Body composition in the rat: interactions of exercise, age, sex, and diet.

Changes in rat body composition (15 components) associated with changes in level of physical activity (cage restraint, ad libitum activity in living cages, and ad libitum wheel running), age at initiation of regimen (22 days-16 wk), sex, and diet (chow and 60% fat) are evaluated. Running regimen initiation at 7 wk of age or younger is associated with a decrease in the fat-free body mass (FFBM) in males and an increase in females. Initiation at 11 wk or older does not change FFBM. Both reduce body fatness in either sex. Age altered the mass of 11 components, diet altered the mass of 10 components, and activity altered the mass of 8 components, and there were 6 interactions between the experimental variables. The significance of interactions in general is discussed. Our data together with those of other investigators indicate that the responses to volitional wheel running and forced treadmill running are similar, but data from swimming rats may not be comparable in some cases.