Soy as a dietary supplement

• PRINCIPAL PROPOSED USE: High cholesterol
• OTHER PROPOSED USES: Allergic rhinitis, cancer prevention, cyclic mastitis, diabetes, high blood pressure, menopausal symptoms, menstrual migraines, osteoarthritis, osteoporosis, ulcerative colitis

DEFINITION: Natural food product promoted as a dietary supplement for specific health benefits.

Overview

The soybean has been prized for centuries in Asia as a nutritious, high-protein food with myriad uses, and it has become 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, hot dogs, and tofu cheese can be found in most grocery stores alongside traditional white blocks of tofu. Also, soy is increasingly used as a protein filler in many prepared foods, including fast-food hamburgers.

Soy appears to reduce blood cholesterol levels. In 1999, the US Food and Drug Administration (FDA) authorized foods containing at least 6.25 grams (g) of soy per serving to carry a heart-healthy label because of their ability to limit the risk for coronary heart disease. However, in 2017, the agency announced a plan to revoke the label authorization following further research reporting inconsistent results concerning soy's role in heart health and the reduction of heart disease. With the evocation set to take effect in January 2020, a debate ensued over the issue in the following years. In 2019, researchers published a study in the Journal of the American Heart Association arguing that a meta-analysis of the forty-six randomized controlled trials considered by the FDA illustrated a reduction in total cholesterol and LDL (bad) cholesterol related to soy consumption.

Despite controversy over health-claim labeling, soy has demonstrated health benefits in many studies. 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, added to soup, or mashed and used instead of ricotta cheese in lasagna. Many other soy products are made from soy protein isolate (SPI) or soy protein concentrate (SPC), including plain soybeans, soy cheese, soy meat substitutes, soy milk, soy dairy substitutes, and tempeh. Soy supplements are also available.

Therapeutic Dosages

Evidence suggests that a daily intake of twenty-five grams of soy protein, recommended by the American Heart Association, is adequate to noticeably reduce cholesterol and lower one’s risk of heart conditions. The American Institute for Cancer Research recommends individuals ingest two servings of whole soy food sources each day. This amount is typically found in about two-and-one-half cups of soy milk or one-half pound of tofu.

Soy has been increasingly added to foods in the United States as a protein filler, and some experts raised concerns that Americans may greatly exceed the amount of soy eaten elsewhere. Even the twenty-five gram dose recommended for reducing cholesterol levels is relatively high, according to some. However, other research supports a high intake of soy protein to support health.

Therapeutic Uses

According to the combined evidence of numerous controlled studies, soy can reduce blood cholesterol levels and improve the ratio of LDL to HDL cholesterol.

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 proteinas opposed to other soy productscould significantly reduce blood pressure.

Soy has not been found helpful for improving the hot flashes that occur in persons who have had breast cancer.

Unlike estrogen, soy appears to reduce the risk of uterine cancer. It can also help reduce the risk of fibroids that can develop in the uterus. Its effect on breast cancer is not as well established, but there are reasons to believe that soy can help reduce breast cancer risk, possibly by reducing estrogen levels and lengthening the menstrual cycle. However, soy can interfere with the breast cancer drug tamoxifen (Nolvadex). Soy has shown inconsistent promise for helping to prevent prostate and colon cancers.

One double-blind trial found evidence that soy protein can reduce symptoms of osteoarthritis. In addition, soy might help prevent osteoporosis.

Some unique types of soy may have nutritional benefits. In preliminary double-blind studies, a special extract of soy sauce called Shoyu polysaccharides has shown promise as a treatment for allergic rhinitishay fever. Another special extract of soy, Bowman-Birk inhibitor concentrate, has shown promise in helping prevent cancer and in treating ulcerative colitis.

Soy may be beneficial in diabetes. A bioactive soy extract, pinitol, may improve blood sugar control in people with diabetes. Also, in addition to its favorable effects on cholesterol, the long-term consumption of a soy-based diet 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 abdominal fat buildup.

