A new “dietary restriction” (not just calorie restriction) theory about how diet affects aging suggests that the drop in calories is not solely responsible for lifespan extension — in some species at least, perhaps it is also the accompanying drop in dietary protein.

Protein restriction is much less difficult to maintain than calorie restriction and may be more powerful in reducing insulin-like growth factor 1 (IGF-1) in humans (a promoter of aging), says Luigi Fontana, a professor of medicine at Washington University and head of the Division of Nutrition and Aging at the Italian National Institute of Health.

Fontana draws his conclusions from his studies of people who are practicing calorie restriction (“CRONies” — short for Calorie Restriction with Optimal Nutrition).

Vegans and vegetarians have another advantage: proteins in meat and other animal products have high levels of methionine; studies show that cutting methionine lengthens life to a similar degree as calorie restriction.