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Soy Protein for Sports & Fitness

Also known as: Daidzein, Genistein, Glycine Max, Isoflavone, Protein [Soy], Soya

Illustration

What is it?

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.

Where is it found?

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.

Why do athletes use it?*

Some athletes say that soy protein

  • provides a source of low-fat, cholesterol-free protein.

What do the advocates say?*

Protein is necessary for rebuilding tissue, which is especially important for bodybuilders. All types of protein provide the body with amino acids.

From an athletic point of view, soy is just another type of protein. Choosing soy protein over other types of protein adds variety to the protein choices available. However, because soy lowers cholesterol, it is an excellent choice for those also trying to lower their cholesterol levels.

Soy is a common ingredient in many meal replacements that are designed to provide a large amount of nutrients in a minimal amount of calories.

How much is usually taken by athletes?

Adding protein to carbohydrate intake immediately after exercise may be helpful for improving recovery of glycogen (carbohydrate) stores after exercise, according to some,1 2 3 though not all,4 5 6 7 8 controlled studies. Preliminary studies have suggested that protein supplements may have biological effects that could improve muscle growth resulting from strength training, especially if liquid supplements (typically containing at least 10 grams of protein in addition to varying amounts of carbohydrate) are taken immediately after exercise.9 10 11 However, controlled studies have found no advantage of protein supplementation (up to about 100 grams per day or about 14 grams immediately following exercise) for improving strength or body composition as long as the diet already supplies typical amounts of protein and calories.12 13 14 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.15 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 to a diet that was adequate in protein during an 8-week training program did not improve lean body mass compared to a placebo.16 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.

Are there any side effects or interactions?

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.17 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.18 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 function19 or produce no change in thyroid function.20 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.21

Most research, including animal studies, report anticancer effects of soy extracts,22 though occasional animal studies have reported cancer-enhancing effects.23 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.24 When pregnant rats were given genistein injections, their female offspring were reported to be at greater risk of breast cancer.25 Although premenopausal women have shown decreases in estrogen levels in response to soy,26 27 pro-estrogenic effects have also been reported.28 When pre-menopausal women were given soy isoflavones, an increase in breast secretions resulted—an effect thought to elevate the risk of breast cancer.29 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.30

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.31 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.

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

Are there any drug interactions?
Certain medicines may interact with soy. Refer to drug interactions for a list of those medicines.

Resources

See a list of books, periodicals, and other resources for this and related topics.

*Athletes and fitness advocates may claim benefits for soy protein based on their personal or professional experience. These are individual opinions and testimonials that may or may not be supported by controlled clinical studies or published scientific articles on soy protein. For more complete and detailed information, including references and safety information, see Soy as a nutritional supplement.

References:

1. Ivy JL, Goforth HW Jr, Damon BM, et al. Early postexercise muscle glycogen recovery is enhanced with a carbohydrate-protein supplement. J Appl Physiol 2002;93:1337–44.

2. Ivy JL. Glycogen resynthesis after exercise: effect of carbohydrate intake. Int J Sports Med 1998;19:S142–5 [review].

3. Cade JR, Reese RH, Privette RM, et al. Dietary intervention and training in swimmers. Eur J Appl Physiol 1991;63:210–5.

4. Jentjens RL, van Loon LJ, Mann CH, et al. Addition of protein and amino acids to carbohydrates does not enhance postexercise muscle glycogen synthesis. J Appl Physiol 2001;91:839–46.

5. van Hall G, Shirreffs SM, Calbet JA. Muscle glycogen resynthesis during recovery from cycle exercise: no effect of additional protein ingestion. J Appl Physiol 2000;88:1631–6.

6. Carrithers JA, Williamson DL, Gallagher PM, et al. Effects of postexercise carbohydrate-protein feedings on muscle glycogen restoration. J Appl Physiol 2000;88:1976–82.

7. van Loon LJ, Saris WH, Kruijshoop M, Wagenmakers AJ. Maximizing postexercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures. Am J Clin Nutr 2000;72:106–11.

8. Roy BD, Tarnopolsky MA. Influence of differing macronutrient intakes on muscle glycogen resynthesis after resistance exercise. J Appl Physiol 1998;84:890–6.

9. Levenhagen DK, Gresham JD, Carlson MG, et al. Postexercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis. Am J Physiol Endocrinol Metab 2001;280:E982–93.

10. Kraemer WJ, Volek JS, Bush JA, Ivy JL. Hormonal responses to consecutive days of heavy-resistance exercise with or without nutritional supplementation. J Appl Physiol 1998;85:1544–55.

11. Chandler RM, Byrne HK, Patterson JG, et al. Dietary supplements affect the anabolic hormones after weight-training exercise. J Appl Physiol 1994;76:839–45.

12. Rozenek R, Ward P, Long S, Garhammer J. Effects of high-calorie supplements on body composition and muscular strength following resistance training. J Sports Med Phys Fitness 2002;42:340–7.

13. Williams AG, van den Oord M, Sharma A, Jones DA. Is glucose/amino acid supplementation after exercise an aid to strength training? Br J Sports Med 2001;35:109–13.

14. Lemon PW, Tarnopolsky MA, MacDougall JD, Atkinson SA. Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. J Appl Physiol 1992;73:767–75.

15. Esmarck B, Andersen JL, Olsen S, et al. Timing of postexercise protein intake is important for muscle hypertrophy with resistance training in elderly humans. J Physiol 2001;535:301–11.

16. Stroescu V, Dragan J, Simionescu L, Stroescu OV. Hormonal and metabolic response in elite female gymnasts undergoing strenuous training and supplementation with SUPRO Brand Isolated Soy Protein. J Sports Med Phys Fitness 2001;41:89–94.

17. Divi RL, Chang HC, Doerge DR. Antithyroid isoflavones from soybean. Biochem Pharmacol 1997;54:1087–96.

18. 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].

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

20. 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.

21. 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.

22. 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.

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

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

25. 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.

26. Lu L-JW, 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.

27. 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.

28. 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.

29. 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.

30. 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.

31. 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.
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