Soy, a staple food in many Asian countries, contains valuable constituents, including protein, isoflavones, saponins, and phytosterols. Soy protein provides essential amino acids. When eaten with rice, soy provides protein comparable with that found in animal products. Soy is low in fat and cholesterol-free.

What Are Star Ratings?

Our proprietary “Star-Rating” system was developed to help you easily understand the amount of scientific support behind each supplement in relation to a specific health condition. While there is no way to predict whether a vitamin, mineral, or herb will successfully treat or prevent associated health conditions, our unique ratings tell you how well these supplements are understood by the medical community, and whether studies have found them to be effective for other people.

For over a decade, our team has combed through thousands of research articles published in reputable journals. To help you make educated decisions, and to better understand controversial or confusing supplements, our medical experts have digested the science into these three easy-to-follow ratings. We hope this provides you with a helpful resource to make informed decisions towards your health and well-being.

3 Stars Reliable and relatively consistent scientific data showing a substantial health benefit.

2 Stars Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.

1 Star For an herb, supported by traditional use but minimal or no scientific evidence. For a supplement, little scientific support.

This supplement has been used in connection with the following health conditions:

Used for Why
3 Stars
High Cholesterol
25 grams soy protein per day
Soy supplementation has been shown to lower cholesterol. Soy contains isoflavones, which are believed to be soy’s main cholesterol-lowering ingredients.

Soy supplementation has been shown to lower cholesterol in humans. Soy is available in foods such as tofu, miso, and tempeh and as a supplemental protein powder. Soy contains isoflavones, naturally occurring plant components that are believed to be soy’s main cholesterol-lowering ingredients. A controlled trial showed that soy preparations containing high amounts of isoflavones effectively lowered total cholesterol and LDL (“bad”) cholesterol, whereas low-isoflavone preparations (less than 27 mg per day) did not. However, supplementation with either soy or non-soy isoflavones (from red clover) in pill form failed to reduce cholesterol levels in a group of healthy volunteers, suggesting that isoflavone may not be responsible for the cholesterol-lowering effects of soy. Further trials of isoflavone supplements in people with elevated cholesterol, are needed to resolve these conflicting results. In a study of people with high cholesterol levels, a soy preparation that contained soy protein, soy fiber, and soy phospholipids lowered cholesterol levels more effectively than isolated soy protein.

3 Stars
10 grams soy protein or 16 ounces soy milk twice per day
Supplementing with soy protein may significantly lower blood pressure.

In a double-blind study of postmenopausal women, supplementing with 10 grams of soy protein twice a day for six weeks significantly reduced diastolic blood pressure by an average of 5 mm Hg, compared with a diet not containing soy protein. In another study, men and women with mild to moderate hypertension consumed 500 ml (approximately 16 ounces) of soy milk or cow's milk twice a day for three months. After three months, the average systolic blood pressure had decreased by 18.4 mm Hg in the soy group, compared with 1.4 mm Hg in the cow's milk group. The reductions in diastolic blood pressure were 15.9 mm Hg with soy milk and 3.7 mm Hg with cow's milk. In another study of people with hypertension who were consuming a low-protein, low-fiber diet, supplementing with a combination of soy protein and psyllium (a fiber source) lowered systolic blood pressure by an average of 5.9 mm Hg. The blood pressure reduction with soy protein alone or with fiber alone was less pronounced than that with combination treatment. Other research has also shown a blood pressure–lowering effect of soy protein.

2 Stars
Athletic Performance and Exercise Recovery
33 to 40 grams daily
In one study, elderly men participating in a strength training program who took a supplement containing protein (part of which was soy protein) immediately following exercise saw significant gains in muscle growth and lean body mass.

In one preliminary study, elderly men participating in a 12-week strength training program took a liquid supplement containing 10 grams of protein (part of which was soy protein), 7 grams of carbohydrate, and 3 grams of fat either immediately following exercise or two hours later. Men taking the supplement immediately following exercise experienced significantly greater gains in muscle growth and lean body mass than those supplementing two hours later, but strength gains were no different between the two groups. A controlled study of female gymnasts found that adding 0.45 grams of soy protein (0.45 grams per pound of body weight per day) to a diet that was adequate in protein during a four-month training program did not improve lean body mass compared with a placebo. No research has compared different sources of protein to see whether one source, such as soy protein, has a better or more consistent effect on exercise recovery or the results of strength training.

1 Star
Refer to label instructions
Some research has found soy isoflavones may help reduce certain menopause symptoms, such as hot flashes, and support mood and thinking performance.

A double-blind study of 78 postmenopausal women (average age, 49.5 years) were randomly assigned to receive 60 mg per day of soy isoflavones (as aglycone) or placebo for six months. Then after a one-month break, each woman received the alternate treatment for an additional six months. Thinking (cognitive) performance and mood were assessed by tests at the end of each treatment period. Compared with placebo, people treated with isoflavones experienced significant improvement on 6 of 17 measures of cognitive function and 7 of 8 measures of mood. In another double-blind study, 177 postmenopausal women (average age, 55 years) who were experiencing five or more hot flushes per day were randomly assigned to receive soy isoflavones (50 mg per day total of an approximately equal mixture of isoflavones genistin and daidzin) or placebo for 12 weeks. The average reduction in hot flash frequency after 6 weeks was significantly greater in the isoflavone group than in the placebo group; however, during the entire 12 weeks, the difference between the two groups was only marginally significant.

