L-carnitine: effect of intravenous administration on fuel homeostasis in normal subjects and home-parenteral-nutrition patients with low plasma carnitine concentrations.

We studied the effects of intravenous L-carnitine on the metabolism of fatty acids, ketone bodies, glucose, and branched-chain amino acids in four normal volunteers and four patients on long-term home parenteral nutrition (HPN) with low plasma carnitine concentrations. Substrate kinetics were determined by use of [1-14C]palmitate, [3,4-13C2]-acetoacetate, [6,6-2H2]glucose, and [5,5,5-2H3]leucine before and during a 3-h intravenous infusion of L-carnitine. HPN patients were restudied after 1 mo of nightly intravenous carnitine administration. HPN patients tolerated the short-term fast well, exhibiting neither hypoglycemia nor hypoketonemia. Intravenous carnitine had no effect on rates of fatty acid oxidation, ketone body production, glucose production, or leucine kinetics in either group. Routine addition of carnitine to the HPN regimen does not appear to be necessary. The failure of L-carnitine administration to have discernable effects on intermediary metabolism in normal volunteers casts doubt on its role in the treatment of a variety of medical conditions.