Vegan for Life - Jack Norris [83]
At present, we can say that in healthy women, soyfoods don’t raise breast cancer risk, and, while it remains speculative, they may offer benefits for women who have had breast cancer. The most promising findings, though, are that young girls who consume soy could have a lower lifetime risk of getting breast cancer.
Prostate Cancer
There is reason to believe that soy lowers the risk for prostate cancer. First, the rates of prostate cancer are low in soyfood-consuming countries compared to western populations.47 Asian men who consume higher amounts of soy are about 30 percent less likely to develop prostate cancer than those who consume little soy.48 Recent clinical research involving prostate cancer patients suggests that isoflavones might inhibit the spread of prostate cancer. This suggests that soyfoods may turn out to be useful for both the treatment and prevention of prostate cancer.49 Since prostate tumors are slow growing and are typically diagnosed late in life, anything that delays tumor onset or growth can profoundly impact prostate cancer mortality. As we mentioned in Chapter 12, there is evidence that a diet high in dairy products could raise the risk for prostate cancer. So it may be that men who adopt a vegan diet and replace milk with soymilk will have added protection against prostate cancer.
Cognitive Function
Since estrogen appears to help maintain cognitive function in older women, there is speculation that soy isoflavones could have a similar effect. However, results from a study of Japanese men living in Hawaii—the Honolulu Heart Study—showed that those who ate the most tofu exhibited more signs of mental decline in their seventies through their nineties.50 The study wasn’t designed to look specifically at cognitive function, and the researchers measured the intake of only a small number of foods—two factors that limit interpretation of the findings. Also, the way in which soy intake was assessed changed over the course of the study. This too may have influenced conclusions about how much soy the men ate. In another study, which looked at women in Indonesia, tofu consumption was linked to memory loss, but tempeh—a fermented soy product that is a staple of Indonesian diets—had the opposite effect. The reason for the difference could be that in Indonesia, tofu has been typically preserved with formaldehyde, a toxic compound that can harm brain function. Recently, there have been successful efforts to prevent the use of formaldehyde in tofu production in Indonesia.51
Another study in Hong Kong found no effect—positive or negative—of soyfood consumption on cognitive function.52 More importantly, most clinical studies have shown improvements in some aspects of cognitive function with soy consumption.53
Thyroid Function
Many foods, including soyfoods, millet, cruciferous vegetables, and some herbs, contain goitrogens. These compounds interfere with thyroid function (and in extreme cases can cause an enlarged thyroid, which is called a goiter). Generally, they cause problems only in parts of the world where iodine intake is low, since iodine is an essential nutrient needed for thyroid function. The effects of iodine deficiency can be made worse if the diet is high in goitrogens. For western vegans, this shouldn’t be a problem as long as your diet includes sufficient iodine—an easy task if you use small amounts of iodized table salt every day or take an iodine supplement.
Concerns about the effects of isoflavones on thyroid function derive mostly from studies