Soy & Your Health

This topic was discussed on the radio show that originally aired on September 11, 2008.

There are many claims made about the health benefits of consuming soy, and there are also many who assert that soy can actually be harmful to your health. The scientific studies themselves often seem to contradict each other and it's challenging to discern what information is valuable. However, by looking at the traditional use of soy foods in Asia provides we gain clearer insight into how we can consume soy foods in such a way as to achieve health benefits without unwanted consequences.



Benefits of Soy

Many benefits have been ascribed to the consumption of soy foods. Some of the most commonly mentioned benefits include: reducing menopausal symptoms, such as hot flashes; reducing high cholesterol; balancing blood sugar in diabetics; and preventing cancer. However, the studies that mention these benefits often do so while mentioning the levels of soy in the diet.

For example, a study in Hong Kong found that chronic diseases (such as certain cancers and ischemic heart disease) were reduced as soy consumption increased. However, in their conclusion, the authors stated that these benefits could be obtained by consuming "traditional levels of soy". 1

Another study investigated the epidemiological and clinical data concerning the effect of soy isoflavones on breast cancer. In this case the authors concluded that the data confirmed that there are no unwanted effects on breast tissue when the consumption of isoflavones is "consistent with historical Asian soyfood intake". 2



Side Effects of Soy

A variety of negative side effects of soy consumption have also been demonstrated. Most commonly mentioned are: premature sexual development in girls; delayed sexual development in boys; and depressed thyroid function.

In the young and the old the consumption of soy appears to have more significant and deleterious effects. For example, the use of soy formula for children has been associated with an increase in autoimmune thyroid disease. 3 One study found that the use of soy lecithin in pregnant rats altered brain function similarly to "disturbances in neural function". 4 While effects in young rats may not directly correlate to similar effects in humans, it has been found that an increased consumption of soy during midlife years with decreased performance on cognitive tests as well as decreased brain weight. 5

In men soy has been shown to have some deleterious effects. For example, soy protein has been shown to decrease testosterone levels in men. 6 More recently, it was found that the more soy men ate the greater their sperm count was decreased, especially in men who were overweight or obese. 7



Soy and Breast Cancer

The confusion around the effects of soy may be highest in the case of breast cancer. Studies repeatedly show both benefit and harm from the use of soy. One study that demonstrates this confusion most clearly examines the effect of certain isoflavones on breast tumor growth. The results (both in in vitro and in vivo animal studies) showed that certain compounds (genistein and daidzein) caused breast tumors to grow when given in low concentrations. However, at higher levels, one of them (genistein) actually prevented tumor growth. As a result, the authors concluded that women "should be aware of the risks of potential tumor growth when taking soy products". 8



Soy consumption in Asia

Originally soy was cultivated in China and was valued for its ability to facilitate the growth of other crops. After it was used as an agricultural tool, it was eventually discovered that it could be eaten regularly as a fermented food and was utilized largely as a condiment or additive to a full meal.

However, in the western hemisphere many vegetarians and vegans eat large quantities of soy in the form of tofu or soy protein isolates (e.g. texturized vegetable protein). In these cases, soy is often being used as a primary source of protein at meals or snacks. Even non-vegetarians, often unwittingly, consume significant amounts of soy in the form of energy bars, snacks, or protein supplements.

A survey conducted in Japan revealed that the average daily consumption of soy protein amounted to approximately 10 grams (2 teaspoons) per day. In these cases, soy was consumed in the form of miso, natto, soymilk and boiled soybeans. 9 Some report that in the 1930's soy only comprised 1.5% of the Chinese diet, whereas pork only constituted 65%. 10



Benefits of Fermentation

Phytic acid, present in many seeds and nuts, is high in soybeans. It binds to important nutrients such as calcium, magnesium, iron, and zinc and preven them from being absorbed in the digestive tract. Enzyme inhibitors prevent digestive enzymes from exerting their effect. Thus, as a result of eating unfermented soy the bioavailability of nutrients in the soy as well as other foods is significantly compromised. In fact abnormal growth was observed in rats consuming unfermented soy. 11 In monkeys, the consumption of soy concentrate resulted in several conditions including pancreatitis. 12

Interestingly, soy formula has been used for years as a substitute for children who are unable to breastfeed and are allergic to dairy foods and was even thought to prevent allergies. However, a survey of studies has determined that soy formula should not be used in an attempt to prevent allergies and food intolerances in children. 13 In fact, the incidence of soy allergies has increased significantly in recent years. However, fermentation greatly enhances the digestibility of soy quite significantly by reducing the content of phytic acid and enzyme inhibitors. And fortunately, researchers have discovered that the fermentation of soy significantly reduces its tendency to cause allergic reactions. 14 In fact, other unexpected benefits are derived from consuming fermented soy. For example, rat studies have demonstrated that the consumption of natto may improve cardiovascular health. 15



Conclusion

Soy foods may offer health benefits, and, in order to prevent any side effects, it appears that there are certain preparations that are best to consume. Small quantities of soy in the form of fermented foods such as natto, tempeh, miso, and shoyu (soy sauce) provide the highest nutrient content while conferring health benefits. Used as condiments or side-dishes, fermented soy can improves both the taste and nutrition of meals.



References

1. July 2006, Preventative Medicine

2. June 2008, Nutrition Journal

3. April 1990, Journal of the American College of Nutrition

4. August 1986, Brain Research Bulletin

5. April 2000, Journal of the American College of Nutrition

6. March 2005, Journal of Nutrition

7. July 2008, Human Reproduction

8. Sep 2001, Annals of Pharmacotherapy

9. The Journal of Nutrition Vol. 128 No. 2 February 1998, pp. 209-213

10. Food in Chinese Culture by K C Chang

11. Weston A. Price Foundation

12. Soy Online Service

13. 2004, Cochrane Database System Review

14. 2008, Journal of Agriculture and Food Chemistry

15. July 2003, Life Sciences