The cold truth: torpor as a confound in studies of caloric restriction

Calorie restriction has been shown to dramatically extend lifespan in a range of species. Beyond longevity, calorie restriction is also reported to improve cognitive function, ameliorate neurodegeneration and peripheral nerve damage, reduce cancer incidence, and is commonly used to increase motivation in studies of behaviour. The mouse has been the most common species for these experiments and whilst efforts are ongoing to demonstrate the benefits of calorie restriction in humans, the evidence in mice is most compelling. Many mechanisms have been proposed for the beneficial effects of calorie restriction, but we note that one potentially important factor has seldom been considered: namely that mice readily enter torpor in response to food restriction. Torpor is a remarkable protective physiological state characterized by profound reductions in body temperature, oxygen consumption, heart rate, and activity. In this review, we describe the dietary protocols used to study the effects of calorie restriction and present the case that mice in these studies are highly likely to have entered torpor. We discuss the extent to which torpor might influence or mediate the measured outcomes. We highlight that induction of torpor is an important confound that is rarely, if at all, considered in calorie restriction research and make recommendations for the design and conduct of future studies.