Maternal protein reserves, diet and lactational performance in rats
Pine, Andrew Paul
The importance of tissue protein reserves to lactating females attempting to sustain milk output under conditions of severe dietary protein restriction was investigated using rats. Four experiments were carried out to study the effect of variation in repletion of tissue protein reserves on lactational performance, rates of body protein mobilisation and changes in tissue protein metabolism involved in promoting protein mobilisation. The extent to which body protein reserves were capable of maintaining milk quantity and quality under such conditions was also considered.The lactational performance of multiparous, female Sprague-Dawley rats, offered isoenergetic diets (21 MJ GE/kg DM), was assessed from growth of a standardised litter of 12 pups. Variation in repletion of protein reserves at parturition was achieved by applying a period of protein restriction during the latter half of gestation. Changes in body composition were estimated from carcass analysis and rates of protein mobilisation were derived from serial slaughter experiments. Tissue protein synthesis was estimated in vivo using a flooding dose of [3H] phenylalanine and tissue Na+,K+-ATPase activity was measured polarographically in vitro . Milk samples were obtained following injection of oxytocin.Females offered a high protein diet (215 g CP/kg DM) during lactation exhibited an increase in both feed intake and lactational performance while not utilising their body protein stores. However, in rats offered imbalanced feeds (low protein/high energy) such an increase in intake was not apparent and dams were forced to draw upon their endogenous protein reserves in an attempt to sustain milk production. Between 15 and 22 % of body protein was lost by dams assumed to be "Fully" protein replete at parturition. When dietary protein was limiting, reductions in the size of the protein reserve had a significant impact on a female's ability to sustain milk production, and dams which were initially "Fully" replete supported greater (P<0.05) litter growth during early lactation, due to a greater endogenous protein supply and feed intake (P<0.05), than their "Depleted" contemporaries. Prior depletion ofiv body protein reserves had no significant impact on lactational performance when an adequate supply of dietary protein was provided.The use of body protein reserves during lactation could not maintain either milk quantity or quality at the level of better fed females and throughout lactation milk protein and fat contents were considerably altered (P<0.05) compared to that of well nourished dams. Such changes limit the applicability of milk yield prediction equations for undernourished dams.Rates of protein mobilisation during lactation varied with the degree of protein inadequacy and ranged from 0 g/d in dams on a 215 gCP/kg DM feed to 0.49 g/d (0.01 d_1) and 1.01 g/d (0.021 d"1) in dams offered a 150 and 90 gCP/kg DM diet respectively.A period of protein undemutrition during early lactation did not prevent dams from improving their lactational performance (P<0.01) in response to an increase in the dietary protein supply and this was favoured by the maintenance of the mammary cell population during this period. The data suggest that the mammary cell population is quite sensitive to nutritional status in rats.Tissue protein mobilisation during lactation was associated with a dramatic increase in muscle protein degradation, to 13.0 %/d, while the decline in protein synthesis was of less importance, although this was reflected in a reduced Na+,K+-ATPase activity (P<0.05). Mammary protein synthesis (FSR and ASR) increased during lactation (P<0.05) in dams offered a high protein/high energy feed, although this was prevented by dietary protein restriction. Whilst liver FSR was less sensitive to dietary protein content, changes in liver ASR reflected the effect of dietary treatment on liver size (P<0.001).n vivo rates of mammary protein synthesis were adversly affected by the use of exogenous oxytocin in the milking procedure used immediately before estimation, although rates of muscle and liver protein synthesis were unaffected.When low dietary protein concentration constrained feed intake there was also a considerable mobilisation of body fat and it appeared that under these conditions intake constraint was associated with the impact of a nutrient imbalance on a dams ability to dispose of surplus ("imbalanced") nutrients.Nutrient balances developed for severely protein restricted dams during lactation indicated that tissue protein reserves support lactation not only through the provision of endogenous amino acids but also by allowing an increased feed intake.