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Harvard Commentaries
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Food for Thought Food for Thought

Soy: Friend or Foe?

October 23, 2014

By Elizabeth Marchlewicz, M.P.H., B.S.
Brigham and Women's Hospital

Soy foods have flooded U.S. markets in the last decade. You've probably heard about their potential health benefits. But are you cautious about eating soy foods because of the conflicting health claims?

You're not alone. Here's a look at the current research on the positive and negative health effects of soy.

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Potential Health Benefits

Heart health

In 1999, the U.S. Food and Drug Administration began allowing food manufacturers to claim on food labels that 25 grams of daily soy protein as part of a diet low in saturated fat and cholesterol may reduce the risk of heart disease.

However, a recent American Heart Association Science Advisory paper reviewed 22 clinical trials on soy. It did not find significant benefits from soy. Isolated soy protein with isoflavones decreased LDL cholesterol by an average of 3%. This reduction was very small relative to the large amount of soy protein tested in the studies – an average of 50 grams or about half the usual total daily protein intake. Soy had no significant effects on HDL cholesterol, triglycerides, lipoprotein(a) or blood pressure.

Bottom line: Soy offers no benefits for cardiovascular disease.

Bone health

Soy isoflavones may have a bone-sparing effect, which suggests that consuming soy isoflavones may help prevent osteoporosis. However, the actual evidence to support this benefit is unclear.

Studies of six months or less showed no difference in bone mineral density (BMD) between women who ate soy and those who didn't. However, a two-year study of post-menopausal women found that drinking 17 ounces of soy milk daily prevented bone loss. For some women, the soy milk actually increased the BMD in their spines.

Bottom line: There are mixed results regarding soy's ability to slow postmenopausal bone loss. More research is needed to provide a definitive result.

Menopausal symptoms

Soy isoflavones may mimic the effect of estrogen, thereby decreasing symptoms of menopause. Women who consume at least 50 milligrams of soy isoflavones and 15 milligrams of genistein (an estrogen-like substance found in soy products) daily report fewer hot flashes than women on placebo, but more hot flashes than women taking estrogen.

Bottom line: Soy may decrease symptoms of menopause, but not as much as estrogen therapy. Results may vary between individual women.

Breast cancer

The evidence is currently inconclusive regarding the positive effect soy foods may have on breast cancer risk.

Studies of women eating specific soy foods found no relationship between soy and breast cancer prevention. In some studies, however, high soy intake during adolescence lowered breast cancer risk in adulthood.

Bottom line: There is insufficient research at this time to know whether certain phytoestrogens are protective or harmful for breast cancer, and at what dose and time period, if any, in a woman's life they are active.

Prostate cancer

Studies are divided on whether or not soy foods can prevent prostate cancer.

Eating 50 grams of soy protein daily has not been shown to lower prostate stpecific antigen (PSA) levels in healthy, middle-aged men.

However, some population studies in Asian countries — where soy products are commonly eaten and prostate cancer rates are relatively low — have shown a positive effect on soy foods and prostate cancer.

Bottom line: There is no definitive research to support eating soy to reduce the risk of prostate cancer.

Colorectal cancer

Results are mixed on whether eating soy foods may help to prevent colorectal cancer.

Among Japanese men, there was no difference in the risk of developing colorectal cancer between men eating the highest and lowest levels of soy foods in their regular diets.

Yet the Shanghai Women's Health Study of post-menopausal women showed a decreased risk of colorectal cancer for women who ate soy.

Bottom line: More research is needed before we can draw any firm conclusions.

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Potential Risks of Soy

Cancer proliferation

Studies from the 1990s raised concerns among many women by suggesting that soy increased the growth rate of estrogen receptor positive (ER+) breast cancer. However, research from the last five years has not supported this concern. Women with ER+ breast cancer should still exercise caution until more long-term data supports the safety of eating soy, but results are promising.

Anti-thyroid effects

Isoflavones make it more difficult for thyroid hormone to send the correct signals to target cells by blocking the receptors on those cells. Therefore, excessive soy intake can disrupt thyroid function. This can cause goiters and an abnormal enlargement of the thyroid gland. Iodine supplementation is often necessary to correct the problem. This effect has been observed in:

  • Iodine-deficient animals
  • Infants fed soy-flour formula that is not iodine-fortified

Reproductive problems

Animals that eat excessive amounts of soy isoflavones have reduced fertility or become infertile. For their body weight, infants given soy-based formulas have blood concentrations of isoflavones six to eleven times greater than amounts known to have hormonal effects in adults. This creates an environment with unnaturally high estrogen-like properties that may damage the reproductive system throughout childhood and adulthood.

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Traditional Soy Foods

Soybeans are low in saturated fat and are full of healthy nutrients, such as protein, fiber, minerals (calcium, folic acid, iron) and isoflavones (genestein, daidzein and glycetein).

Unfortunately, many of the soy products sold in the United States contain large amounts of sugar. They also use isolated soy components, which have less nutritional value than the whole soybean, as consumed in traditional Asian diets. The table below compares the protein and isoflavone content of traditional soy foods. Isoflavones are a class of phytoestrogens, or plant substances that have estrogen-like effects in the body.


Serving size


Protein grams/ 100 grams

Isoflavones (millgrams per serving)

Mature soybeans, uncooked
1/2 cup
Roasted soybeans
1/2 cup
Soy flour
1/4 cup


Textured soy protein, dry
1/4 cup


Green soybeans, uncooked
1/2 cup


Soy milk
1 cup


Tempeh, uncooked
4 ounces


Tofu, uncooked
4 ounces


Soy isolate, dry
1 ounce


Soy concentrate, dry
1 ounce


Source: Presentation at the American Dietetic Association 80th Annual Meeting and Exhibition, held October 27-30, 1997, in Boston, Massachusetts, by James W. Anderson, M.D. Professor of Medicine and Clinical Nutrition University of Kentucky

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The Bottom Line

The current evidence for most of soy's potential health benefits is conflicting. People metabolize daidzein differently, so eating soy foods may affect people differently. This may be one reason for the mixed results found in studies on soy consumption and related health effects.

Avoid using soy protein and soy isoflavone supplements. Adverse effects of soy are seen when consuming amounts in excess of what you get by eating whole soy foods. Including soy foods in your diet will not harm you. If you replace foods high in saturated fat, trans fat, cholesterol and sodium with soy products, you will gain vitamins and maintain adequate protein levels while getting, at the very least, heart benefits.

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Elizabeth Marchlewicz, M.P.H., completed her B.S. in Physiological Science from the University of California, Los Angeles and received a Masters of Public Health in Human Nutrition from the University of Michigan. She is currently a dietetic intern at Brigham and Women’s Hospital.

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