A product containing soy isoflavones and other herbs has shown some promise for migraine headaches associated with the menstrual cycle, known as 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 and perhaps slightly raise HDL cholesterol. It has often been assumed that isoflavones are the active ingredients in soy that are responsible for improving cholesterol profiles. However, studies that used purified isoflavones, as opposed to whole soy or soy protein, 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 review of thirty studies involving almost three thousand people, researchers determined that isolated soy protein, at a concentation of 15 to 40 grams 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 soybeans 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, one study suggests that isoflavone daidzein may only be effective for reducing cholesterol when converted by intestinal bacteria into a substance called equol, and 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, or “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 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 placebomilk 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, many studies have failed to find benefits 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, certain soy formulations may 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 placebosoy protein with isoflavones removedor soy protein with fifty-six or ninety milligrams (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 benefits. On balance, 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 creamanother treatment proposed for preventing or treating osteoporosismay decrease the bone-sparing effect of soy isoflavones. Estrogen and most other medications for osteoporosis work by fighting bone breakdown. Soy may also work in other ways, such as 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 giving healthy postmenopausal women 900 mg of soy isoflavones daily for eighty-four consecutive days. However, soy or its isoflavones could have some potentially harmful effects in certain 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 the 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 on 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 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 the 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, and experts do not consider the results of this study a serious issue. Additionally, several studies on cognitive improvement have found that soy or soy isoflavones either have no effect on mental function or perhaps minimally improve it. Several subsequent studies found soy to have positive impacts on cognitive function, particularly in older adults and individuals with neurodegenerative diseases. Some benefits include improved nonverbal and verbal memory, verbal fluency, attention span, and spatial learning. Though further studies are needed, these findings are promising.

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

Agnieszka, Barańska, et al. “The Role of Soy Isoflavones in the Prevention of Bone Loss in Postmenopausal Women: A Systematic Review with Meta-Analysis of Randomized Controlled Trials.” Journal of Clinical Medicine, vol. 11, no. 16, 2022, p. 4676. doi:10.3390/jcm11164676. Accessed 2 Nov. 2024.

Basson, Abigail Raffner, et al. "Regulation of Intestinal Inflammation by Soybean and Soy-Derived Compounds." Foods, vol. 10, no. 4. doi:10.3390/foods10040774.

Cui, Chendi, et al. “Effects of Soy Isoflavones on Cognitive Function: A Systematic Review and Meta-analysis of Randomized Controlled Trials.” Nutrition Reviews, vol. 78, no. 2, 2020, pp. 134-44. doi:10.1093/nutrit/nuz050.

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, vol. 11, 2007, pp. 155-64.

Goldschmidt, Debra, and Jacqueline Howard. "FDA Moves to Revoke Soy Health Claim." CNN, 31 Oct. 2017, www.cnn.com/2017/10/30/health/fda-soy-heart-health-claim-bn/index.html. Accessed 4 May 2020.

Hooper, L., et al. “Flavonoids, Flavonoid-Rich Foods, and Cardiovascular Risk.” American Journal of Clinical Nutrition, vol. 88, 2008, pp. 38-50.

Jenkins, David J. A., et al. "Cumulative Meta-Analysis of Soy Effect Over Time." Journal of the American Heart Association, vol. 8, no. 13, 2019. doi:10.1161/JAHA.119.012458.

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, vol. 12, 2005, pp. 56-62.

Pawlak, R., et al. “Benefits, Barriers, Attitudes, and Beliefs About Soy Meat-Alternatives Among African American Parishioners Living in Eastern North Carolina.” Ethnicity and Disease, vol. 20, 2010, pp. 118-22.

Phares, Emily H. "Straight Talk about Soy." Harvard school of Public Health, Jan. 2022, www.hsph.harvard.edu/nutritionsource/2014/02/12/straight-talk-about-soy. Accessed 30 Sept. 2024.

Zuo, Xinrong, et al. “Soy Consumption and the Risk of Type 2 Diabetes and Cardiovascular Diseases: A Systematic Review and Meta-Analysis.” Nutrients, vol. 15, no. 6, Mar. 2023, p. 1358. doi:10.3390/nu15061358. Accessed 2 Nov. 2024.