Dietary restriction fails to extend life in stressful environments

Moderate dietary restriction often prolongs life in laboratory animals, and this response has been interpreted as an adaptive strategy that promotes survival during famine. However, dietary restriction can also increase frailty, and it therefore remains unclear whether restricted diets prolong life under stressful conditions like those experienced by wild animals. 
We manipulated adult dietary protein of Drosophila melanogaster across a gradient of ambient temperature and examined effects on survival. To test for trade-offs, we also quantified the reproduction, and performance of F1, F2 and F3 descendants.
We found that protein restriction increased longevity of one or both sexes at benign ambient temperatures (25°C, 27°C) but failed to extend longevity of flies maintained in cold (21°C, 23°C) or hot (29°C) conditions. Instead, in females, protein restriction resulted in strongly elevated mortality at cold temperatures. Protein restriction also generally reduced reproductive performance and did not consistently enhance performance of F1, F2 or F3 descendants. 
Taken together, our results challenge the long-held idea that extended longevity of diet-restricted laboratory animals represents an adaptive survival strategy in natural populations. Our findings suggest instead that this response is an artefact of benign laboratory conditions, and that DR-induced life extension might not be achieved in the more stressful conditions experienced in the wild.