A COMPARISON OF ENERGY METABOLISM IN THE NEW‐BORN INFANT, PIGLET AND LAMB

Abstract

Characteristics of energy metabolism in the new‐born infant, piglet and lamb have been compared quantitatively in order to assess how the constraints imposed by the availabilities of energy substrates in body reserves and colostrum differ between species and affect the new‐born's well being during the first day after birth. Three air temperature ranges, described as thermoneutral (32‐38 °C), moderate (18‐26 °C) and cold (0‐10 °C) and representing the usual birth environments of infants, piglets and lambs, respectively, have been considered. The analysis revealed the following noteworthy points. Carbohydrate and lipid are the major energy substrates for heat production because protein catabolism is minimal during the first day after birth in all three species. The availability of carbohydrate determines how long the new‐born can avoid hypoglycaemia, which threatens well being because it leads to hypothermia or compromised cerebral function, but lipid availability can affect the periods for which the carbohydrate can last. Thus, in unfed piglets and lambs the available reserves of liver and skeletal muscle glycogen (g/kg body weight) are similar in normal (n.) and growth retarded (g.r.) individuals, but glycogen exhaustion occurs earlier in g.r. new‐borns because a reduced lipid availability in them increases their dependence on carbohydrate. In contrast, lipid energy is plentiful in g.r., preterm (p.) and n. infants, so that the faster depletion of glycogen in g.r./p. than in n. individuals is primarily due to a restricted prenatal glycogen deposition in the former. The usual colostrum intakes of n. infants are very low during the first day, but their body reserves can supply the required energy, the major source of carbohydrate being liver glycogen. However, g.r./p. infants require supplementary feeding, the choice of feed being determined by factors such as the intakes the infants can achieve, the carbohydrate content of the feed and the need to ensure a balanced supply of minerals, electrolytes and other substances. In their usual birth environments piglets and lambs, whether growth retarded or not, require colostrum to avoid hypothermia during the first day. The colostrum of both species is rich in lipid, which corrects any deficit in the new‐born and thus extends the availability of glycogen, but at the usual colostrum intakes the amounts of lactose can provide sufficient carbohydrate energy for only about half a day. Piglets and lambs, even when fed fully, are therefore obliged to call on their body glycogen reserves in order to make up the difference. After the first day in all three species milk must rapidly become the only source of carbohydrate, lipid and protein. It is suggested that despite differences between them, the piglet or lamb could prove to be useful animal models of the infant's metabolism, but only if the experimental conditions are chosen carefully.