Overview
The soybean has been prized for centuries in Asia as a nutritious, high-protein
food with myriad uses, and today it is popular in the United States not only in
Asian food but also as a cholesterol-free meat and dairy substitute in traditional
American foods. Soy burgers, soy yogurt, tofu (a soft soy product) hot dogs, and
tofu cheese can be found in a growing number of grocery stores, alongside the
traditional white blocks of tofu. Also, soy is increasingly used as a protein
filler in many prepared foods, including fast-food hamburger.
Soy appears to reduce blood cholesterol levels. The U.S. Food and Drug
Administration (FDA) has authorized allowing foods containing
soy to carry a “heart-healthy” label.
Soybeans contain isoflavones, chemicals that are similar to estrogen. These
are widely thought to be the active ingredients in soy, although there is
substantial evidence that other constituents may be equally or more important.
Sources
It is easy to get a healthy dose of soy by eating Japanese, Chinese, Thai, or Vietnamese foods. Tofu is one of the world’s most versatile foods. It can be stir-fried, steamed, or added to soup. A cake of tofu can be mashed and used in place of ricotta cheese in lasagna. Many other soy products are available, including plain soybeans, soy cheese, soy burgers, soy milk, and tempeh. Soy supplements are also available.
Therapeutic Dosages
The FDA allows soy foods containing 6.5 grams (g) of soy to carry a heart-healthy label. Evidence suggests that a daily intake of 25 g of soy protein is adequate to noticeably reduce cholesterol. This amount is typically found in about two-and-one-half cups of soy milk or in one-half pound of tofu.
Soy is increasingly added to foods in the United States as a protein filler, and there are concerns that some people in the United States may be greatly exceeding the amount of soy eaten anywhere else in the world. Even the 25-g amount recommended for reducing cholesterol levels is relatively high. For comparison, in Asia, the average intake of soy is only about 10 g daily.
Therapeutic Uses
According to the combined evidence of numerous controlled studies, soy can reduce blood cholesterol levels and improve the ratio of LDL (bad) to HDL (good) cholesterol. At an average dosage of 47 g daily, total cholesterol falls by about 9 percent, LDL cholesterol falls by 13 percent, and triglycerides fall by 10 percent.
Soy’s effects on HDL cholesterol itself are less impressive. There is inconsistent
evidence regarding whether soy might help reduce high blood pressure. Indeed, a
comprehensive and careful review of studies investigating the influence of
phytoestrogens (including soy meals) on blood pressure found
no meaningful effect. However, another review found that soy protein (as opposed
to other soy products) could significantly reduce blood pressure.
Soy may reduce the common menopausal symptom known as hot flashes, but study results conflict. Soy has not been found helpful for improving the hot flashes that often occur in persons who have had breast cancer.
Unlike estrogen, soy appears to reduce the risk of uterine cancer. Its effect on breast cancer is not as well established, but there are reasons to believe that soy can help reduce breast cancer risk too, possibly by reducing estrogen levels and lengthening the menstrual cycle. Soy has shown inconsistent promise for helping to prevent prostate and colon cancers.
One preliminary double-blind trial found evidence that soy protein can reduce symptoms of osteoarthritis. In addition, soy might help prevent osteoporosis.
In preliminary double-blind studies, a special extract of soy sauce called Shoyu polysaccharides has shown promise as a treatment for allergic rhinitis (hay fever).
Another special extract of soy, Bowman-Birk inhibitor concentrate, has shown promise for helping to prevent cancer and also for treating ulcerative colitis.
Soy may be beneficial in diabetes. A soy extract, pinitol, may improve blood sugar control in diabetics. Also, in addition to its favorable effects on cholesterol, the long-term consumption of a diet rich in soy may reduce blood sugar and improve kidney function in diabetic persons with renal dysfunction. A small study found hints that soy isoflavones might help reduce the buildup of abdominal fat.
A product containing soy isoflavones and other herbs has shown some promise for migraine headaches associated with the menstrual cycle (menstrual migraines). Weak evidence suggests that soy protein may be helpful for cyclic breast pain. Finally, a fermented soy product called natto has shown some promise for dissolving blood clots.
Scientific Evidence
High cholesterol. Numerous controlled studies indicate that soy can reduce LDL cholesterol by about 10 percent and perhaps slightly raise HDL cholesterol. It has often been assumed that isoflavones are the active ingredients in soy responsible for improving cholesterol profile. However, studies that used purified isoflavones (as opposed to whole soy, or soy protein) have generally failed to find benefit. It is possible that non-isoflavone constituents of soy, such as proteins, fiber, and phospholipids, may be equally or perhaps even more important than the isoflavones in soy.
In a careful review of thirty studies involving almost three thousand people, researchers determined that isolated soy protein (in the range of 15 to 40 g per day) modestly reduced LDL cholesterol levels by an average of 6 percent. In another study involving two soy milk products, one made from whole soy beans and the other from isolated soy proteins, researchers found that both were more effective than cow’s milk at reducing LDL cholesterol levels. In addition, the substance pinitol appears to have cholesterol-lowering properties.
However, there are other possibilities too. One study suggests that the isoflavone daidzein may be effective for reducing cholesterol only when it is converted by intestinal bacterial into a substance called equol. It appears that only about one-third of people have the “right” intestinal bacteria to make equol.
Another study found that soy products may at times have an unusual isoflavone
profile, one containing high levels of the isoflavone glycitein rather than the
more usual genistein and daidzein. Glycitein could be inactive
regarding cholesterol reduction; in other words, variations in the proportions of
specific isoflavone constituents might have made some studied soy isoflavone
products inactive.
