Diurnal regulation of milk lipid production and milk secretion in the rat: effect of dietary protein and energy restriction

Abstract

Lactating rats consuming a control diet (20% casein) ad libitum and kept on a 12-h light-dark cycle produced milk during the dark phase at a rate 50% greater than that during the light period. Diurnal variations in the food consumptions of these animals, maternal live weights and litter weight gains were also observed. This diurnal variation with respect to milk production and food consumption appears to be abolished by feeding a low protein (10% casein) diet. However, the variation was amplified when the amount of the control diet was restricted to 25 g/d and made available at the middark period; this was probably related to the meal-feeding behavior of these rats because they consumed all their food within 6 h after it was made available. The rates of fatty acid synthesis in the mammary glands, livers and adipose tissue measured in vivo from [3H]H2O incorporation also varied diurnally with maximal rates at the middark period. Similar rates were obtained for the rats receiving restricted amounts of the control diet measured in the feeding period, whereas the values for the rats fed the low protein diet in the dark period tended to be significantly lower. The milk fatty acids were also analyzed, and whereas no diurnal differences were detected in the samples taken from control rats, a marked reduction in the proportion of the medium-chain acids was observed in the samples from rats receiving restricted amounts of the control diet taken immediately before the normal feeding period.