Nature's Pharmacopeia: A World of Medicinal Plants

SOY

Glycine max

The soybean is an annual herbaceous legume that originated in eastern China.18 It grows to a height around 1 to 1.5 meters and produces numerous small, white, pink, or purple flowers that give rise to pods, each containing two to four pea-size seeds (figure 14.3).19 The plant has been cultivated for its leaves, which can be cooked and eaten or fed to animals as forage, and for its seeds and seed pods. The immature seed pods can be cooked and eaten, and the mature seeds can be prepared for human and animal consumption. The seeds are processed into numerous products, including soy meal, soy oil, soy milk (cooked, ground soybeans), and tofu (solidified soy proteins).

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FIGURE 14.3   Soybean pods.

Archaeological and literary evidence supports the idea that the soybean was domesticated sometime before about 3000 years ago. Soybeans were first grown in Europe and North America during the eighteenth century and used primarily for animal feed. As a worldwide commodity, soybeans now account for a major part of agricultural production in the United States, Canada, Brazil, and China.20Economically important applications include food additives, such as the extract lecithin, which is used in the food industry to improve the texture of products containing both oil- and water-based ingredients, and soybean oils, which can be used for culinary purposes or for fuel as biodiesel.

PROPERTIES OF THE POLYPHENOLIC COMPOUNDS

Structure

Polyphenols consist of rings of carbon atoms linked together in various ways

Molecular composition

Polyphenols contain carbon, oxygen, and hydrogen atoms; they do not contain nitrogen

Diversity

There are many thousands of types of polyphenols; hundreds can occur in any given plant tissue

Complexity

Some polyphenols are made up of dozens of units of simpler phenolic structures

Solubility

Polyphenols are often water soluble

Biological activity

Many polyphenols bind (chemically attach) to cell surfaces, which may explain antibacterial properties; some can bind to hormone receptors, enhancing or reducing natural hormone-related processes; some can bind to neurotransmitter receptors, mimicking or blocking their functions; many are also antioxidants

Vocabulary

Flavonoids, flavonols, isoflavones, anthocyanins, catechins, and tannins are all polyphenolic compounds

In ancient China, soybean was both a food crop and a medicinal substance.21 Classic texts of Chinese traditional medicine recommend soybean sprouts to clear bodily heat and treat painful obstructions of blood and qi, and cooked seeds as a remedy for illnesses characterized by irritability, headache, and restlessness. Soybean seed coats (the skin of the seed) were suggested as a poultice to treat sores and diaper rash.22

The past several decades have seen considerable interest in dietary soy as a possible preventive measure or therapy against cardiovascular disease, age-related bone loss, and the symptoms of menopause. Drawing on the observation that East Asian women appear to be less susceptible than Western women to postmenopausal bone loss, hot flashes, breast cancer, and other concerns of aging, investigators speculated that some aspect of the East Asian diet or lifestyle might be associated with a lower risk of these conditions.23 Since the Japanese, Koreans, and Chinese, among others, consume larger quantities of soy products than do people in other nations, researchers questioned whether these foods might have properties related to women’s health.

Menopause is accompanied by a decrease in the hormone estrogen, and it is possible that some components of soy might counteract this hormonal change, thereby reducing diverse menopause-associated health risks. Current studies often focus on a class of polyphenolic compounds called the isoflavones, chemicals that are in relatively high abundance in soybean seeds (figure 14.4).24 Certain isoflavones have the property of binding to the cellular receptors for estrogen, thereby activating estrogen-dependent responses in the target tissues. This feature has earned them the name phytoestrogens: plant-derived chemicals that act like the steroid hormone estrogen. In addition to their estrogenic properties, isoflavones can interact with a suite of cellular enzymes, and they are antioxidants.25

Phytoestrogens

Chemicals from plants that act in the body like the female sex hormone estrogen

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FIGURE 14.4   Soy isoflavones: genistein; daidzein.

Because isoflavones can mimic natural estrogens in laboratory tests, soy products have been examined against a wide range of menopausal symptoms in humans, from flushing and emotional changes to vaginal dryness and loss of bone density.26 Most tests to date have been of a small scale and use differing sources and concentrations of soy products, rendering it relatively difficult to generalize findings.27 Some statistically significant results have been obtained on the effect of soy products on bone density. In analyses of the results of several trials collected together, soybean or isoflavone supplementation appears to have moderately counteracted the loss of bone in postmenopausal women.28

While estrogen promotes bone density and staves off menopausal symptoms in women, it can also induce the proliferation of estrogen-sensitive cancer cells, leading some to question whether soy-isoflavone consumption might increase the risk of breast or ovarian cancers.29 Although the mechanism of isoflavone binding to estrogen receptors could conceivably induce cancer in susceptible tissues, human trials have not demonstrated a strong relationship between soy consumption and markers of cancer proliferation.30 Interestingly, some evidence may indicate a possible antiproliferative effect for isoflavones, suggesting that certain types or doses of these chemicals might reduce the growth of cancer. The dual pro- and anticancer cell–growth properties of certain isoflavones is speculated to be a function of their binding to alternative estrogen-receptor subtypes, with different, sometimes opposing roles in regulating cell proliferation. As there are so many factors influencing the development of cancers occurring in diverse tissues, it will take time for investigators to identify any specific components of soy that might play a role in this area.

Researchers have attempted to test associations between soy consumption and numerous aspects of health, including cardiovascular measures such as blood-lipid profile and blood pressure.31 Such studies have generally yielded conflicting or equivocal results, with analysts complaining of small sample sizes and poor experimental designs.32 Since much of the current understanding of soy and population health is built on epidemiological approaches with many possible variables and a relatively small set of controlled trials, there are few conclusive findings on the role of soy as a therapeutic food.