raw eggplant is caused by tiny air pockets
between the cells. When cooked, the air pockets collapse into an
interestingly textured mass (McGee 2004).
Spinach and its constituent compounds have received a good amount of
research attention, which can substantiate spinach’s reputation as a highly
nutritious vegetable. Silver beet probably has similar health benefits, since it
contains many of the same phytochemicals and at reasonable levels, but
because it has been less studied, it is not possible to state this unequivocally.
It is also difficult to estimate whether its more unique compounds, such as the
vitexin family, have particular health attributes. Research does not appear to
have demonstrated particular health attributes for eggplant or eggplant
compounds at this time, although further research may add weight to early
promise identified in compounds such as nasunin.
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okra, beets, asparagus, eggplant, turnips, green beans, carrots, and
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Appendix l Nutritional information on boiled spinach, raw silver beet and
eggplant (per 100 g edible portion) from Athar et al. (2004)
and upper stem,
raw Eggplant, raw
Water g 91.8 90.6 92.4
Energy kcal 22 23 18
Protein g 2.2 2.57 1.1
Total fat g 0.8 0.26 0.3
Carbohydrate, available g 1.5 2.5 2.6
Dietary fibre (Englyst, 1988) g 2.1 2.78 2
Ash g 0.59 1.29 0.71
Sodium mg 120 138 5
Phosphorus mg 28 39.2 31
Potassium mg 230 426 170
Calcium mg 160 65.4 23
Iron mg 1.6 1.18 0.2
Beta-carotene equivalents µg 3840 3170 22
Total vitamin A equivalents µg 640 529 4
Thiamin mg 0.06 0.018 0.05
Riboflavin mg 0.05 0.036 0.04
Niacin mg 0.9 0.253 0.7
Vitamin C mg 8 24.4 3
Cholesterol mg 0 0 0
Total saturated fatty acids g 0.08 0.047 0
Total monounsaturated fatty acids g 0.06 0.057 0
Total polyunsaturated fatty acids g 0.48 0.106 0
Dry matter g 8.2 9.37 7.6
Total nitrogen g 0.35 0.41 0.19
Glucose g 0.8 1.08 1.1
Fructose g 0.5 0.77 0.8
Sucrose g 0 0.4 0.1
Lactose g 0 0 0
Maltose g 0 0 0
Total available sugars g 1.3 2.3 2
Starch g 0.2 0.21 0.6
Alcohol g 0 0 0
Total niacin equivalents mg 2.3 0.8 0.9
and upper stem,
raw Eggplant, raw
polysaccharides g 0.8 1.09 1
polysaccharides g 1.3 1.69 1
Energy kJ 92 94 73
Magnesium mg 34 35.9 8
Manganese µg 500 1350 100
Copper mg 0.01 0.127 0.01
Zinc mg 0.5 0.63 0.1
Selenium µg 0.38 0.238 1
Retinol µg 0 0 0
Potential niacin from tryptophan mg 1.4 0.5 0.2
Vitamin B6 mg 0.09 0.255 0.08
Folate, total µg 140 68 18
Vitamin B12 µg 0 0 0
Vitamin D µg 0 0 0
Vitamin E mg 1.7 2.39 0.03
Appendix ll Major functions of main micronutrients contained in spinach, silver
beet and eggplant
Table1: Main micronutrients in legumes and their physiological functions (Adapted from Medscape 2004;
Name Major function
Retinol (animal origin)
Some carotenoids (plant origin, converted to
retinol in the body)
Important for normal vision and eye health
Involved in gene expression, embryonic development and growth and
health of new cells
Assists in immune function
May protect against cancers and atherosclerosis
Necessary for healthy connective tissues – tendons, ligaments,
cartilage, wound healing and healthy teeth
Assists in iron absorption
A protective antioxidant – may protect against some cancers
Involved in hormone and neurotransmitter synthesis
alpha-tocopherols and tocotrienols
Non-specific chain-breaking antioxidant
Reduces peroxidation of fatty acids
May protect against atherosclerosis
Coenzyme in the metabolism of carbohydrates and branched-chain
Needed for nerve transmission
Involved in formation of blood cells
Important for skin and eye health
Coenzyme in numerous cellular redox reactions involved in energy
metabolism, especially from fat and protein
Nicotinic acid, nicotinamide
Coenzyme or cosubstrate in many biological reduction and oxidation
reactions required for energy metabolism and fat synthesis and
Reduces LDL cholesterol and increases HDL cholesterol
Pyridoxine, pyridoxal, pyridoxamine
Coenzyme in nucleic acid metabolism, neurotransmitter synthesis,
Involved in neuronal excitation
Reduces blood homocysteine levels
Prevents megaloblastic anaemia
Generic term for large group of compounds
including folic acid and pterylpolyglutamates
Coenzyme in DNA synthesis and amino acid synthesis. Important for
preventing neural tube defects
Key role in preventing stroke and heart disease, including reducing
blood homocysteine levels with vitamin B12
May protect against colonic and rectal cancer
Name Major function
Calcium Structural component of bones and teeth
Role in cellular processes, muscle contraction, blood clotting, enzyme
activation, nerve function
Copper Aids in utilization of iron stores, lipid, collagen, pigment
Role in neurotransmitters synthesis
Iron Component of haemoglobin and myoglobin in blood, needed for
Role in cellular function and respiration
Magnesium Component of bones
Role in enzyme, nerve, heart functions, and protein synthesis
Manganese Aids in brain function, collagen formation, bone structure, growth, urea
synthesis, glucose and lipid metabolism and CNS functioning
Potassium Major ion of intracellular fluid
Maintains water, electrolyte and pH balances
Role in cell membrane transfer and nerve impulse transmission
Phosphorus Structural component of bone, teeth, cell membranes, phospholipids,
nucleic acids, nucleotide enzymes, cellular energy metabolism
Major ion of intracellular fluid and constituent of many essential
compounds in body and processes
Zinc Major role in immune system
Required for numerous enzymes involved in growth and repair
Involved in sexual maturation
Role in taste, smell functions