Jan. 23, 2006 -- "Eat soy to protect your heart" has been the message from government and private health agencies for the past six years, but the American Heart Association now says it looks like soy has no independent impact on cardiovascular risk.
Recent clinical studies have failed to show a protective benefit for soy protein in the diet, and, especially, for taking soy-based isoflavones in supplement form, Harvard School of Public Health nutrition researcher Frank M. Sacks, MD, tells WebMD.
Sacks and other members of the AHA nutrition committee reviewed 22 such studies and concluded that eating soy-based foods has only minimal impact on cholesterol and other heart-disease risk factors.
The studies indicated that people who ate about 50 grams of soy protein a day, which represented about half of the usual total daily protein intake, reduced LDL, or "bad," cholesterol by only about 3%. Eating large amounts of soy had no effect on other risk factors such as triglycerides or HDL "good" cholesterol.
Combined data from 19 studies assessing isoflavones -- the plant estrogen derived from soy --suggested that isoflavone supplements had no effect on cholesterol at all.
Is Soy a Bust?
The new research prompted the AHA Nutrition Committee to revisit its scientific advisory on soy protein published in 2000. In the earlier statement the group concluded that "it is prudent to recommend including soy protein foods in a diet low in saturated fat and cholesterol to promote heart health."
That advice is still sound, says Tufts University nutrition researcher Alice Lichtenstein, DSc, who helped write the new statement. But not for the reasons suggested in the earlier report.
The clinical evidence available in 2000 led the AHA nutrition panel to conclude that as little as 25 grams a day of soy protein could reduce LDL cholesterol by as much as 8%. That has not been borne out in the subsequent studies, she says.
"Soy is a great food. It is low in saturated fat and it is a good-quality protein," she tells WebMD. "It is helpful because a lot of the products made with soy can be used to displace foods that are high in saturated fats, like meat and cheese, from the diet."
But Lichtenstein says it is now clear that "the independent benefits of soy, in terms of protecting against heart disease, were overestimated."
The FDA Weighs In
Since 1999, the FDA has allowed manufacturers of soy-based products to claim on their packaging that a daily diet containing 25 grams or more of soy protein can reduce the risk of heart disease.
In an interview Monday, an FDA spokeswoman tells WebMD that the agency could revisit the action in light of the new research.
"[The] FDA may determine that a re-evaluation of the health claims may be needed when new scientific evidence does not support the current claim," Kimberly Rawlings says. "We will take a look at the science that has been made available before we make a decision on how we are going to proceed."
Menopause and More
Sacks says he agrees that there are clear, if indirect, health benefits for replacing hamburgers with tofu burgers and replacing other artery-clogging animal proteins with plant-based soy protein.
But the AHA nutrition panel found no convincing evidence that taking isoflavone supplements conveyed any health benefits at all.
Millions of women take isoflavone-containing supplements with the belief that they reduce hot flashes and other symptoms of menopause, strengthen bone, and even help protect against breast cancer. But none of these claims has been backed up in the recent studies, Sacks says.
"We know that [hormone replacement therapy] can dramatically reduce hot flashes," he says. "But when you look at the isoflavone studies there doesn't look like there is much happening compared to placebo."
Many men also take isoflavones with the belief that they protect against or help slow the progression of prostate cancer.
The AHA nutrition panel concluded that the effectiveness and safety of isoflavones for preventing or treating breast, endometrial (uterine), and prostate cancer have not been established.
The group added that the few studies that have been done raise questions about the safety of isoflavone supplements.
"For this reason, use of isoflavone supplements in food or pills is not recommended," the statement read.