One cycle of reproduction consisting of pregnancy, lactation or no lactation, and recovery: effects on carcass composition in ad libitum-fed and food-restricted rats.

Groups of nine ad libitum-fed and food-restricted Osborne-Mendel rats weighing between 200 and 220 g at mating were killed on day 21 of pregnancy, day 21 of lactation and day 21 of the postweaning recovery period. Restricted rats were fed 70% of the mean daily ad libitum intake. Groups of nine ad libitum-fed animals, not permitted to suckle their young, were killed on day 21 postpartum and day 21 of the recovery period. Groups of nine nonpregnant rats of comparable initial body weight were killed at appropriate times to serve as advancing age and growth controls. Changes in carcass composition were determined in each group at the end of the pregnancy, lactation and recovery periods. Carcass fat was elevated in the nonlactating group on day 21 postpartum compared to lactating animals and nonpregnant controls. At the end of the study, carcass weight was highest in the ad libitum-fed lactating group. This was due to increased carcass water and not to increased carcass fat. In fact, carcass fat was significantly reduced in the ad libitum-fed lactating group compared to nonpregnant controls. During the recovery period, the absolute increase in carcass fat was greater in the restricted group than in the ad libitum-fed group. The data indicate that 1) pregnancy followed by no lactation results in an increase in carcass fat content and 2) the postweaning recovery period is marked by a tendency to increase carcass fat even in food-restricted rats.