The effects of protein nutrition on lactational performance

Abstract

The effects of an inadequate diet, in terms of both protein quality and quantity, on maternal protein metabolism and on the lactational performance of the Wistar rat have been studied. The rats received diets with a high or low protein content of a good quality (milk-based) protein, or poor quality (cereal-based) protein. Reduced food intakes resulted on the latter diets. The experimental design therefore included rats pair-fed the control diet, in amounts equivalent in energy content to that eaten by the cereal-protein fed rats. In an attempt to increase energy intakes, rats fed the high cereal diet were given additional energy in the form of a sucrose solution. Liver, muscle and mammary tissues were examined. Measurements carried out included: protein, RNA and DNA concentrations of the three tissues, and also serum albumin concentrations; protein synthesis in the three tissues; lactose synthetase and galactosyl transferase activities; milk volume and composition and serum insulin and corticosterone concentrations. The effects of exogenous prolactin on lactational performance were also examined. The results suggested that, in liver, reduction in dietary protein quality and quantity reduced the protein synthesising capacity but had no significant effect on the fractional synthetic rate (FSR). Serum albumin secretion was decreased by reduced dietary energy intake but not by the quality or quantity of the dietary protein. The response of muscle and mammary gland to dietary protein insufficiency were similar in many respects. In muscle, cellular protein concentration, protein sythesising capacity and FSR were reduced by poor dietary protein quality. Maternal muscle mass, as represented by tibialis weight, was affected more by quality than quantity of dietary protein. In the mammary gland, the protein synthesising capacity decreased with dietary protein quantity, though the effects were not as pronounced as in liver. Reduced dietary protein quality significantly decreased FSR and had a greater effect on lactose synthetase activity, milk volume and total milk protein content than reduced dietary protein quantity. The reduction in lactose synthetase activity may be due to a specific decrease in mammary α-lactalbumin content. Reduced energy intakes decreased lactose content, whereas differences in protein quality did not. Serum insulin and corticosterone concentrations were reduced by inadequate dietary protein quality and quantity. Insulin was reduced to a greater extent than corticosterone. It could be concluded that, in general, when a diet inadequate in protein is fed to lactating rats, maternal metabolic adaptations take place in order to bring about mobilisation of nutrients, so as to supplement the nutrient requirements of the mammary gland, and thus improve lactation. However, the severe dietary restrictions imposed in this study rendered these adaptations ineffective, due to the high nutrient demands of mammary tissue in small animals like the rat.

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