Soy : Health Benefits

by MedlinePlus

Soy is a subtropical plant, native to southeastern Asia. This member of the pea family ( Fabaceae ) grows from one to five feet tall and forms clusters of three to five pods, each containing two to four beans per pod. Soy has been a dietary staple in Asian countries for at least 5,000 years, and during the Chou dynasty in China (1134-246 B.C.), fermentation techniques were discovered that allowed soy to be prepared in more easily digestible forms such as tempeh, miso, and tamari soy sauce. Tofu was invented in second century China.

Soy was introduced to Europe in the 1700s, and to the United States in the 1800s. Large-scale soybean cultivation began in the United States during World War II. Currently, Midwestern U.S. growers produce approximately half of the world's supply of soybeans.

Soy and components of soy called "isoflavones" have been studied scientifically for numerous health conditions. Isoflavones (such as genistein) are believed to have estrogen-like effects in the body, and as a result are sometimes called "phytoestrogens." In laboratory studies, it is not clear if isoflavones stimulate or block the effects of estrogen, or both (acting as "mixed receptor agonists/antagonists").

Evidence

These uses have been tested in humans or animals. Safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider.

Uses based on scientific evidence

High cholesterol

Numerous human studies report that adding soy protein to the diet can moderately decrease blood levels of total cholesterol and low-density lipoprotein ("bad" cholesterol). Small reductions in triglycerides may also occur, while high-density lipoprotein ("good" cholesterol) does not seem to be significantly altered. The greatest effects seem to occur in people with elevated cholesterol levels, with benefits lasting as long as the diet is continued. Total replacement of dietary animal proteins with soy protein yields the greatest benefits. People on low-cholesterol diets experience further reductions in cholesterol levels by adding soy to the diet.Some scientists have proposed that specific components of soybean, such as the isoflavones genistein and daidzein, may be responsible for the cholesterol-lowering properties of soy. However, this has not been clearly demonstrated in research, and remains controversial. It is not known if products containing isolated soy isoflavones have the same effects as regular dietary intake of soy protein.Dietary soy protein has not been proven to affect long-term cardiovascular outcomes such as heart attack or stroke.

Dietary source of protein

Soy products such as tofu are high in protein and are an acceptable source of dietary protein.

Menopausal hot flashes

Soy products containing isoflavones have been studied for the reduction of menopausal symptoms such as hot flashes. The scientific evidence is mixed in this area, with several human trials suggesting reduced number of hot flashes and other menopausal symptoms, but more recent research reporting no benefits. Overall, the scientific evidence does suggest benefits, although better quality studies are needed in this area in order to form a firm conclusion.Many researchers have attributed these effects to the presence in soy of "phytoestrogens" (plant-based compounds with weak estrogen-like properties), such as isoflavones. An area of concern has been whether phytoestrogens carry the same risks as prescription drug hormone replacement therapy (HRT), which contains estrogens, such as increased risk of hormone-sensitive cancers (breast, ovarian, uterine) or blood clots. This is an important area of concern for patients, as some women may consider soy as an alternative to HRT in order to avoid these risks. Early studies report that soy isoflavones do not cause the same thickening of the uterus lining (endometrium) as estrogen, and therefore may not carry the same risks as HRT. In addition, some scientists theorize that isoflavones may actually reduce the risk of cancer by blocking estrogen effects in the body, based on laboratory studies showing isoflavones to partially block (non-competitively inhibit) estrogen receptors. Additional research is needed in this area before a clear risk-assessment can be conducted.

Diarrhea (acute) in infants and young children

Numerous studies report that infants and young children (ages 2 to 36 months) with diarrhea who are fed soy formula experience fewer bowel movements per day and fewer days of diarrhea. This research suggests soy to have benefits over other types of formula, including cow milk-based solutions. The addition of soy fiber to soy formula may increase the effectiveness. Although many of the trials in this area are not high quality, and some report conflicting results (lack of benefits), overall, the evidence supports this use of soy. Better quality research is needed before a strong recommendation can be made.Parents are advised to speak with a qualified healthcare provider if infants experience prolonged diarrhea, become dehydrated, develop signs of infection such as fever, or experience blood in the stool. A healthcare provider should be consulted for current breastfeeding recommendations, and to suggest long-term formulas with adequate nutritional value.

Breast cancer prevention

Several large population studies have asked women about their eating habits, and reported higher soy intake (such as dietary tofu) to be associated with a decreased risk of developing breast cancer. However, this type of research (retrospective, case-control, epidemiologic) can only be considered preliminary, because people who choose to eat soy may also partake in other lifestyle decisions that may lower the risk of cancer. These other habits, rather than soy, could theoretically be the cause of the benefits seen in these studies (for example, lower fat intake, more frequent exercise, lack of smoking). Controlled human trials are necessary before a firm conclusion can be drawn.Theoretical concerns have been raised that soy may actually increase the risk of breast cancer because of the presence in soy of "phytoestrogens" (plant-based compounds with weak estrogen-like properties), such as isoflavones. This remains an area of controversy. Recently, some scientists have theorized that isoflavones may reduce the risk of cancer by blocking estrogen effects in the body, based on laboratory studies showing isoflavones to partially block (non-competitively inhibit) estrogen receptors. In fact, early research suggests that soy isoflavones do not have the same effects on the body as estrogens, such as increasing the thickening of the uterus lining (endometrium). Genistein has been found in laboratory and animal studies to have other anti-cancer effects, such as blocking new blood vessel growth (anti-angiogenesis), acting as a tyrosine kinase inhibitor (a mechanism of many new cancer treatments), or causing cancer cell death (apoptosis).Until better research is available, it remains unclear if dietary soy or soy isoflavone supplements increase or decrease the risk of developing breast cancer.

Cancer treatment

Genistein, an isoflavone found in soy, has been found in laboratory and animal studies to possess anti-cancer effects, such as blocking new blood vessel growth (anti-angiogenesis), acting as a tyrosine kinase inhibitor (a mechanism of many new cancer treatments), or causing cancer cell death (apoptosis). In contrast, genistein has also been reported to increase the growth of pancreas tumor cells in laboratory research. None of these effects has been adequately assessed in humans.In the past, theoretical concerns have been raised that soy may increase the risk of hormone-sensitive cancers (for example, breast, ovarian, endometrial/uterine) because of the presence in soy of "phytoestrogens" (plant-based compounds with weak estrogen-like properties), such as isoflavones like genistein. This remains an area of controversy. Recently, some scientists have suggested that isoflavones may actually reduce the risk of hormone-sensitive cancers by blocking estrogen effects in the body, based on laboratory studies showing isoflavones to partially block (non-competitively inhibit) estrogen receptors. Preliminary human research suggests that soy isoflavones do not have the same effects on the body as estrogens, such as increasing the thickening of the uterus lining (endometrium).Until reliable human research is available, it remains unclear if dietary soy or soy isoflavone supplements are beneficial, harmful, or neutral in people with various types of cancer.

Cardiovascular disease

Dietary soy protein has not been shown to affect long-term cardiovascular outcomes such as heart attack or stroke. Research does suggest cholesterol-lowering effects of dietary soy, which in theory may reduce cardiovascular risk. Soy has also been studied for blood pressure-lowering and blood sugar-reducing properties in people with type 2 diabetes, although the evidence is not definitive in these areas. Although the addition of soy to a regimen of exercise and diet may theoretically improve cardiovascular outcomes, this has not been scientifically proven. Some studies show an association between soy food consumption and lower risk of coronary heart disease in women, but further investigation is needed.

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