What are the benefits of soy?

Originally Published: June 27, 2003
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What are the benefits of soy?

Dear Reader,

It seems as though soy foods are everywhere, from nuts to chips to beverages. Soy contains a group of plant estrogens (phytoestrogens) called isoflavones. Two of these isoflavones, genestin and daidzen, are the substances believed to have protective effects on our health. Many aspects of health have been associated with eating soy — including cancer prevention and reduced risk of heart disease, osteoporosis, and menopausal symptoms, among others.

In the last ten years, soy has been the subject of numerous research studies:

    Heart Disease
    In 1999, the U.S. Food and Drug Administration (FDA) allowed food manufacturers to claim that soy protein can reduce the risk of coronary heart disease. Studies have shown that 25 grams of soy protein per day, in the context of a low cholesterol/low saturated fat diet, can significantly lower blood cholesterol levels. This amount of soy protein is effective in lowering LDL (the "bad" cholesterol) when substituted for high fat meats and other foods.

    Another promising area for soy is bone health. Some studies have shown that soy foods rich in isoflavones favorably affect spine bone mineral density in peri- and post-menopausal women. It was believed that the isoflavone content contributed more than the soy protein. In these studies, 80 milligrams or more of isoflavones per day were found to be beneficial. The greatest benefits were obtained when soy protein was substituted for animal protein. In these cases, less calcium was excreted from the body, demonstrating diminished calcium loss. So far, studies have been small, and it's not certain whether soy protects one's bones over the long-term. Research is underway, and more information will be available soon.

    Since isoflavones in soy are plant estrogens, they act as weak forms of estrogen in the human body. Some experts disagree as to whether soy prevents or promotes cancer. Genestin may reduce natural estrogen cycles in women, decreasing risk of cancer, while others feel it may promote estrogen receptor positive forms of breast cancer. In laboratory, animal, population, and case-controlled studies, isoflavones have been shown to have protective effects against breast and prostate cancers. Results associated with colon cancer have been mixed. The American Cancer Society (ACS) states that randomized clinical trials are needed to better understand how all of this information applies to cancer prevention in humans.

    For cancer survivors, the ACS acknowledges the conflicting scientific evidence on whether soy products are beneficial or harmful. They recommend that "moderate" amounts of soy can be part of a healthy, plant-based diet. ACS advocates against ingesting a high level intake of soy from pills, powders, and/or other supplements.

Whole soybeans contain most of the naturally occurring isoflavones. As soy is processed, these valuable components may be lost, and not all food manufacturers list isoflavone content. The following can give you an idea of the amount of protein and isoflavones present in soy products:

Soy food Serving size Soy protein
Soybeans, cooked 1/2 cup 9 - 11 40 - 50
Soy milk
1 cup

Textured soy protein 1/4 cup (dry) 11 33
Isolated soy protein 1/2 oz 11 27
Tofu 1/2 cup 10 25
Meat Alternative (soy crumbles) 1/2 cup 11 8.5

Source: Soyfoods Association of North America

A moderate amount of soy is considered to be 25 - 30 grams of soy protein per day. Use soy as a whole food, since it's not known at what level supplements may be harmful. One serving of soy food needs to have 9 - 14 grams of soy protein per serving, and 20 - 35 mg of isoflavones. (The food manufacturers may list this information.) Processed foods, such as veggie burgers, may contain other sources of protein, such as wheat gluten or beans, so the protein content listed in the Nutrition Facts section may not all be from soy protein. Check the ingredients section to be sure of the contents.

For more information about soy and health, you can check out the following web sites: