Sorting Out Facts and Hopes About Soy’s Health Impact
The lowly soybean is winning more attention these days--from researchers, doctors, consumers and the Food and Drug Administration.
A legume that has long been a staple for Asians, the soybean has grown steadily in popularity as a health food in the United States since the 1950s.
Many claims are made for the health benefits of soy: from lowering blood cholesterol to lessening menopausal hot flashes to preventing prostate and breast cancer. Not all the claims are based on equally convincing evidence.
Nutritionists agree that soy is a very good food and a moderate intake of soy is healthful. Soybeans have more protein and less fat than some other beans. They are a cholesterol-free source of beneficial omega-3 fatty acids and calcium, and they supply most of the essential amino acids needed by humans to build proteins. They are a good source of fiber.
The strongest evidence so far of health benefits from soy involves heart disease, the leading killer of Americans. Soy’s ability to help lower cholesterol was first described in the 1960s but has gained widespread attention recently.
The FDA in November proposed allowing food marketers to assert on product labels that soy-containing foods can help reduce the risk of coronary heart disease. The FDA action came in response to a petition filed by Protein Technologies International, a leading marketer of soy products.
In its proposal, the FDA cited studies showing that consumption of 25 grams of soy protein per day can have a cholesterol-lowering effect. For a food to qualify for the health claim, each serving would have to contain at least one-quarter of that amount, or 6.25 grams of soy protein.
Some 50 published studies in humans have suggested that soy can help lower blood cholesterol levels, although scientists are still not sure exactly how it does so. Much of the attention has focused on “active ingredients” called isoflavones in soy.
“The effect on cholesterol is very strong,” said Victoria Persky, an epidemiologist at the University of Illinois School of Public Health in Chicago.
But even the cholesterol-lowering ability of soy comes with a caution, said Ronald Krauss, head of molecular medicine at the Lawrence Berkeley National Laboratory in Berkeley and former chairman of the American Heart Assn.’s committee on nutrition. Although plenty of evidence shows that soy tends to lower cholesterol, Krauss said, that is no guarantee that it will reduce any given person’s individual risk of heart disease or heart attack. That risk depends on many factors.
High levels of total cholesterol in the blood and particularly its most harmful form, known as low-density lipoprotein or LDL, are a proven risk factor for coronary heart disease. Cholesterol is implicated in atherosclerosis, the buildup of artery-clogging plaque, which accounts for three out of four American deaths from heart disease. Controlling cholesterol levels--whether by diet, drugs, exercise or a combination of medical and behavioral approaches--has become a staple of primary care.
As to other claims for soy, such as its possible effect on cancer, bone density or menopausal symptoms, research is more limited. As Persky said, “There’s evidence in both humans and animals, but we need more data before we can make public health recommendations.”
Tantalizing evidence for soy’s ability to prevent cancer comes mainly from laboratory animal studies and from epidemiology--the study of suggestive patterns in large populations, such as the lower rate of breast cancer in Asian women and the lower death rate from prostate cancer in Japan. But these studies do not prove that soy protects humans against cancer.
