Agency News and Notes

New evidence report on soy finds limited evidence for health outcomes

Daily consumption of soy protein found in tofu and other soybean products may result in a small reduction in low-density lipoprotein (LDL, also known as "bad" cholesterol) and triglyceride levels, according to a new evidence review, the Effects of Soy on Health Outcomes, supported by the Agency for Healthcare Research and Quality. In addition, isoflavones found in soy may reduce the frequency of hot flashes in postmenopausal women. However, the available studies on the health impacts of soy were limited in number, of poor quality, or their duration was too short to lead to definite conclusions.

Overall, across the 68 studies that examined the impact of soy on cholesterol levels, consumption of soy products resulted in about a 3-percent reduction in LDL and about a 6-percent decrease in triglyceride levels in the populations studied. These studies, examined a large variety of soy products, doses of soy protein, and doses of soy isoflavones. The average dose of soy protein in the studies was equivalent to about one pound of tofu or three soy shakes daily.

There was some indication that soy consumption may be more effective at lowering LDL among people with higher LDL levels. Also, larger amounts of soy protein, but not soy isoflavones, are more effective in people with abnormally elevated LDL levels. Similarly, soy consumption may be more effective at lowering triglycerides among people with higher triglyceride levels. However, there was no evidence of how much soy protein or isoflavones would be needed to affect triglycerides.

Reviews on the relationship between soy consumption and high-density lipoprotein (HDL, also known as "good" cholesterol) levels and between soy consumption and blood pressure did not find significant effects. Among 21 studies evaluating the consumption of soy isoflavones for menopause-related symptoms, there was a net reduction in hot flash frequency ranging from 7 percent to 40 percent, however, these trials were mostly rated as poor quality. Among studies with statistically significant improvements in symptoms, the dose of soy isoflavones ranged from 17.5 to 100 mg/day.

The evidence review also found insufficient data among the 200 human studies examined as part of this analysis to suggest that soy had an effect on bone health, cancer, kidney disease, endocrine function, reproductive health, neurocognitive function, or glucose metabolism. A wide variety of soy products were studied, including soybeans, soy flour, soy milk, tofu, miso, tempeh, natto, and okara; isolated and textured soy protein that is added to foods; and soy-derived isoflavone supplements. Aside from minor gastrointestinal problems reported in some short-term studies, consumption of soy products by study participants was not associated with adverse events. However, long-term safety data are lacking.

The evidence review was prepared by a team of researchers led by Ethan Balk, M.D., and Joseph Lau, M.D., of AHRQ's Tufts-New England Medical Center Evidence-based Practice Center in Boston. The researchers who conducted the evidence review, which was also supported by the National Institute of Health's National Center for Complementary and Alternative Medicine and Office of Dietary Supplements, considered the type of soy product used, amount consumed, frequency of consumption, and safety issues in their review of health effects.

Editor's Note: There are 13 AHRQ-supported Evidence-based Practice Centers (EPCs). They systematically review the relevant scientific literature on topics assigned to them by AHRQ and conduct additional analyses when appropriate prior to developing their reports and assessments. The goal is to inform health plans, providers, purchasers, and the health care system as a whole by providing essential information to improve health care quality.

All of AHRQ's EPC reports, as well as several technical reviews, that have been published to date are available online and through the AHRQ Clearinghouse. Visit the AHRQ Web site at www.ahrq.gov/clinic/epcix.htm.

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