Summary Dietary restriction (DR) is a key focus in ageing research. Specific conditions and genotypes were recently found to negate lifespan extension by DR, questioning the universal relevance of DR. However, the conceptual framework of dietary reaction norms explains why DR’s effects might not be apparent in some situations. When dietary reaction norms shift with genetic or environmental effects, a specific dyad of diets tested can result in a null effect. Only if a full reaction norm is tested can lifespan (or any trait) be shown to be refractory to diet. We tested comprehensively, for the first time, the importance of dietary reaction norms by measuring longevity and fecundity on five diets in five genotypes, with and without water supplementation, using high sample sizes in the fly (N>25,000). We detected 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 contrary to effects on lifespan, 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.