Insulin sensitivity and fetal growth in ewes maintained in a moderate body condition compared with a lean body condition in late pregnancy

Abstract

The possibility that an increase in maternal fatness might further enhance pregnancy-induced insulin insensitivity was investigated in ewes pregnant with one fetus. After selection of fat and lean ewes, they were fed to maintain maternal energy balance throughout pregnancy. A difference in insulin sensitivity of ewes in either a medium condition [score 2·9, 0·32± 0·031 kg fat/kg fat-free empty body (FFBW), n = 7] or lean body condition (score 2·0, 0·16±0.028 kg fat/kg FFBW, n = 8) at Day 136 of pregnancy was inferred by comparing their insulin response to an injection of glucose (350 mg/kg liveweight). The ewes were slaughtered at Day 146 of pregnancy to allow fetal weight, fetal fatness, and maternal fatness to be related to the capacity of the ewe to release insulin. The fatter ewes released more insulin in response to the injection of glucose (2. 54 log area units under the insulin response curve v. 2·22 for the lean ewes, pooled s.e. = 0·092, P < 0·05), yet cleared the glucose from their blood streams at the same rate as the leaner ewes. Because the fat ewes required more insulin to remove a similar amount of exogenous glucose at a similar rate to the lean ewes, we suggest that the fat ewes were less sensitive to insulin. Further, the fetuses of the fatter ewes, while not different in total weight, had higher levels of body fat than those of the leaner ewes (29·4 v. 23·8 g/kg FFBW, pooled s.e. r = 1·32, P < 0·05). Fetal fatness was also positively correlated to the magnitude of insulin release by the ewe (r = 0·61, P < 0·05). Since glucose is the major precursor of fetal fat, these data support our contention that a reduction in maternal insulin sensitivity, brought on by an increase in maternal fatness, could partition more glucose to the fetus. If fat reserves aid lamb survival, lambs from fat ewes rather than lean ewes may have a better chance of survival, particularly in cold conditions.