How Much Soy is Healthy?

The problem is not so much that soy is not healthy, but that overconsumption of unfermented soy and genetically modified soy (see Who Screwed with My Soybeans?) may not be healthy. If people would limit their consumption of soy isoflavones to the amount the Asians typically eat—10 to 30 milligrams per day at most (and mainly fermented soy)—then there probably wouldn’t be such a strong debate about the health effects of soybeans, or the potential estrogenic effect on prostate tumor growth (see: Can soy cause cancer?)

But in the United States, some people eat as much as 80 to 100 milligrams of soy isoflavones a day in soy milk, protein shakes, soy protein bars and foods enriched with soy. In fact, soy has become so synonymous with ”health” that it is being packaged and marketed in all sorts and forms, often with un-natural and heavily processed ingredients. Kaayla Daniels, PhD, author of The Whole Soy Story, suggests in her book, for example, that the negative impact of soy on the thyroid is mostly seen when soy intake is more than 30 mg per day. (Daniels 2005)

Isoflavone levels in Some Foods (per 100 g, or 3.5 oz)

	Soybeans	200 mg (about ½ cup)
	Tempeh	43.52 mg (about ½ cup)
	Miso	7.0 mg (less than 1/3 cup)
	Peas	2.42 mg (1/2 cup)
	Peanuts	0.26 mg (3/4 cup)
	Navy beans (Haricot)	0.20 mg (1/2 cup)
	Chickpeas	0.10 mg (1/2 cup)
	Lentils	0.10 mg (1/2 cup)

(Source:USDA-Iowa State University Database on the Isoflavone, Rel. 1.3 – 2002)

In some cases, too much of a good thing may not be a good thing. The issue of concern is whether the mild estrogenic effect of soy consumption can add up when there is excessive intake of this food. Anything that has a hormonal effect on the body needs to be managed and monitored closely if you are at risk of a prostate disease.

How Much Soy is Too Much?

To use tempeh as an example, if you follow the suggestion to eat 1 to 2 servings of fermented soy per week, and you want to limit your isoflavone intake to 30 mg or less per day (or 210 mg per week), you can safely eat 2 four-ounce servings of tempeh per week and still be well below the 210 mg per week limit. (Here’s the math: 2 ounces of tempeh contains approximately 24 mg of isoflavones, so 4 ounces equals 48 mg eaten twice a week equals only 96 mg of isoflavones)

See also

Anti-angiogenic activity of soybean extract (Miura-2002)

Effects of a Genistein-Rich Extract on PSA Levels in Men With A History of Prostate Cancer

Genistein Polysaccharide effect on Prostate Tissue and Biochemical Recurrene Following Radical Prostatectomy

Isoflavone Preparation Demonstrates Anti-Prostate Cancer Activity in vivso and in vitro

Regression of Prostate Cancer Following Admin of GCP