Herbs & Supplements:
Soy
Supplement Forms / Alternate Names
•
Soy Protein Extract, Hydrolyzed Soy Protein, Soy Protein
Principal Proposed Uses
•
High Cholesterol
Other Proposed Uses
•
Menopausal Symptoms, Cancer Prevention, Osteoporosis
The soybean has been prized for centuries in Asia as a nutritious, high-protein food with a myriad uses, and today it's 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 a growing number of grocery stores alongside the traditional white blocks of tofu.
Soy appears to reduce blood cholesterol levels, and the U.S. Food and Drug Administration has authorized allowing foods containing soy to carry a "heart-healthy" label.
Soybeans contain chemicals that are similar to estrogen, called isoflavones. These are widely thought to be the active ingredients in soy, although this theory is disputable.
Sources
If you like Japanese, Chinese, Thai, or Vietnamese food, it's easy to get a healthy dose of soy. Tofu is one of the world's most versatile foods. It can be stir-fried, steamed, or added to soup. You can also mash a cake of tofu and use it in place of ricotta cheese in your lasagna. If you don't like tofu, there are many other soy products to try: plain soybeans, soy cheese, soy burgers, soy milk, or tempeh. Or, you can use a soy supplement instead.
Therapeutic Dosages
The FDA suggests a daily minimum intake of 25 g of soy protein to reduce cholesterol, although higher intake is probably more effective. This amount is typically found in about 2 1/2 cups of soy milk or 1/2 pound of tofu. Evidence suggests that substituting as little as 20 g per day of soy protein for animal protein can significantly improve cholesterol levels.1 Other studies have used dosages of up to 40 g daily.
Therapeutic Uses
According to the combined evidence of 38 controlled studies, soy can reduce blood cholesterol levels and improve the ratio of LDL ("bad") versus HDL ("good") cholesterol.2 At an average dosage of 47 g daily, total cholesterol falls by about 9%, LDL cholesterol by 13%, and triglycerides by 10%. Soy's effects on HDL cholesterol itself are less impressive.
Soy may reduce the common menopausal symptom known as "hot flashes,"3,4 although study results conflict.34-38,72 Soy has not been found helpful for improving the hot flashes that often occur in breast cancer survivors.73
Unlike estrogen, soy appears to reduce the risk of uterine cancer.5 Its effect on breast cancer is not as well established, but there are reasons to believe that soy can help reduce breast cancer risk as well,6,7,8 possibly by reducing estrogen levels and lengthening the menstrual cycle.74 (For more information, see the isoflavone article)
Soy may also help prevent prostate and colon cancers.11,12,13 In addition, soy might help prevent osteoporosis.14–22
What Is the Scientific Evidence for Soy?
High Cholesterol
In 1995, a review of 38 controlled studies on soy and heart disease concluded that soy is effective at reducing total cholesterol, LDL cholesterol, and triglycerides.23 A double-blind study (not part of the review mentioned previously), which involved 66 older women, found improvements in HDL cholesterol as well.24 The women were divided into three groups. The first group received 40 g of skim milk protein daily. The second group was given the same amount of soy protein, and the third received 40 g of soy protein with extra soy isoflavones. Compared with the skim milk (placebo) group, both soy groups showed significant improvements in both total cholesterol and HDL cholesterol.
Isoflavones in soy are widely thought to be the active cholesterol-lowering ingredient;25,26,27 however, several studies have failed to find isoflavones themselves effective.28–32,70 One possibility is that isoflavones are not the active ingredient in soy; it has been hypothesized that various proteins may actually be the active cholesterol-loweing ingredients in soy.75 Another possibility is suggested by a study which found that some soy products have an unusual isoflavone profile.71 Researchers discovered that certain soy products contain high levels of the isoflavone glycitein rather than genistein and daidzein; in most soy products glycitein is a minor constituent.
High levels of glycitein could account for some of the negative trials of concentrated soy isoflavones. At present, it is not clear under what circumstances soy might have high glycitein levels, and whether that accounts for the inconsistent study results.