1 Star
Refer to label instructions
Soy appears to have several effects on the body that might help with weight loss.
Animal and human studies have suggested that when soy is used as a source of dietary protein, it may have several biological effects on the body that might help with weight loss. A preliminary study found that people trying to lose weight using a meal-replacement formula containing soy protein lost more weight than a group not using any formula. However, controlled studies comparing soy protein with other protein sources in weight-loss diets have not found any advantage of soy. When soy protein is used for other health benefits, typical daily intake is 20 grams per day or more.
1 Star
Premenstrual Syndrome
Refer to label instructions
Supplementing with soy protein may help relieve premenstrual swelling and cramping.

In a double-blind trial, supplementing with soy protein (providing 68 mg of isoflavones per day) for two menstrual cycles was significantly more effective than a placebo at relieving premenstrual swelling and cramping. The placebo used in this study was cow's milk protein. Some doctors believe that cow's milk, because of its estrogen content, can worsen premenstrual symptoms. If that is the case, then the beneficial effect of soy protein may have been overestimated in this study.

How It Works

How to Use It

The ideal intake of soy is not known. Researchers suggest the equivalent of one serving of soy foods per day supports good health, and the benefits increase as soy intake increases.1 Societies in which large amounts of soy are consumed ingest between 50 and 100 mg per day of soy isoflavones. The cholesterol-lowering effects of soy have been observed at amounts as low as 20 grams of soy protein per day, if it replaces animal protein in the diet.2

Where to Find It

Relatively large amounts of isoflavones are present in whole soybeans, roasted soy nuts, tofu, tempeh, soy milk, meat substitutes, soy flour, and some soy protein isolates. In addition, the isoflavones present in soy are available as supplements, in capsules or tablets.

Possible Deficiencies

Although deficiencies do not occur, people who do not consume soy foods will not gain the benefits of soy.


Interactions with Supplements, Foods, & Other Compounds

Soy contains a compound called phytic acid, which can interfere with mineral absorption.

Interactions with Medicines

Certain medicines interact with this supplement.

Types of interactions: Beneficial Adverse Check

Replenish Depleted Nutrients

  • none

Reduce Side Effects

  • none

Support Medicine

  • none

Reduces Effectiveness

  • Dessicated Thyroid

    Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.

  • Levothyroxine

    Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.

  • Liothyronine

    Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.

  • Liotrix

    Ingestion of soy products simultaneously with the thyroid hormone levothyroxine appears to reduce its absorption. To be safe, people taking thyroid medications should not consume soy products within three hours of taking their medication. In addition, infants with congenital hypothyroidism given thyroid medication must not be given increased or reduced amounts of soy-based formula without consulting a pediatrician or pediatric endocrinologist.

Potential Negative Interaction

  • none

Explanation Required

  • Ipratropium Bromide

    Atrovent® and Combivent® for oral inhalation contain soy lecithin. Rarely, people very sensitive to soy have reacted to these drugs, and life-threatening anaphylactic reaction is possible, though extremely rare. Ipratropium bromide nasal spray and solution for inhalation contain no soy lecithin.

  • Theophylline

    In a study of healthy volunteers given theophylline, ingesting daidzein (one of the major isoflavones in soy) in the amount of 200 mg twice a day for ten days inhibited the metabolism of theophylline, resulted in higher concentrations of the drug. The amount of daidzein used in this study was greater than what would be found in a normal portion of soy foods; it is not known whether consuming average amounts of soy would have a similar effect.

The Drug-Nutrient Interactions table may not include every possible interaction. Taking medicines with meals, on an empty stomach, or with alcohol may influence their effects. For details, refer to the manufacturers’ package information as these are not covered in this table. If you take medications, always discuss the potential risks and benefits of adding a supplement with your doctor or pharmacist.

Side Effects

Side Effects

Soy products and cooked soybeans are safe at a wide range of intakes. However, a small percentage of people have allergies to soybeans and thus should avoid soy products.

