Humans have been eating soybeans for centuries. Unlike most plant foods, soybeans are high in protein. Today, researchers are interested in both the nutritional value and the potential health benefits of soy.
Several studies of people who eat a lot of soy foods, such as people in Asia, suggest that soy may help reduce menopausal symptoms, and lower the risk of heart disease and osteoporosis. Researchers are also examining whether eating soy can help prevent hormone-related cancers, such as breast, prostate, and uterine cancer.
Isoflavones are phytoestrogens -- chemicals that act like the hormone estrogen. Soy phytoestrogens are very complex. They can either strengthen or reduce the effects of estrogen in the body.
Soy contains other nutrients as well. Some researchers believe the health benefits of soy foods may come from the combination of these nutrients and not just the isoflavones.
Studies suggest that soy protein may reduce the risk of heart disease by lowering levels of LDL (bad) cholesterol. For example, in a clinical study of nearly 5,000 men and women living in Japan, those who ate the most soy had the lowest cholesterol levels. Not all studies agree, but those that show benefits seem to indicate that 25 g of soy protein per day may lower LDL by 5%. A review of studies found that people with high cholesterol may reduce their total cholesterol by 9% if they eat 31 to 47 grams of soy protein instead of meat each day.
As a result, in 1999, the U.S. Food and Drug Administration (FDA) allowed soy products to carry a heart-healthy label indicating that soy may reduce cholesterol when consumed as part of a diet that is low in saturated fat and cholesterol. 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. That is about 25% of the amount needed each day, 25 grams of soy, to get the heart-healthy benefit.
It is relatively easy to meet that 25-gram recommendation: 4 ounces of firm tofu contains 13 grams of soy protein; one soy "burger" includes 10 to 12 grams; and an 8-ounce glass of plain soy milk contains 10 grams.
The American Heart Association (AHA) also 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. Studies that offer evidence for this have found:
Studies looking at large numbers of people have found that eating a diet high in soy protein may lower the risk of developing hormone-related cancers, such as breast, prostate, and uterine cancer.
But the findings are complex. For example, most studies that have found a lower risk of breast cancer have looked at Asian women, who typically eat a lot of soy. Studies that have looked at Western women have not all found a lower risk of breast cancer, although an analysis of 18 previously published studies found that in healthy women, soy was associated with a small (14%) reduction in breast cancer risk. Some studies suggest that a woman may get the best protection if she eats a diet high in soy when she is young, rather than when she goes through menopause.
The findings are also complicated when it comes to prostate cancer. Some studies suggest that rates of prostate cancer are similar in Asian and Western countries, but more Western men than Asian men (who eat more soy) die from prostate cancer. But other studies show that men who eat a diet high in soy may have a lower risk of prostate cancer, although researchers are not sure if that is because of what they eat. Other researchers note that as Japanese men age, their prostate size does not grow as much as it does in Western men. A diet high in soy may account for the size differences.
Other studies suggest that consuming soy foods is associated with a lower risk of lung cancer, and an improved chance of survival among those who are diagnosed with lung cancer.
Researchers have started to investigate whether soy foods can help prevent osteoporosis by increasing bone mineral density (BMD). Most of the research has focused on the isoflavones in soy protein. Although there are not many studies, most suggest that women approaching menopause who eat isoflavone-rich soy protein are more likely to boost BMD than women whose diets are low in soy isoflavones. Some studies suggest that 80 to 90 mg of isoflavones in 40 g of soy protein worked the best.
However, other studies have found that soy isoflavones did not increase bone mineral density in early postmenopausal women.
Soy isoflavones may help reduce hot flashes and night sweats that many women have during menopause. In clinical studies, postmenopausal women who eat high amounts of dietary soy protein (20 to 60 g per day) generally have fewer and less intense hot flashes and night sweats than those who eat less soy.
Results of studies about whether soy supplements reduce menopausal symptoms have been mixed. In several studies, women who took the placebo also had fewer menopausal symptoms.
Some researchers also think that using soy extracts with different isoflavones in different studies can cause conflicting results. The studies with the most positive results have used soy products with at least 15 mg of genistein (an isoflavone) per day.
