The protein theory is bad news for people on low-carbohydrate weight-loss plans like the Atkins diet. "I'd be wary of diets that put a heavy emphasis on protein," says Piper. "It's hard to see how that could be healthy."
Fontana goes one step further, saying that high-protein diets could risk accelerated ageing and cancer. It's good news, however, for people already on low-protein diets, like vegans, who avoid eating meat, eggs and dairy products. In 2007, Fontana showed that vegans have lower levels of IGF-1 than meat-eaters.
There may be another reason for vegans to celebrate. Studies on flies and rodents suggest that cutting intake of one particular amino acid, called methionine, lengthens life to a similar degree as calorie restriction. Proteins in meat and other animal products have high levels of methionine, so a vegan diet would score well by that measure, too.