Dietary protein modulates circadian changes in core body temperature and metabolic rate in rats

e assessed the contribution of dietary protein to circadian changes in core body temperature (T-b) and metabolic rate in freely moving rats. Daily changes in rat intraperitoneal temperature, locomotor activity (LMA), whole-body oxygen consumption (VO2), and carbon dioxide production (VCO2) were measured before and during 4 days of consuming a 20% protein diet (20% P), a protein-free diet (0% P), or a pair-fed 20% P diet (20% P-R). Changes in T-b did not significantly differ between the 20% P and 20% P-R groups throughout the study. The T-b in the 0% P group remained elevated during the dark (D) phase throughout the study, but VO2, VCO2, and LMA increased late in the study when compared with the 20% P-R group almost in accordance with elevated T-b. By contrast, during the light (L) phase in the 0% P group, T-b became elevated early in the study and thereafter declined with a tendency to accompany significantly lower VO2 and VCO2 when compared with the 20% P group, but not the 20% P-R group. The respiratory quotient (RQ) in the 0% P group declined, throughout the D phase and during the early L phase. By contrast, RQ in the 20% P-R group consistently decreased from the late D phase to the end of the L phase. Our findings suggest that dietary protein contributes to the maintenance of daily oscillations in T-b with modulating metabolic rates during the D phase. However, the underlying mechanisms of T-b control during the L phase remain obscure.