The Soy Questions

Looking at the research behind soy’s potential pitfalls.

There are two camps regarding soy and its protein derivatives. One group lauds soy for its protein content and suggested ability to help fight cancer, obesity, heart disease, and insulin resistance. The other disparages soy for being an incomplete source of protein that raises estrogen levels and causes manboobs. Let’s take a closer look at the research to determine whether or not this form of protein is actually good for you and your body composition.

Does Soy Boost Estrogen Levels?

1R’s own Carolyn Brown shot down this supposed soy drawback in June. Nothing’s changed since then.

Is Soy an “Anti-Nutrient?”

Soybeans must be cooked before eating. One reason for this is the presence of enzyme-blocking proteins called trypsin inhibitors that prevent the body from absorbing nutrients. , Some speculate commercial products may not be adequately cleared of trypsin inhibitors, though their well-known nature, coupled with the sheer volume of research and monitoring performed on all grades of soy products, makes this unlikely. Several studies using commercial products discount this theory by indicating clear muscle growth. ,

Does Soy Cause Hypothyroidism?

Inadequate thyroid function, or hypothyroidism, can promote fat gain since thyroid hormones tell your body to burn calories. It can also cause depression, cholesterol problems, hair loss, and a host of other problems. Soy phytoestrogens have been linked to lowered thyroid hormones in human subject tests, though the effect is seen primarily in people who already have hypothyroidism, and to lessening the effect of thyroid drugs. One study on subjects with mild hypothyroidism found that two months of consuming extra soy and soy-derived substances in reasonable amounts significantly increased their risk of developing a more severe form.

This effect is less apparent in healthy people. A 2006 analysis of fourteen separate soy/thyroid studies showed little or no negative impact. Since then, two fairly large lab studies have supported this conclusion. One, a short-term study, gave subjects a daily portion of soybeans designed to emulate a soy-heavy Asian-inspired diet; among participants, there was no change in actual thyroid hormones, though in men one marker was slightly elevated. The second study looked at several months of soy protein powder consumption and its effect on men, and found no impact on thyroid hormones or markers.

How Effective is Soy for Muscle Growth?

Soy protein in various forms has been demonstrated to improve muscle growth in conjunction with training. While this is good for soy, some studies show soy versions coming up slightly short versus milk , , , and milk-derived proteins like whey and casein. It’s probably something that won’t see much more scientific attention because the difference (when reported) is so small.

One reason might be simple digestibility. Another is that soy protein’s amino acid profile, which doesn’t contain as much muscle-important aminos as whey, leads to smaller gains. The authors of one study believe the difference is caused entirely by soy proteins being routed away from skeletal muscle to repairing internal organs, while a second credits a combination of both this and soy’s weaker amino profile.

The Takeaway

Soy, like any other food, has drawbacks and benefits. Like most staple foods, its drawbacks seem to be apparent only when over-consumed, looked at under a microscope, or eaten by specific, limited populations. If you follow a vegan diet, soy intake is probably a requirement for maximal performance and fitness. Non-vegans can enjoy soy foods like tofu and edamame as part of their diet but folks with thyroid issues should avoid soy. However all of that said, I’d recommend supplementing whey protein over soy protein, though this is due more to soy’s cost than muscle-building efficacy.

References
Friedman, M and and Brandon, DL. Nutritional and Health Benefits of Soy Proteins. Journal of Agricultural and Food Chemistry.2001 49 (3), 1069-1086.
van der Ven, C., et al. Inactivation of Soybean Trypsin Inhibitors and Lipoxygenase by High-Pressure Processing.
Journal of Agricultural and Food Chemistry, 2005 53 (4), 1087-1092
Brown, EC, et al. Soy versus whey protein bars: effects on exercise training impact on lean body mass and antioxidant status. Nutrition Journal.2004 3:22.
Hartman, JW et al. Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters. American Journal of Clinical Nutrition. 2007 Aug;86(2):373-81.
Sathyapalan, T., et al. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study. The Journal of Clinical Endocrinology and Metabolism.2011 May;96(5):1442-9.
Messina, M and Redmond, G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature.Thyroid.2006 Mar;16(3):249-58.
Hampl, R. et al. Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects. Endocrine Regulations.2008 Jun;42(2-3):53-61.
Dillingham, BL et al.Soy protein isolates of varied isoflavone content do not influence serum thyroid hormones in healthy young men.Thyroid.2007 Feb;17(2):131-7.
Kalman, D. et al. Effect of protein source and resistance training on body composition and sex hormones. Journal of the International Society of Sports Nutrition.2007 Jul 23;4:4.
Phillips, SM et al. Dietary Protein to Support Anabolism with Resistance Exercise in Young Men. Journal of the American College of Nutrition.2005 Apr;24(2):134S-139S.
Phillips, SM et al. The Role of Milk- and Soy-Based Protein in Support of Muscle Protein Synthesis and Muscle Protein Accretion in Young and Elderly Persons .Journal of the American College of Nutrition.2009 Aug;28(4):343-54.
Hartman.
Wilkinson, SB et al.Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. The American Journal of Clinical Nutrition.2007 Apr;85(4):1031-40.
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Phillips, SM. The Role […]