Dietary restriction (DR), limiting calories or specific nutrients without malnutrition, extends lifespan across diverse taxa. Traditionally, this lifespan extension has been explained as a result of diet‐mediated changes in the trade‐off between lifespan and reproduction, with survival favored when resources are scarce. However, a recently proposed alternative suggests that the selective benefit of the response to DR is the maintenance of reproduction. This hypothesis predicts that lifespan extension is a side effect of benign laboratory conditions, and DR individuals would be frailer and unable to deal with additional stressors, and thus lifespan extension should disappear under more stressful conditions. We tested this by rearing outbred female fruit flies (Drosophila melanogaster) on 10 different protein:carbohydrate diets. Flies were either infected with a bacterial pathogen (Pseudomonas entomophila), injured with a sterile pinprick, or unstressed. We monitored lifespan, fecundity, and measures of aging. DR extended lifespan and reduced reproduction irrespective of injury and infection. Infected flies on lower protein diets had particularly poor survival. Exposure to infection and injury did not substantially alter the relationship between diet and aging patterns. These results do not provide support for lifespan extension under DR being a side effect of benign laboratory conditions.

9Dietary restriction (DR), limiting calories or specific nutrients, extends lifespan across 10 diverse taxa. This lifespan extension has been explained as diet-mediated changes in the trade-11 off between lifespan and reproduction, with survival favoured with scarce resources. Another 12 evolutionary hypothesis suggests the selective benefit of the response is the maintenance of 13 reproduction. This hypothesis predicts that lifespan extension is a side effect of benign 14 laboratory conditions, where DR individuals are frailer and unable to deal with additional 15 stressors, and thus lifespan extension should disappear under more stressful conditions. We 16 tested this by rearing outbred female Drosophila melanogaster on 10 different 17 protein:carbohydrate diets. Flies were either infected with a bacterial pathogen (Pseudomonas 18 entomophila), injured or unstressed. We monitored lifespan, fecundity and ageing measures. 19 DR extended lifespan and reduced reproduction irrespective of injury and infection. These 20 results do not support lifespan extension under DR being a side effect of benign laboratory 21 conditions. 22 23