Caloric restriction does not enhance longevity in all species and is unlikely to do so in humans

Calorie restriction is known to increase lifespan in many but not all species and may perhaps not do so in humans. Exceptions to life extension have been identified in the laboratory and others are known in nature. Given the variety of physiological responses to variation in food supply that are possible, evolutionary life history theory indicates that an increased investment in maintenance in response to resource shortage will not always be the strategy that maximises Darwinian fitness. Additionally, for the well-studied species in which life extension is observed, there is considerable variation in the response. This suggests that it is not an ancient ancestral response, which has been conserved across the species range. Although calorie restriction does not increase lifespan in all species, it remains a fascinating and valuable tool to study ageing at the whole organism level.