The soybean has been a part of the human diet for almost 5,000 years. Unlike most plant foods, the soybean is high in protein and is considered equivalent to animal foods in terms of the quality of the protein it contains. Today, researchers are interested in both the nutritional value and the potential health benefits of soy. Several studies of populations in which soy foods are a major component of the diet (particularly Asian populations) suggest that soy may be beneficial for reducing menopausal symptoms, and risk of heart disease and osteoporosis. A possible relationship between dietary soy and the prevention of hormone-related cancers (such as breast, prostate, and endometrial cancer) is also being evaluated.

Studies investigating the value of soy have focused on the protein and isoflavone content of soy foods. Isoflavones are phytoestrogens or plant compounds that weakly mimic the action of estrogen (a reproductive hormone). Soy phytoestrogens are very complex; they bind to estrogen receptors and either enhance or diminish the effects of the hormone. Soy contains other nutrients in addition to protein and isoflavones, and some researchers believe that the healthful benefits of soy foods may be due to the combined action of these nutrients. This possibility is being investigated.

Heart Disease

Various studies suggest that soy protein may reduce the risk of heart disease by lowering blood levels of cholesterol. For example, in a study of nearly 5,000 men and women living in Japan, those with the highest soy intake had the lowest cholesterol levels. A recent scientific review also found that individuals with high cholesterol may reduce their total cholesterol by 9% if they replace their daily intake of animal protein with 31 to 47 grams of soy protein. This and other evidence led the United States Food and Drug Administration (FDA) in 1999 to allow for the use of a health claim regarding an association between dietary soy and reduced risk of heart disease. Health claims represent a strong statement about an association between diet and a particular disease condition. The soy protein health claim states that including 25 grams of soy protein per day in a diet low in saturated fat and cholesterol may reduce the risk of heart disease by lowering blood cholesterol levels. In order for a particular food to carry this claim, one serving of the food must contain at least 6.25 grams of soy protein per serving. In fact, it is relatively easy to add enough soy to the daily diet to meet the 25 gram recommendation: 4 ounces of firm tofu contains 13 grams of soy protein; one soy "burger" includes 10 to 12 grams of protein; and an 8-ounce glass of plain soy milk contains 10 grams of protein.

Cancer

The evidence suggesting that soy may be helpful in preventing hormone-related cancers, such as breast, prostate, and endometrial cancer, comes from studies of large populations of individuals. These types of studies suggest that populations who consume high quantities of soy have a lower incidence of hormone-related cancers.

For example, Asian-American women who eat tofu at least once a week have a lower risk of developing breast cancer than those who do not eat tofu. Yet, when Asian women migrate to the United States, their intake of tofu tends to decrease and their risk of developing breast cancer tends to increase. In comparing the soy intake of Hawaiian women with their risk of developing endometrial cancer, the women who consumed the least amount of soy had the highest risk of endometrial cancer. Incidence of prostate cancer appears similar in Asian and Western countries, but more Western men than Asian men die from prostate cancer. Other findings that suggest soy foods may have health benefits against prostate disease include the observation that prostate size in Japanese men does not increase so dramatically with age as it does in Western men.

Heart Disease

The American Heart Association (AHA) recommends adding at least 25 grams of soy protein per day to a diet low in saturated fat and cholesterol to reduce the risk of heart disease. These recommendations are also believed to improve blood cholesterol levels in people with elevated cholesterol. The main study findings that prompted these recommendations include the following:

• Asian populations who routinely eat soy foods (up to 55 grams of soy protein per day) have lower incidences of heart disease than populations who eat a typical Western diet (fewer than 5 grams of soy protein per day)
• Substituting 31 to 47 grams of animal protein with soy protein in a diet low in saturated fat and cholesterol can lower blood cholesterol levels
• Including at least 20 grams of soy protein per day in a diet low in saturated fat and cholesterol reduces blood fat levels in both men and women

Osteoporosis

Researchers have begun to investigate whether soy foods can be helpful in preventing osteoporosis by increasing bone mineral density. Most of the research has focused on the isoflavone content of soy protein. Although the studies to date are limited, they suggest that women approaching menopause who eat isoflavone-rich soy protein are significantly more likely to boost their bone mineral density than women whose diets are low in soy isoflavones. Postmenopausal Japanese women whose diets include high levels of soy isoflavones have greater bone mineral density than those whose diets include low levels of soy isoflavones.

Menopause Symptoms

Soy isoflavones may help alleviate hot flashes and night sweats that many women experience during menopause. In the studies that have been conducted to date, postmenopausal women who consume high amounts of dietary soy protein generally experience fewer and less intense hot flashes and night sweats than those with low intakes. Studies evaluating the use of soy supplements for the reduction of menopausal symptoms, however, have been more controversial.

Soy foods are available in a variety of forms. The whole soybean (called edamame) is popular in Japanese and Chinese cuisines but is not commonly consumed in the typical Western diet. Tofu, which is a curd made from cooked, pureed soybeans, is increasingly popular in Western cuisine. Tofu can be found in various textures, ranging from soft to extra firm. The softer tofus are popular for shakes, salad dressings, and creamy dishes; the firm tofus are easy to dice or crumble into mixed dishes such as stir fries or pasta. Soy milk, soy cheese, soy yogurt, and soy nuts and nut butters are also increasingly popular with Western consumers. Other soy foods available in Western markets include miso, which is a mixture of fermented soybean paste and a grain such as rice or barley; natto, which are whole soybeans that have been cooked and fermented; and tempeh, made from a mixture of whole, cooked soybeans and grains that have been cultured with an edible mold. The oil from soybeans is widely used throughout the food industry. Soy oil is a popular choice for cooking and frying oils and as a base for mayonnaise, salad dressing, and margarine.