Soy isoflavones have been reported to reduce thyroid function in some people.3 A preliminary trial of soy supplementation among healthy Japanese, found that 30 grams (about one ounce) per day of soybeans for three months, led to a slight reduction in the hormone that stimulates the thyroid gland.4 Some participants complained of malaise, constipation, sleepiness, and even goiter. These symptoms resolved within a month of discontinuing soy supplements. However, a variety of soy products have been shown to either cause an increase in thyroid function5 or produce no change in thyroid function.6 The clinical importance of interactions between soy and thyroid function remains unclear. However, in infants with congenital hypothyroidism, soy formula must not be added, nor removed from the diet, without consultation with a physician, because ingestion of soy may interfere with the absorption of thyroid medication.7

Most research, including animal studies, report anticancer effects of soy extracts,8 though occasional animal studies have reported cancer-enhancing effects.9 The findings of several recent studies suggest that consuming soy might, under some circumstances, increase the risk of breast cancer. When ovaries have been removed from animals—a situation related to the condition of women who have had a total hysterectomy—dietary genistein has been reported to increase the proliferation of breast cancer cells.10 When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.11 Although premenopausal women have shown decreases in estrogen levels in response to soy,12 , 13 pro-estrogenic effects have also been reported.14 When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.15 In yet another trial, healthy breast cells from women previously given soy supplements containing isoflavones showed an increase in proliferation rates—an effect that might also increase the risk of breast cancer.16

Of 154 healthy postmenopausal women who received 150 mg of soy isoflavones per day for five years, 3.9% developed an abnormal proliferation of the tissue that lines the uterus (endometrial hyperplasia). In contrast, none of 144 women who received a placebo developed uterine hyperplasia.17 Although no case of uterine cancer was diagnosed during the study, endometrial hyperplasia is a potential forerunner of uterine cancer. The amount of isoflavones used in this study is two to three times as much as that used in many other studies. Nevertheless, the possibility exists that long-term use of isoflavones could cause uterine hyperplasia, and women taking isoflavones should be monitored appropriately by their doctor.

Some postmenopausal women taking the soy isoflavone genistein have experienced gastrointestinal side effects (abdominal pain, epigastric pain, dyspepsia, vomiting, or constipation).18


1. Messina M. To recommend or not to recommend soy foods. J Am Diet Assoc 1994;94:1253-4.

2. Teixeira SR, Potter SM, Weigel R, et al. Effects of feeding 4 levels of soy protein for 3 and 6 wk on blood lipids and apolipoproteins in moderately hypercholesterolemic men. Am J Clin Nutr 2000;71:1077-84.

3. Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from soybean: isolation, characterization and mechanisms of action. Biochem Pharmacol 1997;54:1087-96.

4. Ishizuki Y, Hirooka Y, Murata Y, Togashi K. [The effects on the thyroid gland of soybeans administered experimentally in healthy subjects.] Nippon Naibunpi Gakkai Zasshi 1991;67:622-9. [in Japanese].

5. Forsythe WA. Soy Protein, thyroid regulation and cholesterol metabolism. Forsythe WA. Soy protein, thyroid regulation and cholesterol metabolism. J Nutr 1995;125:619S-23S.

6. Bennink MR, Mayle JE, Bourquin LD, Thiagarajan D. Evaluation of soy protein in risk reduction for colon cancer and cardiovascular disease: Preliminary results. Second International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. September 15-18, 1996. Brussels, Belgium.

7. Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: the influence of soy-based formula. J Am Coll Nutr 1997;16:280-2.

8. Messina MJ, Persky V, Setchell KD, Barnes S. Soy intake and cancer risk: a review of the in vitro and in vivo data. Nutr Cancer 1994;21:113-31.

9. Rao CV, Wang C-X, Simi B, et al. Enhancement of experimental colon cancer by genistein. Cancer Res 1997;57:3717-22.

10. Barnes S. The chemopreventive properties of soy isoflavonoids in animal models of breast cancer. Breast Cancer Res Treat 1997;46:169-79 [review].

11. Hilakivi-Clarke L, Cho E, Onojafe I, et al. Maternal exposure to genistein during pregnancy increases carcinogen-induced mammary tumorigenesis in female rat offspring. Oncol Rep 1999;6:1089-95.

12. Lu LJ, Anderson KE, Grady JJ, Nagamani M. Effects of soya consumption for one month on steroid hormones in premenopausal women: implications for breast cancer risk reduction. Cancer Epidemiol Biomarkers Prev 1996;5:63-70.

13. Nagata C, Takatsuka N, Inaba S, et al. Effect of soymilk consumption on serum estrogen concentrations in premenopausal Japanese women. J Natl Cancer Inst 1998;90:1830-5.

14. Hargreaves DNF, Potten CS, Harding C, et al. Two-week dietary soy supplementation has an estrogenic effect on normal premenopausal breast. J Clin Endocrinol Metab 1999;84:4017-24.

15. Petrakis NL, Barnes S, King EB, et al. Stimulatory influence of soy protein isolate on breast secretion in pre- and postmenopausal women. Cancer Epidemiol Biomarkers Prev 1996;5:785-94.

16. McMichael-Phillips DF, Harding C, Morton M, et al. Effects of soy-protein supplementation on epithelial proliferation in the histologically normal human breast. Am J Clin Nutr 1998;68(suppl):1431S-6S.

17. Unfer V, Casini ML, Costabile L, et al. Endometrial effects of long-term treatment with phytoestrogens: a randomized, double-blind, placebo-controlled study. Fertil Steril 2004;82:145-8.

18. Marini H, Minutoli L, Polito F, et al. Effects of the phytoestrogen genistein on bone metabolism in osteopenic postmenopausal women: a randomized trial. Ann Intern Med 2007;146:839-47.