While animal proteins increase levels of the cancer-promoting growth hormone IGF-1, and most plant proteins bring levels down, “high quality” plant proteins such as soy may not significantly affect levels in either direction. This, however, may depend on the quantity consumed.
If animal proteins are bad because they boost our blood levels of the cancer promoter IGF-1, what about the few plant proteins that just coincidently happen to have amino acid ratios similar to animal proteins, like soy? One of soy’s selling points is that it has quote unquote “high quality” protein. But now we know that from a cancer perspective higher quality may mean a higher quantity of cancer risk, thanks to IGF-1.
Let’s go back to this study. Those who ate a lot of animal protein had significantly higher levels of IGF-1. And those that ate lots of nonsoy plant protein had significantly lower levels, presumably because it substituted for some of the animal protein in their diets. The same thing might have happened a little with soy—hey at least you’re not eating animal protein, but this was not a significant decrease, meaning if all we do is just swap out animal protein and swap in soy protein, we may not see that beautiful drop in IGF-1 enjoyed by those replacing animal protein instead with a variety of different plant proteins.
Indeed the more soy milk, for example, that vegan women drank, the higher their IGF levels tended to be, but the trend was only of borderline statistical significance, meaning it could have just been due to chance. To test this once and for all would require the combined might of both the Ornish and Pritikin research teams. a study we'll cover tomorrow.
To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring watch the above video. This is just an approximation of the audio contributed by Kerry Skinner.
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See Monday's video Protein Intake and IGF-1 Production and yesterday's video-of-the-day Higher Quality May Mean Higher Risk for some immediate background and IGF-1 as One-Stop Cancer Shop for the reason we'd like to see these levels low in adulthood (though not in childhood—see Cancer-Proofing Mutation). Is there a level of soy food consumption at which one might see IGF-1 levels comparable to those induced by animal protein? The title of tomorrow's video-of-the-day kind of gives it away: Too Much Soy May Neutralize Plant-Based Benefits.
For some context, please check out my associated blog post: How Much Soy Is Too Much?
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