For the most part, the controversy has swirled around whether soy protects us from or promotes breast cancer. But the controversy doesn’t stop here. Several studies suggest that soy consumption contributes to a lower incidence of chronic diseases, such as cardiovascular disease, bone loss, prostate cancer, and Type-2 diabetes, as well as, helps ease menopause symptoms like hot flashes. Critics, on the other hand, say soy has no positive association with heart health, may affect thyroid function, and increase the risks of various cancers.
So is soy good for us or not? With more than 10,000 soy-related articles published in the last 20 years, trying to sort through the evidence is confusing. Let’s look at what science has to say regarding specific health-related issues:
Soy and Breast Cancer
The molecular structure of isoflavones is very similar to that of our own estrogen. The controversy over soy and breast cancer revolves largely around whether soy isoflavones stimulate breast cancer cells in the same way that human estrogen does. It is clear that increased estrogen exposure correlates with increased breast cancer risk. For example, beginning your period at a younger age, beginning menopause at an older age, having your first child at an older age, obesity and drinking alcohol all increase a woman’s risk for breast cancer by increasing a woman’s exposure to estrogen. While isoflavones in soy have been shown to promote breast cancer in mice, many experts claim that the metabolism of soy isoflavones in a mouse model may differ from human, meaning the results aren’t necessarily applicable to women. Moreover, doses of soy isoflavones used in studies were in quantities much larger than usually ingested in a typical diet. Furthermore, the type of soy product used in animal studies may also be key factor; the more processed the soy product, the greater the risk of tumor growth. Whole soy products did not show such effect.
Human clinical studies generally have not shown that the adult consumption of soy increases the markers of breast cancer risk. Conversely, several studies have detected that soy can inhibit tumor growth. Furthermore, a possible protective effect has also been suggested in large population studies linking soy with lower risk of breast cancer.
With that said, since in theory, soy isoflavones may act like human estrogens, women at high risk for breast cancer may consider avoiding or limiting their soy.
Soy and Heart Protection
In 1995, the FDA approved a claim stating that “consuming 25 grams of soy protein daily in a diet low in saturated fat and cholesterol could reduce the risk of coronary heart disease by lowering cholesterol”. Since then, studies have found additional beneficial effects on other markers of heart health, such as lower blood pressure and increased dilation of blood vessels.
However, a recent analysis found that the cholesterol-lowering effect of soy protein is lower than originally presumed. Although modest, the cholesterol lowering effect of soy protein is similar to that of soluble fiber, which makes soy a valuable part of a cholesterol-lowering diet.
Keep in mind that the suggested 25 grams of soy protein a day—about a pound of tofu—is much higher than the average consumption of soy foods in Japan and China—about 10 grams or two teaspoons a day. According to some recent studies, the heart-protective effects are apparent with just half the amount of whole soy protein currently being recommended per day.
Soy contains substances called goitrogens that can interfere with thyroid function; but most of the studies have shown no harmful effect in adults with healthy thyroids. Theoretically, soy could be a risk factor for goiter, especially if iodine is lacking in the diet, but so far there’s no convincing evidence that it’s a problem.
Cooling off Menopause
Soy protein isolate and isoflavone pills have been found to ease the symptoms associated with menopause, such as hot flashes. However, the benefit is relatively small—only about a 40 percent to 50 percent reduction compared to a 90 percent reduction in patients on hormone replacement therapy. On the other hand, soy may offer an advantage over hormone replacement therapy by not disrupting natural hormones.
Words to the Wise
Michael Pollard succinctly said what many of us are finding to be a recurrent theme in nutrition, “Eat (natural, unprocessed) food. Not too much. Mostly plants.” Let’s remember that soybeans are a nutritious food and that people have eaten for centuries. They supply high-quality lean protein that can replace at times animal protein. This is especially beneficial for vegetarians that need to bolster their protein intake. Like other beans, soybeans are high in fiber when eaten in their whole form. And they are the only legume that provides ample amounts of ALA, an essential omega-3 fatty acid. Focus on forms of soy that are whole or less processed, such as whole soybeans (like Edamame), tofu, tempeh, and unsweetened soy milk. As a general rule, the more closely food resembles its natural state, the higher the nutritional value and the less potential for harm. Since little is known about possible damage and consequence of over-processing, try to limit concentrated forms of soy, such as soy protein concentrates, isolates, and textured soy protein products, which are ever present in cereals, energy bars, smoothie powders, and processed foods. Be particularly wary of soy supplements, as little is known about its effects.
7 Smart Ways to Add Soy to Your Diet
1) Add dry-roasted soy nuts to salads (instead of croutons).
2) Add cooked edamame to stir-fries and salads.
3) Use unsweetened soy milk or silken tofu in shakes and smoothies.
3) In recipes calling for cream cheese, replace half with pureed tofu.
4) Mash tofu with a fork and add to scrambled eggs and omelets.
5) Add cubed firm tofu to chili, soups and stews.
6) Marinate chunks of firm tofu in soy sauce, ginger and garlic. Sautee in a lightly oiled nonstick pan or cook on a grill.
7) Marinate cubes of tempeh in a mixture of soy sauce, sesame oil, ginger, garlic and a dash of splenda/stevia for 30 minutes. Skewer for Shish Kebobs.
Doerge DR and Sheeham DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002; 110[suppl 3]:349-353.
Hooper L et al. Flavonoids, flavonoid-rich foods, and cardiovascular risk: A meta-analysis of randomized controlled trials. Am J Clin Nutr. 2008;88:38-50.
Jenkins DJ et al. A dietary portfolio approach to cholesterol reduction: Combined effects of plant sterols, vegetable proteins, and viscous fibers in hypercholesterolemia. Metabolism. 2002; 51: 1596-1604.
Matthau J. The many sides of soy. Biodiversity, 2004; March-April:8-9, May-June:12-17, July-August:6-9.
Messina M et al. Can clinicians now assure their breast cancer patients that soyfoods are safe? Women’s Health. 2010; 6:335-338.
Pollan, M. In Defense of food. Penguin, 2009