It has also been suggested that soy protein must be kept in its original state to be effective. Ordinary soy protein extracts are somewhat damaged (“denatured”). In a double-blind study of 120 people, a special soy protein extract, in which the proteins were protected from damage, proved more effective for improving cholesterol profile than did standard denatured soy protein extracts.
Hot flashes. Although study results are not entirely consistent,
soy may be helpful for symptoms of menopause, especially hot flashes.
For example, a double-blind, placebo-controlled study involving 104 women found
that isoflavone-rich soy protein provided significant relief of hot flashes
compared with placebo (milk protein). Improvements in hot flashes and other
menopausal symptoms, such as vaginal dryness, were also seen in several other
studies of soy or soy isoflavones. However, about as many studies have failed to
find benefit with soy or concentrated isoflavones. Furthermore, in three
double-blind, placebo-controlled trials, isoflavone-rich soy failed to reduce hot
flashes among those who had had breast cancer.
Another double-blind study, of 241 women experiencing hot flashes, found equivalent benefits for isoflavone-free and isoflavone-rich soy products. The high rate of the placebo effect seen in many studies of menopausal symptoms may account for these discrepancies. In addition, it is possible that certain formulations of soy contain unidentified ingredients beyond isoflavones that play an important role.
Osteoporosis. In one study that evaluated the benefits of soy in
osteoporosis, sixty-six postmenopausal women took either
placebo (soy protein with isoflavones removed) or soy protein with 56 or 90 mg of
isoflavones daily for six months. The group that took the higher dosage of
isoflavones showed significant gains in spinal bone density. There was little
change in the placebo or low-dose isoflavone groups. This study suggests that the
soy isoflavones in soy protein may be effective for osteoporosis. Nearly the same
results were seen in a similar study. This twenty-four-week, double-blind study of
sixty-nine postmenopausal women found that soy can significantly reduce bone loss
from the spine.
Similar benefits with soy or soy isoflavones have been seen in other human and animal trials; however, other studies have failed to find benefit. On balance, it is probably fair to say that isoflavones (either as soy, purified isoflavones, or tofu extract) are likely to have a modestly beneficial effect on bone density at most.
One small but long-term study suggests that progesterone cream (another treatment proposed foruse in preventing or treating osteoporosis) may decrease the bone-sparing effect of soy isoflavones. Estrogen and most other medications for osteoporosis work by fighting bone breakdown. It has been hypothesized that soy may also work in other ways, by helping to increase new bone formation.
Safety Issues
Studies in animals have found soy essentially nontoxic. Researchers found no evidence of ill effects when they gave healthy postmenopausal women 900 mg of soy isoflavones a day for eighty-four consecutive days. However, soy or its isoflavones could conceivably have some potentially harmful effects in certain specific situations.
Soy appears to have numerous potential effects involving the thyroid
gland. When given to persons with impaired thyroid function, soy
products have been observed to reduce absorption of thyroid medication. In
addition, some evidence hints that soy isoflavones may directly inhibit the
function of the thyroid gland, although this inhibition may be significant only in
persons who are deficient in iodine. However, studies of healthy humans and
animals given soy isoflavones or other soy products have generally found that soy
either had no effect on thyroid hormone levels or actually increased levels.
In view of soy’s complex effects regarding the thyroid, persons with impaired thyroid function should not take large amounts of soy products except under the supervision of a physician.
One study found that soy products may decrease testosterone levels in men. This could conceivably cause problems for men with infertility or erectile dysfunction.
Soy may reduce the absorption of the nutrients zinc, iron, and calcium. To avoid absorption problems, one should probably take these minerals a minimum of two hours before or after eating soy.
Other concerns relate to the estrogenic properties of soy isoflavones. For example, while soy is thought to reduce the risk of developing breast cancer, it is possible that soy might not be safe for women who have already had breast cancer. In addition, there are concerns that intensive use of soy products by pregnant women could exert a hormonal effect that impacts unborn fetuses. Finally, fears have been expressed by some experts that soy might interfere with the action of oral contraceptives. However, one study of thirty-six women found reassuring results.
One observational study raised concerns that soy might impair mental function. However, observational studies are highly unreliable by nature, and experts do not consider the results of this study a serious issue. Additionally, a number of studies looking at cognitive improvement have found that soy or soy isoflavones either have no effect on mental function or perhaps minimally improve it.
Important Interactions
It may be best to eat soy at a different time of day to avoid problems absorbing zinc, iron, or calcium supplements. Also, persons taking a thyroid hormone should consult a physician before increasing their intake of soy products.
Bibliography
Arjmandi, B. H., et al. “Soy Protein May Alleviate Osteoarthritis Symptoms.” Phytomedicine 11 (2005): 567-575.
Fournier, L. R., et al. “The Effects of Soy Milk and Isoflavone Supplements on Cognitive Performance in Healthy, Postmenopausal Women.” Journal of Nutrition, Health, and Aging 11 (2007): 155-164.
Hooper, L., et al. “Flavonoids, Flavonoid-Rich Foods, and Cardiovascular Risk.” American Journal of Clinical Nutrition 88 (2008): 38-50.
Kok, L., et al. “A Randomized, Placebo-Controlled Trial on the Effects of Soy Protein Containing Isoflavones on Quality of Life in Postmenopausal Women.” Menopause 12 (2005): 56-62.
Pawlak, R., B. Malinauskas, and A. Corbett. “Benefits, Barriers, Attitudes, and Beliefs About Soy Meat-Alternatives Among African American Parishioners Living in Eastern North Carolina.” Ethnicity and Disease 20 (2010): 118-122.
Sites, C. K., et al. “Effect of a Daily Supplement of Soy Protein on Body Composition and Insulin Secretion in Postmenopausal Women.” Fertility and Sterility 88 (2007): 1609-1617.
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