Whole countries use soy as a primary source of protein and many processed foods add it. In 1999, the FDA allowed products containing at least 6.25 g of soy to be labeled as having heart healthy benefits, a position the American Heart Association reviewed in 2006, saying that earlier benefits had not been confirmed. Many health experts are pondering the question of how much soy is good and how much is too much.
Ingredients
The ingredients in soy that may have both benefits and risks include isoflavones, a type of phytoestrogen. Phytoestrogens have similar properties to estrogen, but do not always behave like estrogen in the body. In some cases, they bind to estrogen receptors and block the effects of estrogen. Soy is also a good source of dietary protein and is high in polyunsaturated fats, the type of fat that can reduce cholesterol levels and fiber, which can also reduce cholesterol levels.
Amounts
In Asian countries where soy is a main source of dietary protein, people consume up to 10 to 50 g per day, Stephen Barnes, Ph.D., of the University of Alabama at Birmingham, reports. Asian cultures also eat soy differently; soy is fermented, in many cases, before eating. The Harvard School of Public Health suggests eating in soy in moderation as a replacement for red meat two to four times a week. Both the USDA and the American Heart Association recommend an intake of 25 g per week.
Concerns
Concerns about the cumulative effects of eating large amounts of isoflavones center around its effects on reproductive organs and cancers, particularly breast and prostate cancer. Several cases of hypothyroidism, or low thyroid functioning have occurred in infants taking soy formula. Soy can lower iodine levels, according to the University of Maryland Medical Center. Because soy also contains more phosphorus and potassium than meat, people with kidney disease may need to restrict their use of soy. Phosphorus and potassium levels can rise to dangerous levels when the kidney's ability to filter waste products decreases. Ask your doctor about eating soy if you have kidney disease.
Evidence
A meta-analysis of 15 studies conducted by researchers from the USDA on the effects of soy on prostate cancer was published in the February 2009 issue of "The American Journal of Clinical Nutrition." Researchers concluded that soy consumption reduced the risk of prostate cancer overall by 26 percent in the studies reviewed. However, because laboratory studies are still not clear on whether soy blocks the effects of estrogen or stimulates them, use soy cautiously if you have or are at risk for reproductive cancers.
References
- University of Maryland Medical Center: Soy; Steven Ehrich; December 2009
- "Circulation"; Soy Protein, Isoflavones, and Cardiovascular Health ; Frank Sacks, M.D., et al.; 2006
- MayoClinic.com: Soy; December 2010
- "The American Journal of Clinical Nutrition;" Soy Consumption and Prostate Cancer Risk in Men: A Revisit of Meta-Analysis.; L. Yan; February 2009
- Harvard School of Public Health: Protein: Moving Closer to Center Stage; 2011
- NIH Record; Researcher Barnes Offers Compelling Reasons to Eat Soy; Valerie Lambros; January 2011
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