Nutritional Quality of Fruits, Nuts, and Vegetables
and their Importance in Human Health
Adel A. Kader1, Penelope Perkins-Veazie2, and Gene E. Lester3
1Department of Pomology, University of California, Davis CA
2South Central Agricultural Laboratory, USDA/ARS, Lane OK
3Kika de la Garza Subtropical Agricultural Research Center, USDA/ARS, Weslaco TX
Fruits, nuts, and vegetables play a significant role in human nutrition, especially as sources
of vitamins [C (ascorbic acid), A, thiamine (B1), niacin (B3), pyridoxine (B6), folacin (also
known as folic acid or folate) (B9), E], minerals, and dietary fiber (Craig and Beck, 1999;
Quebedeaux and Bliss, 1988; Quebedeaux and Eisa, 1990; Wargovich, 2000). Their contribution
as a group is estimated at 91% of vitamin C, 48% of vitamin A, 30% of folacin, 27% of vitamin
B6, 17% of thiamine, and 15% of niacin in the U.S. diet. Fruits and vegetables also supply 16%
of magnesium, 19% of iron, and 9% of the calories. Legume vegetables, potatoes, and tree nuts
(such as almond, filbert, pecan, pistachio, and walnut) contribute about 5% of the per capita
availability of proteins in the U.S. diet, and their proteins are of high quality as to their content of
essential amino acids. Nuts are a good source of essential fatty acids, fiber, vitamin E, and
minerals. Other important nutrients supplied by fruits and vegetables include riboflavin (B2),
zinc, calcium, potassium, and phosphorus. For more information on food composition and
nutrition, access one of the following Internet websites: http://www.nal.usda.gov/fnic/cgi-
bin/nut_search.pl; http://www.nutrition.gov. A recommended daily intake (RDI) for the U.S. of
dietary nutrients can be found at http://www.usaid.gov/hum_response/crg/annex-4.htm. Fruits
and vegetables remain an important source of nutrients in many parts of the world, and offer
advantages over dietary supplements because of low cost and wide availability.
Dietary supplements, while advantageous for conditions where specific nutrients are needed
in abundance such as with iron deficiency, may be poorly absorbed, and many are derived
chemically rather than from natural sources. Climatic conditions, particularly temperature and
light intensity, have an especially strong effect on the nutritional quality of fruits and vegetables
oxidation (Mozafar, 1994). Low temperatures favor synthesis of sugars and vitamin C (glucose
being the precursor to ascorbic acid) and at the same time decrease the rate of ascorbic acid
oxidation. Maximum β-carotene (vitamin A) content in tomatoes occurs at a temperature range
of 15 to 21 ºC, (59 to 70 ºF) but β-carotene content is reduced if temperatures are higher or lower
than this range, principally due to the temperature sensitivity of lycopene, the precursor to β-
carotene and lutein.
The B vitamins are crop specific when it comes to temperature sensitivity. Warm season
crops (beans, tomatoes, peppers, melons, etc.) produce more B vitamins at high (27 to 30 ºC; 81
to 86 ºF) versus low (10 to 15 ºC; 59 to 70ºF) temperatures. Conversely, cool season crops
(broccoli, cabbage, spinach, peas etc.) produce more B vitamins at low versus high temperatures.
Light intensity has little effect on the B vitamins, but as light intensity increases, vitamin C
increases and total carotenoids (vitamin A precursors) and chlorophyll decrease (Gross, 1991).
Higher light intensities produce more sugars, leading to more vitamin C, and also increase plant
temperatures, inhibiting beta carotene (vitamin A) production, which protects chlorophyll from
photo bleaching. Soil type, the rootstock used for fruit trees, mulching, irrigation, fertilization,
and other cultural practices influence the water and nutrient supply to the plant, which can affect
the composition and quality attributes (appearance, texture, taste and aroma) of the harvested
plant parts (Goldman et al., 1999). Other environmental factors that impact fruit and vegetable
nutritional quality are altitude, soil pH and salinity, production practice (organic versus
conventional, and greenhouse versus field), ozone, insect injury, and plant diseases.
Maturity at harvest, fruit size and harvesting method influence the commodity’s quality and
extent of physical injuries. Delays between harvest and consumption or processing can result in
losses of flavor and nutritional quality. The magnitude of these losses increases with exposure to
temperatures, relative humidities, and/or concentrations of O2, CO2, and C2H4 outside the ranges
that are optimum for each commodity during the entire postharvest handling system (Lee and
Kader, 2000). Furthermore, processing and cooking methods can greatly affect the nutritional
value of fruits and vegetables. For instance, water-soluble vitamins such as vitamin C and folate
are lost at high rates when cooking water is discarded, while fat-soluble compounds such as
lycopene may be stabilized or enhanced by cooking.
Fruits, nuts, and vegetables in the daily diet have been strongly associated with reduced risk
for some forms of cancer, heart disease, stroke, and other chronic diseases (Goldberg, 2003;
Hyson, 2002; Prior and Cao, 2000; Produce for Better Health Foundation, 1999; Quebedeaux
and Bliss, 1988; Quebedeaux and Eisa, 1990; Southon, 2000; Tomas-Barberan and Espin, 2001;
Wargovich, 2000). Some components of fruits and vegetables (phytochemicals) are strong
antioxidants and function to modify the metabolic activation and detoxification/disposition of
carcinogens, or even influence processes that alter the course of the tumor cell (Wargovich,
2000). Although antioxidant capacity varies greatly among fruits and vegetables (Prior and Cao,
2000; Perkins-Veazie and Collins, 2001; Kalt, 2002) it is better to consume a variety of
commodities rather than limiting consumption to a few with the highest antioxidant capacity.
