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How Much Soy Is Too Much?

If animal proteins promote cancer because they boost our blood levels of the growth hormone IGF-1, what about the few plant proteins that have amino acid ratios similar to animal proteins such soy foods? One of soy’s selling points is that it has “high quality” protein, but as we saw in my Care2 article on Wednesday, Animal Protein and IGF-1, higher quality may mean a higher risk of cancer.

In my 2-min. video, Animalistic Plant Proteins, I show that animal protein consumption is associated with significantly higher levels of the cancer promoter IGF-1 and non-soy plant protein is associated with significantly lower levels. There was no significant association with soy protein, though. This suggests that if all we do is replace animal protein with soy protein, we may not see as dramatic a drop in IGF-1 as that enjoyed by those replacing meat, eggs, and dairy with a variety of plant proteins. You can see what lower IGF-1 levels can mean for prostate and breast cancer growth in my videos Ex Vivo Cancer Proliferation Bioassay and The Answer to the Pritikin Puzzle.

In my video pick above I show that vegans consuming the equivalent of 7-18 servings of soy foods per day may end up with circulating IGF-1 levels comparable to those who eat meat. For more on the cancer reversal study mentioned in the video, see Cancer Reversal Through Diet. It’s important to remember in this discussion that soy food consumption is associated with an array of health benefits. See, for example, Breast Cancer Survival and SoyThe Effect of Soy On Precocious Puberty, and Soy Foods & Menopause. But how much may be too much?

From the above video it seems that 7 to 18 servings of soy a day may neutralize some of the beneficial effects of avoiding animal protein. At the same time, studies have repeatedly found that women who eat a lot of soy appear to have a lower risk of getting breast cancer and a better chance of surviving breast cancer than those who don’t. So is there some magic number of soyfood servings we should shoot for?

In my 2-min. video, How Much Soy Is Too Much, I run through all the studies to date that have measured the effects of varying levels of soy consumption on IGF-1 levels. Five to ten servings per day increased IGF-1 levels, but two to three servings did not. The bottom-line is that legumes are one of the healthiest things we can eat and should be a part of everyone’s daily diet. This means lentils, peas, and/or beans every day—in fact, ideally every meal! Soy is an excellent choice, but we should probably stick to no more than 3-5 servings a day.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my videos for free by clicking here and watch my full 2012 presentation Uprooting the Leading Causes of Death.

Image credit: Andrea_Nguyen / Flickr

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Dr. Michael Greger

A founding member of the American College of Lifestyle Medicine, Michael Greger, M.D., is a physician, author, and internationally recognized speaker on nutrition, food safety, and public health issues. Currently Dr. Greger serves as the Director of Public Health and Animal Agriculture at The Humane Society of the United States. Hundreds of his nutrition videos are freely available at


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5:47PM PDT on Mar 29, 2014

When it comes to a vegetarian diet legumes (excluding soybeans) particularly lentils and chickpeas are far superior to soy in protein content, taste ,and health benefits.

5:58AM PDT on Mar 29, 2013

Balance is the trick

3:33PM PDT on Mar 28, 2013

Again, it would seem that moderation in everything we eat is the best policy. I'm guessing humans are the only animal to take a good thing and overdo until it becomes a bad thing.

5:10PM PDT on Mar 27, 2013


11:34AM PDT on Mar 27, 2013

How much soy is too much? I ask myself this every week!

Soy is so addictive with the veggie burgers, veggie hot dogs, veggie sandwiches, veggie this and veggie that.

6:52PM PST on Dec 13, 2012

Thank you Dr, Michael Greger, for Sharing this!

9:09PM PDT on Oct 23, 2012

thanks didnt know that

12:21AM PDT on Oct 22, 2012

interesting things, tks.

8:04AM PDT on Oct 18, 2012

ANY soy unfermented (as 3000+ years of chinese history tested) is too much soy. period.

Falling for the soy ploy

Ravi, Daiasolgaia

2:41AM PDT on Oct 15, 2012

I read in one of my women's magazines (Womans Day) where a study was cited at a University(sorry I don't remember the University, only the content of the article) that stated "more than 1.5 grams of soy flavanoids significantly inhibits thyroid function". I was suprised at the amount as well, but I swear it stated 1.5 grams of soy isoflavanoids. In fact it was compared to flouride(never swallow any toothpaste or flouride rinse) in its ability to inhibit thyroid function.

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