Circadian rhythms in the Zucker obese rat: assessment and intervention

Body temperature (Tb) and activity were recorded by telemetry in obese and lean Zucker rats in light-dark (LD), constant dark (DD) and constant light (LL). In LD, obese rats, by comparison with lean rats, exhibited a 2-4-h phase advance and attenuated amplitude of Tb and activity rhythms. These differences persisted on the first day of DD, and thus were not due to differential sensitivity to masking effects of light. In LL, obese and lean rats exhibited similar free-running periods, thus the phase advance in LD was also not due to a short intrinsic period. In LD, obese rats exhibited more diurnal food intake and a reduced LD intake ratio. To assess the role of diurnal feeding in weight gain, one group of obese rats was fed ad libitum, and another fed only at night. Food intake did not differ significantly between groups, but ad libitum fed rats gained 23% more weight (60 g) over 60 days, suggesting that excessive diurnal feeding may contribute adversely to body weight regulation in this animal model of obesity.