There are so many questions to ask about how soy affects our health. With all the products now made from soy from infant formula to milk substitues and ice cream, Americans are concerned about how consuming increasing amounts of soy protein will affect our health now and as we age, as well as the health of our children. Without getting too in-depth into any of the specific topics, here is a general summary of how soy can affect some of the major diseases present in the US. I have included links for additional studies that address more specific topics, and it is likely that I will write more more about the effects of soy. In defense of soy, I would like to point out that Eastern cultures have been eating large amounts of it for generations and they seem to be doing just fine, with the exception of increasing levels of lifestyle related diseases as they adopt a more Western diet.
Reader’s warning: this is a long one.
Phytochemicals are plant-based compounds that are biologically active, meaning that they affect chemical reactions in the human body. One group of these chemicals found in significant amounts in soy and flaxseed products is phytoestrogens. Of particular interest in nutrition research is the group of phytoestrogens called isoflavones. These phytoestrogens are non-steroids that can mimic the steroid estrogen’s behaviors. The amount of isoflavones soy products contain will depend on the species, environment where it is grown, and industrial processing, such as tofu compared to tempeh.
When comparing the steroid activity of estrogens to isoflavones the role of these phytoestrogens is very weak, however they are involved in many other nonhormonal actions. Effects of isoflavones seem to affect women in particular, due to the effects of estrogen on cancer risk, cardiovascular disease, and bone health. While some studies have been done on the health effects in men, more studies seems to be showing the effects of soy consumption on women. It is also recognized that the mechanism of effects of isoflavones on these diseases is not well-understood and seem to be multi-factorial.
Consumption of these compounds at normal dietary rates is associated with lower levels of hormone related diseases, and lower rates of these diseases in countries where high amounts of soy is consumed (i.e., China, Japan, Indonesia). Consumption of soy has been shown to affect cancer growth in several cases. Studies where breast cancer was induced showed that consumption of soy decreased tumor growth, and the effect was lost when consumption ceased. They have also been shown to decrease carcinogenic activity of compounds affecting DNA.
Soy protein is also associated with decreased levels of LDL (bad) cholesterol. After 1 month of consuming an average of 47g of soy protein per day, premenopausal women saw decreases of almost 10% of blood cholesterol levels. Part of the reason for this could be due to substituting soy protein, which contains no cholesterol or saturated fat, for animal proteins that contain varying levels of both. Consumption of pure isoflavones in pill form had no effect on cholesterol, indicating the complicated way these compounds function in the body when consumed as soy protein.
Rates of osteoporosis and hip fracture are also significantly lower in China and Japan than in Western women, indicating that there may be some connection to diet and consumption of soy. Estrogen is important for preventing bone breakdown, which explains why the risk of osteopenia and osteoporosis increases in postmenopausal women. Although the mechanism of isoflavaones in protecting from bone loss are not well understood, animal studies show positive effects on bone density. One study showed that soy protein was equally as effective as estradiol in maintaining bone density 1 month after ovariectomy.
Setchell, KDR. Phytoestrogens: the biochemistry, physiology and implications for human health of isoflavones. American Journal of Clinical Nutrition; 1998;68(suppl):1333S–46S
Zhan, S. & Ho, S. Meta analysis of the effects of soy protein containing isoflavones on the lipid profile. American Journal of Clinical Nutrition; Feb 2005. 81(2) 397-408.
Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis
Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile
META-ANALYSIS OF THE EFFECTS OF SOY PROTEIN INTAKE ON SERUM LIPIDS
Soy Protein Reduces Serum Cholesterol by Both Intrinsic and Food Displacement Mechanisms
Biological effects of a diet of soy protein rich in isoflavones on the menstrual cycle of premenopausal women
Ingestion of whey hydrolysate, casein, or soy protein isolate: effects on mixed muscle protein synthesis at rest and following resistance exercise in young men
Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar spine of perimenopausal women