Heat balance of rats acclimated to diurnal 2-hour feeding.

Heat production (M) and heat loss (H) of ad lib fed rats and of rats fed for 2-hr daily for 2 months (2-hr fed rats) were simultaneously measured by direct and indirect calorimetry over a period of 48 hours. The ad lib fed rats showed a clear nocturnal increase in M and H, which consisted of several discrete increases associated with discrete rises in feeding activity. In each increase, however, M and H were somewhat out of phase with each other. During feeding, a positive shift of heat storage occurred, which was repaid by H being higher than M thereafter. No such increases of M and H were observed in either the fasted ad lib fed rats or the 2-hr fed rats. The 2-hr fed rats showed a very large increase in M and H following the meal, which obscured the nocturnal increase of metabolic rate. Without food, this diurnal increase was greatly reduced but not to the minimum level, and two definite peaks of M and H associated with increased food exploratory activity, one in the day and the other at night, were obtained. There was a sharp fall in RQ 1-2 hr prior to the diurnal feeding. These observations suggest that, besides diet-induced heat production, feeding can be an additional time cue for increasing energy metabolism, but a more basic biological clock mechanism synchronizing the day-night cycle drives circadian rhythms of physical activity and biochemical processes related to energy expenditure in rats.