Health risks
[edit] Phytoestrogen
Main article: Phytoestrogens
Soybeans contain isoflavones called genistein and daidzein, which are one source of phytoestrogens in the human diet. Because most naturally occurring estrogenic substances show weak activity, normal consumption of foods that contain these phytoestrogens should not provide sufficient amounts to elicit a physiological response in humans.[citation needed]
Plant lignans associated with high fiber foods such as cereal brans and beans are the principal precursor to mammalian lignans which have an ability to bind to human estrogen sites. Soybeans are a significant source of mammalian lignan precursor secoisolariciresinol containing 13–273 µg/100 g dry weight.[44] Another phytoestrogen in the human diet with estrogen activity is coumestans, which are found in beans, split-peas, with the best sources being alfalfa, clover, and soybean sprouts. Coumestrol, an isoflavone coumarin derivative is the only coumestan in foods.[45][46]
Soybeans and processed soy foods do not contain the highest "total phytoestrogen" content of foods. A study in which data were presented on an as-is (wet) basis per 100 g and per serving found that food groups from highest to lowest levels of total phytoestrogens per 100 g are nuts and oilseeds, soy products, cereals and breads, legumes, meat products, various processed foods that may contain soy, vegetables, and fruits.[47]
[edit] Women
A 2001 literature review suggested that women with current or past breast cancer should be aware of the risks of potential tumor growth when taking soy products, based on the effect of phytoestrogens to promote breast cancer cell growth in animals.[48]
A 2006 commentary reviewed the relationship with soy and breast cancer. They stated that soy may prevent breast cancer, but cautioned that the impact of isoflavones on breast tissue needs to be evaluated at the cellular level in women at high risk for breast cancer.[49]
[edit] Men
Because of the phytoestrogen content, some studies, but not all, have suggested that there is an inverse correlation between soybean ingestion and testosterone in men.[50] For this reason, they may protect against the development of prostate cancer.[51] A theoretical decrease in the risk of prostate cancer should, however, be weighed against the possible side-effects of decreased testosterone, which are still unclear. The popular fear that soybeans might cause reduced libido and even feminine characteristics in men has not been indicated by any study; the popularity of the notion seems to be based on the simplistic misapprehension that estrogen and testosterone have a simple, inverse relationship in sexual hormone systems and sex-related behaviour. Their interplay is very complicated and largely still unknown.[52]
Studies published in July 2008 shows that Soy products and more specifically the phytoestrogen it contains have been shown to significantly lower a mans sperm count.[53]
[edit] Infant formula
There are some studies that state that phytoestrogen in soy can lead to alterations in the proliferation and migration of intestinal cells. The effects of these alterations are unknown.[54] However, some studies conclude there are no adverse effects in human growth, development, or reproduction as a result of the consumption of soy-based infant formula.[55] Other reviews agree, but state that more research is needed to answer the question of what effect phytoestrogens have on infants.[56][57] Soy formula has also been linked to autoimmune disorders of the thyroid gland.[58]
[edit] Cancer
The soy industry companies claim soy to have anticancer benefits. The statement is mainly based on statistics which shows that Japanese, who have soy based products as a part of a daily nutrient, suffer less from breast cancer, uterus and prostate. The fact that Asian population, with the Japanese among them, have much higher rates of cancer related to dejection system and thyroid are less popularized.[citation needed]
[edit] Allergens
Main article: Soy allergy
About 9% of children in the USA are allergic to soybean proteins.[citation needed] The major soy allergen has been identified by scientists at USDA.[citation needed] Both transgenic and conventional soybean varieties without the allergenic protein have been prepared.[citation needed] Soy allergy, typically, will manifest itself approximately a day after consumption of the beans. Common symptoms are urticaria, rash, itching, and redness of the skin.[59][60]
[edit] Promotion as health food
Soy consumption has been promoted by natural food companies and the soy industry's aggressive marketing campaign in various magazines, television ads and in health food markets. Research has been conducted examining the validity of the beneficial health claims with regard to the increase in consumption of soybeans which mimic hormonal activity.
A practice guideline published in the journal Circulation questions the efficacy and safety of soy isoflavones for preventing or treating cancer of the breast, endometrium, and prostate (although the same study also concludes that soy in some foods should be beneficial to cardiovascular and overall health) and does not recommend usage of isoflavone supplements in food or pills.[61] A review of the available studies by the United States' Health and Human Services' Agency for Healthcare Research and Quality (AHRQ) found little evidence of substantial health improvements and no adverse effects, but also noted that there was no long-term safety data on estrogenic effects from soy consumption.[62]
[edit] Brain
Estrogen helps protect and repair the brain during and after injury.[63] The mimicry of estrogen by the phytoestrogens in soy has introduced a controversy over whether such a replacement is harmful or helpful to the brain. Several studies have found soy to be harmful for rats.[64] Nevertheless the cited study was based on rats fed with concentrated phytoestrogens and not common soybeans and it is already well known that concentrated estrogens cause negative effects in males. The common amounts of phytoestrogens in soy beans are not to be compared to concentrated estrogen. One study followed over 3000 Japanese men between 1965 and 1999, and that showed a positive correlation between brain atrophy and consumption of tofu.[65]
[edit] Carcinogen
Raw soy flour is known to cause pancreatic cancer in rats.[66] Whether this is also true in humans is unknown because no studies comparing cases of pancreatic cancer and soy intake in humans have yet been conducted, and the doses used to induce pancreatic cancer in rats are said to be larger than humans would normally consume. Heated soy flour may not be carcinogenic in rats.[67][68] Existing cancer patients are being warned to avoid foods rich in soy because they can accelerate the growth of tumours.[69]
[edit] Iron
A study conducted in China found that "A Vegetarian Diet Rich in Soybean Products Compromises Iron Status in Young Students"[70]
