Growth of rats fed by a continuous intragastric infusion containing amino acids and keto acids

A nutrient mixture was designed to minimize electrolyte and nitrogen excretion in rats while permitting growth. It contained lysine, threonine, histidine, tryptophan, ornithine, N-free analogues of the other essential amino acids, sucrose, corn oil, minerals, and vitamins. Intragastric infusion of this mixture for 25 days into 150-g rats with no access to food or water produced an average weight gain (after a 4-day lag period) of 3.48 +/- 0.09 g/day with proportionate increase in tail length. Urinary excretion rates of N, urea, Na, K, and P became very low. Fecal N fell to 4 mg/day. Urine pH averaged 6.1. Plasma amino acid concentrations changed markedly. Carcass analysis showed that an average of 63% of the 144 mg/day of administered N was retained for growth. Thus, this nutrient mixture is unusually efficiently utilized and leads to minimal excretion of waste products.