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:
- People in Asia who often eat up to 55 grams of soy protein per day have lower rates of heart disease than people who eat a typical Western diet with fewer than 5 grams of soy protein per day.
- Substituting 31 to 47 grams of soy protein for meat in a diet low in saturated fat and cholesterol may lower blood cholesterol levels.
- Including at least 20 grams of soy protein per day in a diet low in saturated fat and cholesterol may reduce blood fat levels in both men and women, although not all studies agree.
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.
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.
Pregnancy and Breastfeeding
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.
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