Short-term metabolic response in wethers to high dietary protein intake prior to fasting

Twenty crossbred wether lambs (average body weight 38.9 kg) were examined for selected metabolic responses during fasting as affected by dietary protein level prior to fasting. Animals were fed diet analyzed at 10.4 and 20.3% crude protein (diets A and B, respectively) for 3 weeks and then fasted for 7 days. jugular blood samples were collected 4, 28, 52, 76, 100, 124 and 148 hr after the last feeding. Plasma glucose declined (P less than .05) between 4 and 52 hr, after which no further differences in plasma glucose occurred within or between treatment groups. Plasma urea N (PUN) was higher (P less than .05) in lambs fed diet B at 4, 28, and 52 hr postfeeding. In wethers fed diet A, PUN increased (P less than .05) from 28 to 76 hr postfeeding and then declined to prefasting levels. In wethers fed diet B, PUN did not begin to decrease until 76 hr postfeeding and then gradually declined (P less than .05) until the end of the fast. Plasma ammonia N increased (P less than .05) and plasma albumin decreased (P less than .05) during fasting in both treatment groups, but no differences were observed between the two groups in either parameter. The data support the existence of a "glucose-alanine" cycle in that alanine decreased (P less than .05) in both groups whereas glucose remained constant and PUN increased. Together, these factors suggest enhanced gluconeogenesis from amino acids. Increased branch-chain amino acid oxidation may be responsible for the decrease (P less than .05) in total branched-chain amino acids observed during fasting. The higher dietary protein intake of lambs fed diet B apparently afforded some nutritional advantage, since most of the changes observed occurred approximately 24 hr later in these lambs than in those fed the lower protein diet A before fasting.