There are many soy foods. The whole soybean, called edamame, and tofu can be found in many Japanese and Chinese dishes and is growing popular in American cooking. Soft tofus are popular for shakes, salad dressings, and creamy dishes; firm tofus are easy to dice or crumble into stir fries or pasta.
Soy milk, soy cheese, soy yogurt, and soy nuts and nut butters can also be found at many grocery stores.
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 cultured with an edible mold. Soy oil is a popular choice for cooking and frying oils and as a base for mayonnaise, salad dressing, and margarine.
In general, whole soybean-based foods, such as tofu and soy milk, have the highest protein and isoflavone content. The soy germ 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. Researchers are studying one supplement, ipriflavone, to see if it may help strengthen bones.
Soy foods and soy-based infant formula are considered safe. However, soy supplements are not recommended for children because there are no studies to say whether they are safe in children, and there is some concern about the long-term hormonal effects (soy contains plant estrogens called phytoestrogens).
Heart Disease: Most studies use between 20 to 80 grams of soy protein per day. Both the U.S. Food and Drug Administration and the American Heart Association (AHA) recommend 25 grams of soy protein as part of a diet that is low in cholesterol and saturated fat.
Cancer: Phytoestrogens in dietary soy may help prevent hormone-related cancers, such as breast, endometrial, and prostate cancer. But there are no details about dosage, frequency of use, or safety. Most studies have examined soy foods, not soy supplements. Some researchers suggest that soy foods, not soy supplements, should be part of a healthy diet for preventing cancer. If you are being treated for cancer, talk to your doctor about the possible risks and benefits of eating soy foods. There is some concern that soy may be harmful in hormone-related cancers. Talk to your doctor, and do not take soy supplements without your doctor's supervision.
Menopause: According to the North American Menopause Society, eating whole foods containing the following amounts of isoflavones may have health benefits for postmenopausal women:
Some experts believe that it is still too early to recommend soy isoflavones for the treatment of menopausal symptoms. There isn't specific information about the right dose, how long to take it, and what the long-term effects of phytoestrogens may be.
People who are allergic to soy should avoid soy foods and supplements containing soy components, such as soy isoflavones.
Findings from laboratory studies have raised concerns that soy consumption may stimulate the growth of breast cancer cells. So while researchers believe soy reduces the risk of breast cancer, it might not be safe for women who have already had breast cancer. More research is needed. Speak to your doctor about your risk profile and whether soy would be beneficial or harmful to you.
Soy supplements may make the breast cancer medication tamoxifen (Nolvadex) less effective. Women taking tamoxifen should not take soy supplements.
Soy has more phosphorus and potassium than similar amounts of meat, poultry, or fish. People with kidney disease should talk to their nephrologist (kidney specialist) or renal dietitian before eating soy foods.
Isoflavones may lower the amount of iodine in the body, which can lead to underfunctioning of the thyroid, called hypothyroidism. A few cases of hypothyroidism have been reported in infants fed soy formula. It is rare in the United States where most people use iodized salt.
Women with uterine (endometrial) cancer should not take soy supplements or consume a large amount of soy in their diet without first asking their doctor.
For people 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, such as constipation and diarrhea.
Eating soy foods during pregnancy and while breastfeeding is considered safe. Women who are pregnant or breastfeeding should not take soy supplements without asking their doctor.
Since phytoestrogens may affect certain hormones in the body, there has been some concern that infant soy formulas could cause reproductive or health problems later on. For example, preliminary studies suggest that girls fed soy products in early infancy have an increased risk of menarche during early adolescence compared to girls who were not given soy products during infancy. Soy intake during adolescence, however, does not appear to be related to onset of menarche, according to recent studies. Heavy soy consumption may also increase the chances of developing soy allergies.
The phytoestrogens in soy may interfere with the drug tamoxifen (Nolvadex), used in breast cancer treatment, and the drug raloxifene (Evista), as well as other drugs used to prevent and treat osteoporosis. Soy may interact with estrogen medications and other hormonal medications. Speak with your physician. The minerals zinc and iron may also interfere with the absorption of soy and could lower soy levels in the body.
Soy may increase the risk of bleeding, especially if you also take blood thinners such as warfarin (Coumadin), clopidogrel (Plavix), or aspirin.
People who take antidepressants called MAOIs, such as phenelzine (Nardil) and tranylcypromine (Parnate), should not eat soy foods or take soy supplements.
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