Edible Plants and Phytochemicals

Because their consumption is known to enhance health, vegetables, fruits, cereal grains, nuts, and seeds are the most widely researched functional foods. The health benefits of a plant-based diet are usually attributed to the content of fiber and of a variety of plant-derived substances (phytonutrients and phytochemicals) with antioxidant, enzyme-inducing, and enzyme-inhibiting effects. Some phyto-chemicals may also exert their health effects by modifying gene expression. Carotenoids, for example, enhance expression of the gene responsible for production of Connexin 43, a protein that regulates intercellular communication. The protective effect of carotenoid consumption against the development of cancer is more strongly related to the ability of individual carotenoids to upregulate Connexin 43 expression than their antioxidant effects or conversion to retinol. Dietary supplementation with beta-

carotene reduces the blood levels of other carote-noids, some of which are more potent inducers of Connexin 43 than is beta-carotene. The unexpected and highly publicized increase in incidence of lung cancer among smokers taking beta-carotene supplements may be explained by this mechanism.

Phytochemicals associated with health promotion and disease prevention are described in Table 2. The most studied food sources of these phytonutrients are soy beans (Glycine max) and tea (Camellia sinensis leaves), but tomatoes (Lycopersicon esculen-tum), broccoli (Brassica oleracea), garlic (Allium sativum), turmeric (Curcuma longa), tart cherries (Prunus cerasus), and various types of berries are also receiving considerable attention as functional food candidates. An overview of the research on soy and tea illustrates some of the clinical issues encountered in the development of functional foods from edible plants.

Soy protein extracts have been found to lower cholesterol in humans, an effect that appears to be related to amino acid composition. Soy protein extracts frequently contain nonprotein isoflavones, which have received considerable attention because of their structural similarity to estrogen. Soy isofla-vones are weak estrogen agonists and partial estrogen antagonists. Epidemiologic and experimental data indicate that isoflavone exposure during adolescence may diminish the incidence of adult breast

Table 2 Phytochemicals associated with health promotion and disease prevention

Group

Typical components

Biological activities

Food sources

Carotenoids

Alpha- and beta-carotene

Quench singlet and triplet oxygen,

Red, orange and yellow fruits and

cryptoxanthin, lutein,

increase cell-cell communication

vegetables, egg yolk, butter fat,

lycopene, zeaxanthin

margarine

Glucosinolates,

Indole-3-carbinol

Increase xenobiotic metabolism,

Cruciferous vegetables, horseradish

isothiocyanates

sulphoraphane

alter estrogen metabolism

Inositol

Inositol hexaphosphate

Stimulate natural killer cell function,

Bran, soy foods

phosphates

(phytate)

chelate divalent cations

Isoflavones

Genistein, daidzein

Estrogen agonist and antagonist,

Soy foods, kudzu

induce apoptosis

Lignans

Enterolactone,

Estrogen agonists and antagonists,

Flax seed, rye

enterolactone

inhibit tyrosine kinase

Phenolic acids

Gallic, ellagic, ferulic,

Antioxidant, enhance xenobiotic

Diverse fruits, vegetables

chlorogenic, coumaric

metabolism

Phytoallexins

Resveratrol

Antioxidant, platelet inhibition,

Red wine, grape seed

induce apoptosis

Polyphenols

Flavonoids, chalcones,

Antioxidant, enhance xenobiotic

Diverse fruits, vegetables, red wine,

catechins, anthocyanins,

metabolism, inhibit numerous

tea

proanthocyanidins

enzymes

Saponins

Glycyrrhizin, ginsenosides

Antimicrobial, immune boosting,

Legumes, nuts, herbs

cytotoxic to cancer cells

Sterols

Beta-sistosterol,

Bind cholesterol, decrease colonic

Nuts, seeds, legumes, cereal grains

campestrol

cell proliferation, stimulate

T-helper-1 cells

Sulfides

Diallyl sulfides

Antimicrobial, antioxidant

Garlic, onions

cancer. In vitro studies show conflicting effects. On the one hand, soy isoflavones induce apoptosis of many types of cancer cells; on the other hand, estrogen receptor-bearing human breast cancer cells proliferate in tissue culture when exposed to isoflavones. Although the widespread use of soy in Asia is cited in support of the safety of soy foods, the intake of isoflavones among Asian women consuming soy regularly is in the range of 15-40 mgday-1, significantly less than the isofla-vone content of a serving of soymilk as consumed in the US. In clinical trials, soy isoflavones have not been effective in relieving hot flashes of menopausal women but do diminish the increased bone resorption that causes postmenopausal bone loss. In pre-menopausal women, soy isoflavones may cause menstrual irregularities. The successful development of soy derivatives as functional foods will require that these complex and diverse effects of different soy components in different clinical settings be better understood.

Regular consumption of tea, green or black, is associated with a decreased risk of heart disease and several kinds of cancer. These benefits are attributed to tea's high content of catechin polymers, especially epigallocatechin gallate (ECGC), which has potent antioxidant and anti-inflammatory effects, that may lower cholesterol in hyperli-pidemic individuals and alter the activity of several enzymes involved in carcinogenesis. Catechin content is highest in young leaves. Aging and the fermentation used to produce black tea oxidize tea catechins, which polymerize further to form the tannins, theaflavin and thearubigen. Although ECGC is a more potent antioxidant than theaflavin, theaflavin is far more potent an antioxidant than most of the commonly used antioxidants, like glu-tathione, vitamin E, vitamin C, and butylated hydroxytoluene (BHT). Both ECGC and theaflavin are partially absorbed after oral consumption, but a clear dose-response relationship has not been established. Tea-derived catechins and polymers are being intensively studied as components of functional foods, because the results of epidemiologic, in vitro, and animal research indicate little toxicity and great potential benefit in preventing cancer or treating inflammation-associated disorders. Clinical trials have shown a mild cholesterol-lowering effect and perhaps some benefit for enhancing weight loss.

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