The relationship between longevity and diet is genotype dependent and sensitive to desiccation

Dietary restriction (DR) is a key focus in ageing research. Specific conditions and genotypes were recently found to negate lifespan extension by DR, questioning its universal relevance. However, the conceptual framework of dietary reaction norms explains why DRs effects might not be apparent in some situations. We tested comprehensively the importance of dietary reaction norms by measuring longevity and fecundity on five diets in five genotypes, with and without water supplementation in the fly (N>25,000). We found substantial genetic variation in the reaction norm between diet and lifespan. Environments supplemented with water rescued putative desiccation stress but only at the richest diets. Fecundity declined at these richest diets, but was unaffected by water and is thus most likely caused by nutritional toxicity. Our results demonstrate empirically that any conclusion on the absence of DR is only justified when a range of diets is considered in a reaction norm framework.