Substrate Interaction in Intravenous Feeding Comparative Effects of Carbohydrate and Fat on Amino Acid Utilization in Fasting Man

Abstract

The metabolic and endocrine effects of i.v. infusions in normal fasting man were observed under highly controlled conditions over a period of 6-8 days duration. Comparative data on a variety of i.v. feeding programs were presented. The data on low dose glucose, high dose glucose, glycerol, fat emulsion and amino acids, each given separately, demonstrated changes seen with simple infusion of a single substrate in fasting. Data were compared with the utilization of amino acid infusions when accompanied by low dose glucose, high dose glucose, glycerol and fat emulsion. Nine experimental i.v. feeding programs were based on data from 35 subjects observed over a total of 370 subject days. A strong interaction between glucose or lipid and protein metabolism was found. In fasting, glucose had protein sparing effect, most evident when given at high dose. Glycerol, in an amount equal to that contained in 2000 ml of 10% fat emulsion, had a mild protein sparing effect. Fat emulsion was no more effective. When amino acids were given alone, normal fasting subjects were always in negative N balance with the daily N loss half that seen in starvation alone. Although amino acids given alone have a protein sparing effect, this was accomplished only at the expense of a high N excretion including an amount equivalent to the entire infusion plus an additional loss from the body''s native proteins. The provision of energy yielding non-protein substrates with the amino acids markedly improved N economy in the following order: glycerol, low dose glucose, fat emulsion and high dose glucose. Several of the plasma amino acid concentrations responded to glucose when added to amino acid infusion. This lowering of amino acid concentration was unaccompanied by an increase in urinary excretion. The assumption was made that the provision of the added glucose favors the incorporation of amino acid into protein. There was no evidence to suggest that a rising concentration of ketones in the blood favors amino acid utilization or protein synthesis.