Mating reconciles fitness and fecundity by switching diet preference in flies.

Protein-rich diets shorten lifespan but increase fecundity in many organisms. Animals actively adjust their feeding behavior to meet their nutritional requirements. However, the neural mechanisms underlying the dynamic regulation of protein consumption remain unclear. Here we find that both sexes of fruit flies exhibit a preference for protein food before mating to prepare for reproduction. Mated female flies display an increased appetite for yeast to benefit their offspring, albeit at the cost of stress resistance and lifespan. In contrast, males show a momentarily reduced yeast appetite after mating likely to restore their fitness. This mating state-dependent switch between sexes is mediated by a sexually dimorphic neural circuit labeled with leucokinin in the anterior brain. Furthermore, intermittent yeast consumption benefits both the lifespan and fecundity of males, while maximizing female fecundity without compromising lifespan.