What Rate of Infusion of Intravenous Nutrition Solution Is Required to Stimulate Uptake of Amino Acids by Peripheral Tissues in Depleted Patients?

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Abstract
We examined the effect of varying the quantities (0, 0.1, 0.2, 0.3, and 0.4 gl .cntdot. kg-1 .cntdot. [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 .cntdot. kg-1 .cntdot. [day]-1). There was negative nitrogen balance and net loss of amino acids from the limb during fasting. An infusion of 0.2 gN .cntdot. kg-1 .cntdot. [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 .cntdot. kg-1 .cntdot. [day]-1 and 56 kcal .cntdot. kg-1 .cntdot. [day]-1. This was associated with a fivefold increase in 3MH 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 .cntdot. kg-1 .cntdot. [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 < 0.001) on Glu flux, indicating that even at high infusion rates the maximal capacity of peripheral tissues to take up Glu had not been reached.