Interactions between major nutrients in the diet and the lactational performance of rats

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Abstract
The effect on lactational performance of replacing feed carbohydrate with fat at two different protein levels was studied. Lactating Sprague–Dawley rats with a standardized litter size of thirteen pups were allocated one of eight feeds containing either 300 or 150 g protein/kg organic matter (OM) and ranging in fat content from 100 to 550 g/kg OM from day 2 until day 14 of lactation. Daily food intake, live-weight gains, and changes in body composition of both dams and litters were measured. Feeds of low protein content resulted in a significant decline (P < 0.001) in lactational performance despite a significant increase (P < 0.001) in maternal protein mobilization. Maternal lipid mobilization was not significantly affected by feed composition. Litter lipid gain was significantly increased (P < 0.05) as fat replaced carbohydrate in the high-protein feeds, due to an increase in maternal energy intake. In contrast, lactational performance was severely depressed (P < 0.001) as fat replaced carbohydrate in the low-protein feeds. This interaction between feed components on lactational performance was in accordance with the hypothesis that the heat production of lactating rats is maximal and, hence, constraining intake.