"One of the benefits and concerns about high protein intake, especially animal protein, is that it tends to make cells multiply faster. That’s good in early life, when you’re a growing child. But in later life, this is one of the fundamental processes that increase the risk of cancer,” says Dr. Walter Willett, chairman of the department of nutrition at the Harvard School of Public Health.
Roni Caryn Rabin discusses in her New York Times article why too much protein might not be a good idea at all. Protein powders and dietary supplements can be especially troublesome.