Ageing impairs protein leveraging in a sex-specific manner in Drosophila melanogaster

Modifying the relative proportions of macronutrients in an animal's diet has noteworthy effects on its reproduction, lifelong health and life span. Because of this, a wide range of animals carefully regulate their nutrient intake towards species and stage-specific targets. However, when animals are unable to reach their nutrient target from their existing food resources, they will compromise between overconsuming one nutrient and under-consuming the nutrient in deficit. In this study, we used capillary feeding (CAFE) assays to understand the rules of compromise of adult fruit flies, Drosophila melanogaster, of different sex, mating status and age when constrained to single diets. We found that young male and female D. melanogaster compromised by consuming more food on diets with low protein to carbohydrate (P:C) ratios compared to diets with high P:C ratios. Further, young male and female flies varied their carbohydrate intake significantly more than their protein intake, and female flies varied their carbohydrate intake significantly more than males. To test for effects of mating status on nutrient intake, we compared food intake of young mated and virgin females. We found that both virgin and mated females compromised by consuming more food on the low P:C diet compared to high P:C diets; however, mated females consumed more food than virgin females. As flies aged, they decreased their overall food intake and showed more modest alterations in their food intake across varying P:C diets. Further, mated females ceased to compromise for the protein deficit at a younger age than males. These findings provide new understanding about differences in protein leveraging behaviour across the sexes, and how these behaviours change with age.