Caloric restriction increases longevity substantially only when the reaction norm is steep.

Evolutionary theory leads to the general expectation that dietary restriction will often result in increased survival probabilities, and thus increased lifespan. The reaction norm is a basic tool of evolutionary analysis that quantifies the relationship between environmental parameters and functional characters, including reproduction and longevity. In rodents, the reaction norm connecting adult longevity to caloric intake is fairly steep; small changes in intake lead to large changes in longevity. If this strong quantitative relationship were evolutionarily conserved among all mammals, then the prospects for a substantial increase in human lifespan from caloric restriction would be very good. In theory, however, reaction norms are expected to evolve for fitness related characters such as reproduction and survival. It has been shown experimentally in Drosophila that dietary reaction norms readily evolve in the laboratory, suggesting that they can do so among mammals as well, particularly over the millions of years since contemporary rodents and primates last shared a common ancestor. Our previous work crudely estimates that the dietary reaction norms of rodents and humans have diverged substantially, with a very flat dietary reaction norm for human longevity. These general principles and our specific results suggest that the benefits from human caloric restriction would be minor.