Why do the well-fed appear to die young?

Dietary restriction (DR) famously extends lifespan and reduces fecundity across a diverse range of species. A prominent hypothesis suggests that these life-history responses evolved as a survival-enhancing strategy whereby resources are redirected from reproduction to somatic maintenance, enabling organisms to weather periods of resource scarcity. We argue that this hypothesis is inconsistent with recent evidence and at odds with the ecology of natural populations. We consider a wealth of molecular, medical, and evolutionary research, and conclude that the lifespan extension effect of DR is likely to be a laboratory artifact: in contrast with captivity, most animals living in natural environments may fail to achieve lifespan extension under DR. What, then, is the evolutionary significance of the suite of responses that extend lifespan in the laboratory? We suggest that these responses represent a highly conserved nutrient recycling mechanism that enables organisms to maximize immediate reproductive output under conditions of resource scarcity.