The USDA 2000 Dietary Guidelines (USDA, 2000) encourage consumers to: (1) enjoy five a
day, ie., eat at least 2 servings of fruits and at least 3 servings of vegetables each day, (2) choose
fresh, frozen, dried, or canned forms of a variety of colors and kinds, and (3) choose dark-green
leafy vegetables, orange fruits and vegetables, and cooked dry beans and peas often. In some
countries, consumers are encouraged to eat up to 10 servings of fruits and vegetables per day.
There is increasing evidence that consumption of whole foods is better than isolated food
components such as dietary supplements and nutraceuticals. For example, increased consumption
of carotenoid-rich fruits and vegetables was more effective than carotenoid dietary supplements
in increasing LDL oxidation resistance, lowering DNA damage, and inducing higher repair
activity in human volunteers who participated in a study conducted in France, Italy, Netherlands,
and Spain (Southon, 2000). In another study, adding antioxidant (vitamins A, C and E) dietary
supplements into the diet of cancer treatment patients, who were eating a balanced diet of fruits
and vegetables, negatively impacted their radio- and chemotherapies (Seifried et al, 2003). High
consumption of tomatoes and tomato products has been linked to reduced carcinogenesis,
particularly prostate cancer, and has been thought to be due to the presence of lycopene, which
gives red tomatoes their color (Giovannucci, 2002). However, use of tomato powder effectively
reduced prostate carcinogenesis in rats, while lycopene supplements, considered the primary
active ingredient of tomatoes, had no effect (Boileau et al., 2003). Similar comparative studies
are needed on other constituents of fruits and vegetables and on the bioavailability of nutrients
taken as dietary supplements or as foods that contain these nutrients.
Examples of the phytochemicals in fruits and vegetables that have established or proposed
positive effects on human health and their important sources are shown in Tables 1 and 2. Some
changes in these tables are likely as the results of additional studies on effects of phytochemicals
and their bioavailability on human health become available in the next few years. Meanwhile it is
important to evaluate the validity and dependability of the results of every study before reaching
conclusions for the benefit of consumers.
Table 1. Nutritive constituents of fruits and vegetables that have a positive impact on human
health and their sources
proposed effects on
broccoli, cabbage, cantaloupe, citrus fruits, guava,
kiwifruit, leafy greens, pepper, pineapple, potato,
strawberry, tomato, watermelon
prevents scurvy, aids
wound healing, healthy
dark-green vegetables (such as collards, spinach, and
turnip greens), orange vegetables (such as carrots,
pumpkin, and sweet potato), orange-flesh fruits (such
as apricot, cantaloupe, mango, nectarine, orange,
papaya, peach, persimmon, and pineapple), tomato
fatigue, psoriasis, heart
Vitamin K nuts, lentils, green onions, crucifers (cabbage,
broccoli, brussel sprouts), leafy greens
synthesis of pro-
nuts (such as almonds, cashew nuts, filberts,
macadamias, pecans, pistachios, peanuts, and
walnuts), corn, dry beans, lentils and chickpeas, dark-
green leafy vegetables
most fresh fruits and vegetables, nuts, cooked dry
beans and peas
diabetes, heart disease
dark-green leafy vegetables (such as spinach, mustard
greens, butterhead lettuce, broccoli, brussels sprouts,
and okra), legumes (cooked dry beans, lentils,
chickpeas and green peas), asparagus
birth defects, cancer
heart disease, nervous
cooked vegetables (such as beans, greens, okra and
tomatoes) peas, papaya, raisins, orange, almonds,
snap beans, pumpkin, cauliflower, rutabaga
skeletal, teeth, blood
Magnesium spinach, lentils, okra, potato, banana, nuts, corn,
system, teeth, immune
Potassium baked potato or sweet potato, banana & plantain,
cooked dry beans, cooked greens, dried fruits (such as
apricots and prunes), winter (orange) squash, and
Table 2. Non-nutritive plant constituents that may be beneficial to human health
proposed effects on
Proanthocyanins tannins apple, grape, cranberry,
Anthocyanidins cyanidin, malvidin,
red, blue, and purple fruits
(such as apple, blackberry,
blueberry, cranberry, grape,
nectarine, peach, plum &
raspberry, and strawberry)
citrus (oranges, grapefruit,
lemons, limes, tangerine)
Flavones Luteolin, apigenin celeriac, celery, peppers,
rutabaga, spinach, parsley,
artichoke, guava, pepper
onions, snap beans,
broccoli, cranberry, kale,
Phenolic acids Caffeic acid,
strawberry, apple, peach,
papaya, Brazilian guava,
Autumn olive, red
α-carotene sweet potatoes, apricots,
green beans, lima beans,
broccoli, brussel sprouts,
cabbage, kale, kiwifruit,
lettuce, peas, spinach,
prunes, peaches, mango,
papaya, squash and carrots
β-carotene cantaloupes, carrots,
apricots, broccoli, leafy
greens (lettuce, swiss
chard), mango, persimmon,
red pepper, spinach, sweet
Xanthophylls Lutein, zeaxanthin,
sweet corn, spinach, corn,
okra, cantaloupe, summer
squash, turnip greens
Monoterpenes limonene citrus (grapefruit,
Sulfur compounds glucosinolates,
broccoli, Brussels sprouts,
horseradish, garlic, onions,
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