The effect of wheel running and the estrous cycle on energy expenditure in female rats.

In female rats, food intake and wheel running fluctuate with the estrous cycle, creating a state of oscillating energy balance. The effect of the estrous cycle on the energy expenditure of conventionally housed rats (S) and of rats given access to an activity wheel (A) was compared. Over 24 weeks, the weight gain of the groups was similar although the body fat content of A was 30 g lower than that of S at the end of the study. During the assessment of energy expenditure by indirect calorimetry estrous cycles were disrupted and food intake and wheel running reduced. However, a correlation between expenditure and wheel running was obtained from which the cost of activity was derived (21 kJ/kg/km) and from which data were extrapolated to normal circumstances. This revealed that A, running on average 6214 rev/day, expended approximately 64 kJ/day more than S. This they compensated for by increasing their food intake by 6 g/day (70 kJ/day). Over the estrous cycle wheel running was maximal at proestrus and minimal at metestrus--the reverse trend in food intake was observed. This cyclic difference in energy balance between metestrus and proestrus was estimated to be equivalent to 23% of the 'maintenance' requirement. Thus voluntary activity bears a marked impact on day to day energy balance but has little effect in the long-term since in the female rat intake is precisely regulated to meet expenditure.