We've been told eating soy-based foods lowers cholesterol, chills hot flashes, prevents breast cancer and prostate cancer, promotes weight loss, and wards off osteoporosis. That’s not the whole soy story, says the Harvard School of Public Health (HSPH).
“While some of soy’s touted benefits are attributed to a unique characteristic: its high concentration of isoflavones, a type of plant-made estrogen (phytoestrogen) that mimics naturally-produced estrogen, many health claims about soy go far beyond the available evidence,” HSPH said.
In 1999, the Food and Drug Administration let companies claim that foods containing soy protein "may reduce the risk of heart disease." The claim was based on early research showing that soy protein lowered levels of low-density lipoproteins or LDL (bad) cholesterol.
A 1995 meta-analysis, published in the New England Journal of Medicine, 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.
If such reductions were sustained over time, heart attack, stroke or other cardiovascular disease could be reduced by 20 percent.
The American Heart Association’s (AHA’s) nutrition committee looked at more recent soy-cholesterol research and says the results aren’t so glowing. It found eating 50 grams of soy daily—more than half the average person’s daily protein requirement— lowers LDL about 3 percent.
To get 50 grams of soy protein, a person has to eat one-and one-half pounds of tofu or drink 64 ounces of soy milk.
There’s still a silver lining. The AHA committee says that even though soy protein itself has little direct effect on cholesterol, soy foods are still good for the heart and blood vessels.
This is because they usually replace less healthful choices, like red meat while delivering plenty of polyunsaturated fat, fiber, vitamins, and minerals, and low saturated fat.
In theory, it makes sense that soybeans could chill hot flashes and other menopausal problems after all, they’re phytoestrogen-rich. Yet, carefully controlled studies haven’t found this to be the case.
The AHA committee concluded there is no evidence to date to support the claim.
Despite the claims, 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 or worse, that soy could actually spur growth of breast cancer cells.
Timing of soy intake may make a difference: The Shanghai Women's Health Study, for example, found that women with the highest soy protein intake throughout adolescence and early adulthood had nearly a 60 percent lower risk of pre-menopausal breast cancer than women with the lowest intakes.
However, a 2011 study says the same benefits didn’t hold true for western populations.
Several studies also warn against using any soy-based food if you are a current breast cancer patient as it may hinder your treatment.
While soy may conceivably protect against endometrial cancer, ovarian cancer, colorectal cancer and prostate cancer, to date there is no good evidence to validate the claim.
Memory and cognitive skills
A few studies have suggested eating soy can slow or prevent a decline in age-related mental function, such as memory loss or intellectual reasoning. Recent trials have yielded contradictory results. Other studies suggest that too much soy could lead to memory problems for older adults.
One study found that older women of Japanese ancestry living in Hawaii and who relied on a traditional soy-based diet were more likely to have brain impairment than those switching to a Western diet.
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, according to the researchers.
If you don’t like the taste of soy and opt for a supplement instead, you are likely wasting your money. The AHA says there's no evidence that pills containing isoflavones extracted from soybeans offer any benefits, and some studies raise concerns about harmful side effects.