Soy

Clearing Up the Controversies

By Alan Aragon

The soy bean, also called soya bean, is a legume native to East Asia. It was introduced to the United States in the mid 1700s but has been consumed throughout Asia long before written records. In more recent times, soy has gained a dual-reputation of being a “health” food as well as some sort of evil thing to avoid. In this article, I’ll look at the common positive and negative lore behind soy and clear up the confusion with current research-based evidence.

Does soy have feminizing effects on men?

Soy contains isoflavones, which have potentially estrogenic effects. This has raised the concern that men consuming soy might experience adverse hormonal changes. A recent meta-analysis (systematic examination of multiple studies) by Johns Hopkins University researchers found that neither soy foods nor isoflavone supplements significantly alter testosterone levels in men [1]. Of interest to the athletic population, 12 weeks of supplementation with soy protein did not decrease testosterone levels or hinder lean body mass gains in men engaged in a resistance training program [2]. Therefore, the evidence does not support the idea that soy is likely to cause feminizing effects in men.

Does soy protect against breast cancer in women?

Breast cancer rates among women in Asian countries are substantially lower than those among women in Western countries. This has lead to a lot of investigation of dietary factors that might be involved. Since soy is a staple food throughout Asia, it’s under ongoing study as a possible protective food. Three separate meta-analyses came to the similar conclusion that soy intake may be associated with a decreased risk of breast cancer [3-5]. Although this area of study has been focused on postmenopausal women, there’s fairly strong research evidence that soy is protective against breast cancer in premenopausal women as well [6].

Does soy protect against prostate cancer?

Prostate cancer is the leading type of cancer in men, occurring mostly in the older population. The most recent meta-analyses involving various study designs show that consumption of soy foods is associated with a reduction in prostate cancer risk in men [7,8].

Does soy adversely affect thyroid function?

A comprehensive literature review done by researchers at Loma Linda University concluded that there’s not enough evidence that in iodine-replete individuals with normally functioning thyroids, soy foods, or isoflavones, do not adversely affect thyroid function [9]. However, in hypothyroid patients, some evidence suggests that soy foods taken in excess may increase the dose of thyroid hormone medication required by hypothyroid patients. The review also states that hypothyroid adults need not avoid soy foods completely. Still, there’s a theoretical concern that in individuals with compromised thyroid function, soy foods may increase the risk of developing hypothyroidism. Therefore, it’s important for soy consumers who have known thyroid issues to make sure they moderate their intake of soy. It’s also been suggested to make sure that iodine is adequate, but iodine deficiency is rare in industrialized countries due to enrichments in the food supply.

Does soy protect against osteoporosis?

The majority of the research done on soy’s effect on bone has been on menopausal women. As indicated by two recent meta-analyses [10,11], the bulk of the evidence shows that the intake of soy increases bone mineral density and stimulates bone formation.

Does soy protect against cardiovascular disease?

This area of study is not as clear-cut as the rest, since most of it is based on epidemiological studies. This type of research is uncontrolled, and thus it’s limited by many possible unaccounted variables. Nevertheless, soy intake has been positively correlated with the prevention of cardiovascular disease [12]. Part of this mechanism may be attributed to soy isoflavones’ ability to lower LDL and total cholesterol [13].

Tying it together

Other than the potential concerns in those with pre-existent hypothyroidism, some individuals are allergic to soy and soy-based products. This is more common in children than adults. Although it’s not common, some individuals might find difficulty digesting soy foods. As well, soy might not suit everyone’s personal taste. Soy has been a staple part of some of the healthiest regions in the world, including Asian Pacific populations like the Okinawans – the longest-living population on the planet. It’s not surprising that research has for the most part supported its numerous health claims, including protection against osteoporosis (in menopausal women), prostate cancer, and breast cancer. Finally, concerns of soy having a testosterone-lowering effect on men have not been solidly supported in the scientific literature. Bottom line: consumption of soy foods as a part of a balanced diet can contribute to good health, and much of indictments by the alarmist media are unfounded.

Alan Aragon has over 15 years of success in the fitness field. He currently is the lead nutritionist at Elite Fitness Plus in Westlake Village, California. www.elitefitnessplus.com

References

Hamilton-Reeves JM, et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2009 Jun 11. [Epub ahead of print]
Kalman D, et al. Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr. 2007 Jul 23;4:4.
Qin LQ, et al. Soyfood intake in the prevention of breast cancer risk in women: a meta-analysis of observational epidemiological studies. J Nutr Sci Vitaminol (Tokyo). 2006 Dec;52(6):428-36.
Trock BJ, et al. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006 Apr 5;98(7):459-71.
Badger TM, et al. Soy protein isolate and protection against cancer. J Am Coll Nutr. 2005 Apr;24(2):146S-149S.
Lee SA, et al. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 2009 Jun;89(6):1920-6. Epub 2009 Apr 29.
Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr. 2009 Apr;89(4):1155-63. Epub 2009 Feb 11.
Hwang YW, et al. Soy food consumption and risk of prostate cancer: a meta-analysis of observational studies. Nutr Cancer. 2009;61(5):598-606.
Messina M, 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.
Ma DF, et al. Soy isoflavone intake inhibits bone resorption and stimulates bone formation in menopausal women: meta-analysis of randomized controlled trials. Eur J Clin Nutr. 2008 Feb;62(2):155-61. Epub 2007 Mar 28.
Ma DF, et al. Soy isoflavone intake increases bone mineral density in the spine of menopausal women: meta-analysis of randomized controlled trials. Clin Nutr. 2008 Feb;27(1):57-64. Epub 2007 Dec 11.
Rimbach G, et al. Dietary isoflavones in the prevention of cardiovascular disease--a molecular perspective. Food Chem Toxicol. 2008 Apr;46(4):1308-19.
Taku K, et al. Effects of extracted soy isoflavones alone on blood total and LDL cholesterol: Meta-analysis of randomized controlled trials. Ther Clin Risk Manag. 2008 Oct;4(5):1097-103.