Non-canonical Metabolic and Molecular Effects of Calorie Restriction Are Revealed by Varying Temporal Conditions

Calorie restriction (CR) extends lifespan and healthspan in diverse species. However, comparing ad libitum (AL) and CR-fed mice is challenging due to their significantly different feeding patterns. CR-fed mice consume their daily meal in approximately 2 hours, subjecting themselves to a prolonged self-imposed fast each day. To gain deeper insights into the effects of CR, we conducted a comprehensive examination of how AL and CR-fed mice respond to tests performed at various times relative to the completion of their once-daily CR meal. Our findings reveal that many well-known effects of CR, including its impact on insulin sensitivity, result from the specific temporal conditions. CR animals exhibit a divergent response to insulin, and this response varies based on the time elapsed since the CR-fed mice consumed their food. Utilizing an unbiased metabolomics approach, we discovered that the effects of CR on circulating metabolites are heavily dependent upon the time-of-day and feeding regimen. Finally, while it is widely believed that CR functions in part by reducing activity of the kinase mTORC1, our study suggests that the observed differences in mTORC1 activity between AL and CR-fed mice are dependent upon both fasting duration and the specific tissue examined. Furthermore, we find that the metabolic effects of CR are independent of hepatic mTORC1. Our results shed new light on the physiological, metabolic, and molecular effects of a CR diet, and highlight that much of our understanding of the effects of CR are related to when, relative to feeding, we choose to examine the mice.