The availability of nutrients varies with the type of soy food. In general, whole soybean-based foods such as tofu and soy milk have the highest protein and isoflavone content. The soy germ, found in whole soybeans, is particularly high in isoflavones.

In addition to soy foods, dietary supplements containing one or more components from soy are also on the market. One supplement, ipriflavone, is a synthetic derivative of a soy isoflavone and is being investigated for its benefits in improving bone health. At this time, however, most experts agree that the information on the health benefits of soy supplements is limited and prefer to recommend whole soy foods instead.

Soy foods and soy-based infant formula are widely used in children, but there are no studies that have determined whether isolated soy protein or isoflavone supplements are useful or safe in this population. Therefore, isolated soy products are not recommended for use with children at this time.

Heart Disease

Most studies that have examined the effects of soy on heart disease have used between 20 and 80 grams of soy protein per day. Both the FDA and the AHA recommend 25 grams of soy protein when incorporated into a diet that is low in cholesterol and saturated fat.

No formal recommendations for isoflavone intake have been put forward but, based on the amount present in the typical Asian diet, the intake would be between 20 and 50 milligrams per day. According to the AHA report, 2 grams of soy protein contains approximately 1 milligram of soy isoflavones. As mentioned previously, 4 ounces of firm tofu contains 13 grams of soy protein, one soy "burger" includes 10 to 12 grams of protein, and an 8-ounce glass of plain soy milk contains 10 grams of protein.

Cancer

Researchers speculate that the phytoestrogens in dietary soy may be beneficial for the prevention of hormone-related cancers (such as breast, endometrial, and prostate cancer), but specifics about dosage, frequency of use, or safety are not known. The available evidence has been derived from soy foods rather than supplements or isolated components of soy. For this reason, some researchers suggest that soy foods should be recommended as part of a healthy diet for cancer prevention rather than isoflavone supplements.

Menopause

In 2000, the North American Menopause Society (NAMS), a nonprofit organization dedicated to promoting the understanding of menopause, issued a consensus statement that addressed the potential benefits of isoflavones on conditions associated with menopause. The society reported that the consumption of whole foods containing the following amounts of isoflavones may have health benefits for postmenopausal women:

• 50 mg/day to reduce cholesterol
• 40 to 80 mg/day to strengthen blood vessels and possibly improve blood pressure
• 50 mg/day for bone health
• 40 to 80 mg/day to reduce hot flashes

Other experts believe that it is still too early to recommend soy isoflavones for the treatment of menopausal symptoms. They cite the lack of specific information about effective dosage, length of administration, and long-term effects of using phytoestrogens to manage menopausal symptoms. The substitution of phytoestrogens for estrogen replacement therapy is also considered to be premature at this point in our understanding of phytoestrogens and menopause.

Soy Allergy

Those individuals who are allergic to soy should avoid soy foods and any supplements containing soy components, such as soy isoflavones.

Breast Cancer

Findings from laboratory studies have raised concerns that soy consumption may stimulate the growth of breast cancer cells. Although the issue is not completely resolved, the bulk of the evidence presently available suggests that women who can safely take hormone replacement therapy may also use soy phytoestrogens without increasing their risk of breast cancer. Phytoestrogens may, however, reduce the effectiveness of the breast cancer medication tamoxifen and should not be used by women with breast cancer who are also taking tamoxifen.

Kidney Disease

Although soy protein is considered nutritionally equivalent to animal-based protein, the phosphorus and potassium content of soy is greater than that found in comparable amounts of meat, poultry, or fish. Individuals with kidney disease should consult with their nephrologist or renal dietitian before including soy foods in their diet.

Hypothyroidism

Isoflavones may reduce the iodine content in the body, which can lead to underfunctioning of the thyroid (hypothyroidism). A few cases of hypothyroidism have been reported in infants fed soy formula. This effect, however, is not widespread in the United States where most people use iodized salt.

For individuals who are not allergic to soy, no serious short-term or long-term side effects have been reported from eating soy foods. Common mild side effects include stomach upset and digestive problems including constipation and diarrhea.

The consumption of soy foods during pregnancy and while breastfeeding is considered to be safe, but further studies are needed to demonstrate beyond any reasonable doubt that soy is safe during these life stages.

Because phytoestrogens influence the activity of certain hormones in the body, there has been some concern that infants fed soy formula may experience negative effects on general health or reproduction later in life. A recent study of over 800 young adults who (as infants) were fed either soy formula or cow's milk formula, however, found that there were no differences between the two groups in over 30 different parameters measured, including fertility and general health.

The phytoestrogen components of soy may interfere with the drug tamoxifen, used in breast cancer treatment for men and women, and the drug raloxifene, used to prevent and treat osteoporosis. The minerals zinc and iron may also interfere with the absorption of soy and could theoretically lower soy levels in the body. Further research is needed.

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