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Is Soy Protein Harmful?

Soy Protein Supplements in Weight Training

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Updated June 10, 2014

soya beans

Soya beans

Photo © Chrys Omori, Flickr

The soya bean is a widely grown crop throughout the world. It's value is derived from its high protein content, which is higher than all other beans grown for human consumption. The soya bean is also a complete protein, having all essential amino acids in useful proportions.

"Soy" and "soya" are interchangeable, although some cultures tend to use one more than the other.

Soy products are food staples in some parts of Asia, and soy protein is used extensively in manufactured products in many countries. These products include soy milk, meat substitutes tofu and fermented products miso, tempeh and soy sauces.

Protein Powders from Milk, Egg and Soy

Bodybuilders and weight trainers like to take commercial protein supplement powders to support their training and, hopefully, build muscle. Most protein powders are derived from either the milk proteins casein and whey, egg protein, or the extracted soy protein from soya beans.

Of course, there is great debate among purists on the best type of protein powder and also the best commercial brands for athletes and weight trainers. Many varied formulas of protein powder supplements exist. Whether you actually need to take special protein powder supplements is another discussion. Usually, skim milk powder will do a satisfactory job at less cost.

The Trouble with Soy

Soy contains plant estrogens called "soy isoflavones." These chemical compounds have biological effects similar to human estrogen hormones but are mostly of lesser potency. Soy products are often recommended to women to blunt the effects of menopause. This recommendation is controversial as to its worth.

Bodybuilders don't like excess estrogen because, they theorize, too much may inhibit the effects of testosterone or make them store fat. They want to maximize muscle and strength and minimize fat. As a result, many bodybuilders, weight trainers and athletes won't use soy protein foods or protein supplements because they fear this negative effect on body and performance. This could well apply to women and men.

The discussion of soy safety and efficacy comes up time and time again on weight training forums.

Can Soy be Eaten Safely?

In my view, certainly it can. The optimum quantity of isoflavone consumption for health is not known. An important point to note is that plant estrogens (phytoestrogens) don't necessarily add to the estrogenic effect when you consume them. Each body cell has what are called "receptors" to catch and draw in chemicals, such as hormones, in to the cell so that they can do their work. Consuming soy isoflavones could actually compete with and replace the more potent natural body estrogens for the cell receptor and result in a lessening of estrogenic effect. In this case, the muscle builders — at least theoretically — may benefit from soy consumption.

Scientists have studied the effects of soy consumption on male and female fertility, menopause effects, male hormones and thyroid effects. A reasonable summary of this evidence would be that there is no acute or outstanding hazard from moderate soy consumption in either sex. In fact, some studies suggest benefits for heart health and in cancer protection. Moderate consumption could be two or fewer servings a day.

Even so, one recent study found a decrease in testosterone levels in 12 men consuming two scoops (56 grams, 2 ounces) of soy protein powder a day for 28 days. This is a very small number of participants on which to base any firm conclusions. Most other studies have found no effects or small or nonsignificant effects on male hormones.

I know of no convincing evidence that soy protein powder, in moderate amounts, inhibits muscle development at all in weight trainers and bodybuilders.

Soy and Cancer

The evidence is mixed on this one, but it is possible that low to moderate levels of soy may protect against breast cancer. The caution is that higher levels of isoflavones could have the opposite effect, especially in women with existing and perhaps undiagnosed breast cancer. In men, some evidence exists for a protective effect of soy on prostate cancer.

Moderate Soy Consumption

If you want to eat soy for its benefits but worry about possible adverse effects, here is a possible plan:
  • Don't take concentrated isoflavone supplements in tablet or capsule form or in super-concentrated soy bars or modified foods.
  • If you use soy protein powders, cycle soy protein, say, only every second or third day, or, one week on, one week off.
  • Consume low-fat soy milk in preference to full fat.
  • Limit soy consumption to no more than 2 servings a day.
  • The Soy Food Council has a list of isoflavone values in soy foods.

Sources:

Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002 Jun;110 Suppl 3:349-53. Review.
Goodin S, Shen F, Shih WJ et al. Clinical and biological activity of soy protein powder supplementation in healthy male volunteers. Cancer Epidemiol Biomarkers Prev. 2007 Apr;16(4):829-33.
Kurzer MS. Hormonal effects of soy in premenopausal women and men. J Nutr. 2002 Mar;132(3):570S-573S.
Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006 Apr 5;98(7):459-71.
iYan L and Spitznagel E. Meta-analysis of soyfood and risk of prostate cancer in men. International Journal of Cancer. Volume 117, Issue 4, pages: 667-669, November, 2005.

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