One indisputable benefit from eating soy is that, unlike animal sources of protein, it contains no saturated fat. However, soy appears to produce benefits above and beyond substituting for less healthful forms of protein.33
A recent double-blind, placebo-controlled trial failed to find concentrated soy isoflavones themselves effective for reducing cholesterol in postmenopausal women. In this study, 36 postmenopausal women on hormone replacement therapy took either placebo or a relatively high dose of soy isoflavones (50 mg) 3 times daily for 6 months. The results showed no statistically significant changes in lipid levels. It is possible that the use of hormones in this study "overwhelmed" the effects of soy. Another possibility is that the number of participants was too small to detect an effect.
Menopausal Symptoms ("Hot Flashes")
Soy may relieve "hot flashes," a common symptom of menopause, but the results of studies have been contradictory.
One double-blind, placebo-controlled study involving 104 women found that soy provided significant relief compared to placebo (milk protein). After 3 weeks, the women taking daily doses of 60 g of soy protein were having 26% fewer hot flashes.34 By week 12, the reduction was 45%. Women taking placebo also experienced a big improvement by week 12 (30% fewer hot flashes), but soy gave significantly better results.
A 12-week study that enrolled 114 women found evidence of benefit for hot flashes and vaginal dryness, but not for menopausal symptoms as a whole.35 Reduction in hot flashes were seen in two 6-week, double-blind trials as well.36,37 However, a 24-week, double-blind study of 69 women found no benefit with either isoflavone-rich or isoflavone-poor soy.38 Similarly, no benefits were seen in a 3-month trial of 94 women with mild menopausal symptoms.72 The very high rate of placebo effect seen in most studies of menopausal symptoms may account for these discrepancies.
Soy has also failed to prove effective for treating the hot flashes that frequently occur in breast cancer survivors.73
It is generally thought that the isoflavones in soy are responsible for any benefits in menopause. However, similar isoflavones from red clover have not been found effective;39,40 for this reason, along with the results of studies involving soy and cholesterol, researchers are beginning to think there may be other important but unrecognized factors in soy. High levels of glycitein (see previous section) are another possibility.
Osteoporosis
In one study that evaluated the benefits of soy in osteoporosis, a total of 66 postmenopausal women took either placebo (soy protein with isoflavones removed) or soy protein with 56 or 90 mg of isoflavones daily for 6 months.41 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.
Very nearly the same results were also seen in a similar study. This 24-week, double-blind study of 69 postmenopausal women found that soy can significantly reduce bone loss from the spine.42
Similar benefits have been seen in some, but not all, animal studies as well as other human trials. .43–49, 50-52, 75 These conflicting results make it impossible to conclude, as yet, whether soy or soy isoflavones are helpful for osteoporosis.
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.53,54
Safety Issues
Studies in animals have found soy essentially nontoxic.55However, the isoflavones in soy could conceivably have some potentially harmful hormonal effects in certain specific situations.
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. There are also concerns that soy isoflavones could have an immune suppressive effect. For more information on these and other safety issues regarding the isoflavones in soy, see the full isoflavones article.
Fears have been expressed by some experts that soy might interfere with the action of oral contraceptives. However, one study of 36 women suggests that such concerns are groundless.59
Soy appears to have numerous potential effects involving the thyroid gland. When given to individuals with impaired thyroid function, soy products have been observed to reduce absorption of thyroid medication.62,63,76 In addition, some evidence hints that soy isoflavones may directly inhibit the function of the thyroid gland, although this inhibition may only be significant in individuals who are deficient in iodine.77,78 However, to make matters even more confusing, 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.69,79-82 The bottom line: In view of soy’s complex effects regarding the thyroid, individuals with impaired thyroid function should not take large amounts of soy products except under the supervision of a physician.
Soy may reduce the absorption of the nutrients zinc, iron, and calcium.64–68 To avoid absorption problems, you should probably take these minerals at least 2 hours apart from eating soy.
One observational study raised concerns that soy might impair mental function.83 However, observational studies are less reliable than controlled trials, and other studies hint that soy or soy isoflavones might actually have a beneficial effect on cognition.75
Interactions You Should Know About
If you are taking
- Zinc, iron, or calcium supplements: It may be best to eat soy at a different time of day to avoid absorption problems.
- Thyroid hormone: You should consult your physician before increasing your intake of soy products.
View References
Last reviewed September 2002 by Medical Review Board
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