Effects of acute hypercarnitinemia during increased fatty substrate oxidation in man.

To test whether carnitine availability is rate-limiting for fat oxidation under conditions of augmented oxidative use of fatty substrates, two series of studies were performed. In study no. 1, L-carnitine (1 g + 0.5 g/h intravenously [i.v.]) or saline was given to eight volunteers during a 4-hour infusion of a 10% triglyceride emulsion, thereby increasing plasma free-carnitine levels from 38 +/- 4 to 415 +/- 55 mumol/L. Fat infusion increased plasma triglyceride levels (80%) and lipid oxidation (30%), and decreased (28%) carbohydrate oxidation (as measured by indirect calorimetry); hypercarnitinemia had no influence on these responses. In study no. 2 in 12 healthy subjects a bolus of L-carnitine (3 g) or saline was administered 40 minutes before aerobic exercise (bicycling for 40 minutes at 60 W), followed by 2 minutes of anaerobic exercise (250 W) and 50 minutes of recovery. Oxygen consumption (VO2), increased to 18.3 +/- 0.7 mL.min-1 x kg-1 during aerobic exercise, reached a maximum of 46.0 +/- 0.8 mL.min-1 x kg-1 during the anaerobic bout, and returned to baseline within a few minutes, with no difference between control and carnitine. At virtually identical mean energy expenditure rates (196 +/- 7 v 197 +/- 7 J.min-1 x kg-1, saline v carnitine), after carnitine administration the entire exercise protocol was sustained by a lower mean carbohydrate oxidation rate (42.1 +/- 3.6 v 36.5 +/- 2.3 mumol.min-1 x kg-1, P < .03) and a higher mean lipid oxidation rate (6.7 +/- 1.0 v 8.3 +/- 0.7 mumol.min-1 x kg-1, P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)