As is so often the case, some of the claims made for soy were based on preliminary evidence, while others go far beyond the available evidence. Back in 1999, the Food and Drug Administration let companies claim that foods containing soy protein “may reduce the risk of heart disease.” (15) The claim was based on early research showing that soy protein lowered levels of LDL (bad) cholesterol. A number of solid studies done since then have tempered this finding, (16) as well as those regarding soy’s effects on other conditions.
Heart disease: A 1995 meta-analysis of 38 controlled clinical trials showed that eating approximately 50 grams of soy protein a day in place of animal protein reduced total cholesterol levels by 9.3 percent, LDL cholesterol by 12.9 percent, and triglycerides by 10.5 percent. (17) Such reductions, if sustained over time, could have meant a 20 percent reduction in the risk of heart attack, stroke, or other forms of cardiovascular disease. An updated look at the soy story, which includes several strong studies published since 2000, isn’t so bullish on soy and cholesterol. According to this comprehensive update of soy research by the American Heart Association’s (AHA’s) nutrition committee, eating 50 grams of soy a day lowers LDL only about 3 percent. (16) Keep in mind that 50 grams of soy protein is more than half the average person’s daily protein requirement. It’s the equivalent of 1½ pounds of tofu or eight 8-ounce glasses of soy milk a day.
All this doesn’t mean you need to turn up your nose at tofu, tempeh, or soy milk, or ignore edamame (a fancy name for soybeans). The AHA committee says that even though soy protein itself has little direct effect on cholesterol, soy foods are good for the heart and blood vessels because they usually replace less healthful choices, like red meat, and because they deliver plenty of polyunsaturated fat, fiber, vitamins, and minerals, and are low in saturated fat.
Hot flashes: Soy has also been investigated as a treatment for hot flashes and other problems that often accompany menopause. In theory, this makes sense. Soybeans are rich in phytoestrogens. In some tissues, these substances mimic the action of estrogen. So they could cool hot flashes by giving a woman an estrogen-like boost during a time of dwindling estrogen levels. Yet carefully controlled studies haven’t found this to be the case, (18, 19) and the AHA committee concludes that soy hasn’t been shown to ease hot flashes and other symptoms of menopause. (16)
Breast cancer: Phytoestrogens don’t always mimic estrogens. In some tissues they actually block the action of estrogen. If such estrogen-blocking action occurs in the breast, then eating soy could, in theory, reduce the risk of breast cancer because estrogen stimulates the growth and multiplication of breast and breast cancer cells. But studies so far haven’t provided a clear answer, with some showing a benefit and others showing no association between soy consumption and breast cancer. (16, 20-22) In fact, a handful of unsettling reports suggests that concentrated supplements of soy proteins may stimulate the growth of breast cancer cells. (23, 24) Timing of soy intake may make a difference: The Shanghai Women’s Health Study, for example, found that women with the highest soy protein intakes throughout adolescence and early adulthood had nearly a 60 percent lower risk of pre-menopausal breast cancer than women with the lowest intakes. (32)
Other cancers: Although substances in soy could conceivably protect against endometrial, ovarian, colorectal, prostate, and other cancers, there is no good evidence for this.
Memory and thinking ability: A few studies have raised the possibility that eating soy could help prevent the age-related loss of memory or decline in cognitive function. Recent trials have yielded contradictory results in this area, with one showing a benefit for soy, (25) and others showing no benefit. (26-28, 33) Other studies suggest that too much soy could lead to memory problems. Among older women of Japanese ancestry living in Hawaii, those who relied on the traditional soy-based diet were more likely to have cognitive problems than those who switched to a more Western diet. (29) This finding, which has yet to be confirmed by other long-term studies, could result from excessive intake of phytoestrogens or inadequate intake of something found in animal products, such as vitamin B-12.
Finally, there’s no evidence that pills containing isoflavones extracted from soybeans offer benefits, and some studies raise concerns about harmful side effects. (16)