What rate of infusion of intravenous nutrition solution is required to stimulate uptake of amino acids by peripheral tissues in depleted patients?

We examined the effect of varying the quantities (0, 0.1, 0.2, 0.3, and 0.4 gN·kg^-1·[day]^-1) of nitrogen input on N balance, 3-methylhistidine (3MH) excretion, plasma amino acid concentration, and the net flux of amino acids across the leg in depleted patients requiring parenteral nutrition. The calorie-to-nitrogen ratio was 140 to 1 (kcal:1 gN) and consequently the patients received varying amounts of calories (8, 14, 28, 42, and 56 kcal·kg^-1·[day]^-1). There was negative nitrogen balance and net loss of amino acids from the limb during fasting. An infusion of 0.2 gN·kg^-1·[day]^-1 of IVN reversed the net catabolic process and resulted in equilibrium of peripheral total amino acid flux and of tyrosine flux without a decrease in 3MH excretion. Net uptake of total amino acids and tyrosine in peripheral tissues was achieved with 0.4 gN·kg^-1·[day]^-1 and 56 kcal·kg^-1·[day]^-1. This was associated with a fivefold increase in 3 MH excretion (p <0.01), indicating that net anabolism occurred with increased protein turnover. Fifty per cent of the amino acids taken up by peripheral tissues during infusions of 0.4 gN·kg^-1·[day]^-1 was due to the uptake of glutamate (Glu) and 20% was due to the uptake of branched chain amino acids (BCAA). Plasma Glu concentration, [Glu], did not increase with increasing IVN infusion, but BCAA concentrations did. Although the mean plasma [Glu] did not change with IVN infusion, there was an independent effect of plasma [Glu] (p <0.0001) and of N input (p <0.0001) on Glu flux, indicating that even at high infusion rates the maximal capacity of peripheral tissues to take up Glu had not been reached.