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Release Date: January 25, 2006; Valid for credit through February 28, 2008

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Jan. 25, 2006 —The American Heart Association (AHA) nutrition committee found a lack of benefit from soy and therefore does not recommend isoflavone supplements in food or pills, according to a report in the January 17 Rapid Access issue of Circulation.

"Soy protein and isoflavones (phytoestrogens) have gained considerable attention for their potential role in improving risk factors for cardiovascular disease," write Frank M. Sacks, MD, and colleagues from the AHA Nutrition Committee. "This scientific advisory assesses the more recent work published on soy protein and its component isoflavones."

In most of 22 randomized trials reviewed, isolated soy protein with isoflavones, as compared with milk or other proteins, decreased low-density lipoprotein (LDL) cholesterol concentrations by an average amount of about only 3%. This reduction was small relative to the large amount of soy protein tested in these studies, which was about 50 g, or half the usual total daily protein intake. There were no significant effects on high-density lipoprotein (HDL) cholesterol, triglycerides, lipoprotein(a), or blood pressure.

In 19 studies of soy isoflavones, there was no effect on LDL cholesterol or on other lipid risk factors. The panel also noted no demonstrated benefit on vasomotor symptoms of menopause, and mixed results in terms of slowing postmenopausal bone loss. Similarly, the efficacy and safety of soy isoflavones for prevention or treatment of cancer of the breast, endometrium, and prostate are undetermined, and evidence from clinical trials is limited but suggests a possible adverse effect.

"For this reason, use of isoflavone supplements in food or pills is not recommended," the authors write. "Thus, earlier research indicating that soy protein has clinically important favorable effects as compared with other proteins has not been confirmed. In contrast, many soy products should be beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats, fiber, vitamins, and minerals and low content of saturated fat."

Circulation. Posted online January 17, 2006.

Learning Objectives for This Educational Activity

• Describe the 1999 US Food and Drug Administration (FDA) recommendations regarding soy consumption.
• Specify any beneficial health effects of soy protein or soy isoflavone consumption.

Clinical Context

In 1999, the US FDA approved labeling of soy protein as protective against coronary heart disease. The authors of the current advisory review the FDA's statement. The decision to support the health effects of soy was based on data that at least 25 g/day of soy protein had been demonstrated to reduce total and LDL cholesterol, and the recommendation assumed intake of 6.25 of soy protein per serving 4 times daily would satisfy this requirement. This same announcement noted that soy isoflavones were not critical for the cholesterol-lowering effects of soy protein.

There has been a considerable amount of research into the health effects of both soy protein and isoflavones since the FDA report. The authors of the current Scientific Advisory summarize this research and make recommendations regarding the use of soy.

Study Highlights

• The authors reviewed 22 randomized trials comparing consumption of isolated soy proteins with other proteins. The range of soy consumption was 25 to 135 g/day. Only 6 of these trials demonstrated a significant reduction in LDL cholesterol associated with soy protein intake, with a cumulative reduction of only approximately 3% vs control groups. There was no apparent dose effect of soy protein. The authors of the current advisory note that intake of 50 g/day of soy protein would account for at least half of all average daily protein intake for Americans, a ratio that might be difficult to achieve. Soy protein tended to reduce triglyceride levels by a mean of 5% and raise HDL cholesterol levels by 1.5%, but these values fail to achieve overall statistical significance. Soy proteins also had no significant effect on lipoprotein(a).
• Soy protein without isoflavones generally reduced LDL cholesterol to an even lower degree than whole soy protein.
• Isoflavones given alone had negligible effects on cholesterol.
• While one study suggested that patients with higher degrees of hypercholesterolemia might derive greater benefit from soy protein, this was not borne out in another trial. The overall cholesterol content of diet does not significantly alter soy protein's effects on serum cholesterol.
• Soy protein lowered blood pressure in only 1 of 6 studies examining this variable.
• A minority of trials have demonstrated that soy protein or isoflavones can improve the vasomotor symptoms of menopause for longer than 6 weeks, and studies following women for 24 weeks and 2 years have demonstrated no benefit of soy.
• Results of research as to whether soy isoflavones can reduce postmenopausal bone loss have been inconclusive. These studies have examined either bone density studies or biochemical markers of bone turnover, and inconsistent results should prompt further investigations into this issue.
• Soy isoflavones have been hypothesized to reduce the risk for breast and endometrial cancer through reducing the effects of endogenous estrogens. Some epidemiologic studies have demonstrated a protective effect of soy isoflavones against breast cancer, whereas other studies have failed to demonstrate a benefit. While some research has suggested that increased consumption of soy isoflavones during adolescence can reduce the later risk for breast cancer, isoflavones can also stimulate breast epithelial cell proliferation in premenopausal women, which is a potential precancerous condition. Overall, there is insufficient evidence to recommend soy isoflavones as a means to prevent breast cancer.
• There is less evidence for the efficacy of soy isoflavones in the prevention of endometrial cancer. However, one placebo-controlled trial demonstrated that soy isoflavones can increase the rate of endometrial hyperplasia in postmenopausal women.
• There is insufficient evidence to recommend soy isoflavones in the prevention of prostate cancer.
• Because of the lack of significant benefit to health outcomes, the authors recommend against the use of isoflavone supplements. However, they do advocate consumption of foods containing soy, as these foods generally have high contents of polyunsaturated fats, fiber, and vitamins.

Pearls for Practice

• Soy protein, but not soy isoflavones, was recommended by the US FDA in 1999 for their ability to lower total and LDL cholesterol.
• Soy protein and, particularly, soy isoflavones do not appear to confer significant health benefits, but foods containing soy are recommended because of their high contents of polyunsaturated fats, fiber, and vitamins.

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This article is intended for primary care clinicians, cardiologists, obstetrician-gynecologists, oncologists, and other specialists who care for patients who use soy to improve health outcomes.

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News Author

Laurie Barclay, MD
is a freelance reviewer and writer for Medscape.

Disclosure: Laurie Barclay, MD, has disclosed no relevant financial relationships.

CME Author

Charles P Vega, MD
Associate Professor, Residency Director, Department of Family Medicine, University of California, Irvine

Disclosure: Charles Vega, MD, FAAFP, has disclosed that he has received grants for educational activities from Pfizer.

Clinical Reviewer

Gary Vogin, MD
Senior Medical Editor, Medscape

Disclosure: Gary Vogin, MD, has disclosed no relevant financial relationships.

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