Previous L-Carnitine studies have used a range of supplementation duration's from 1-hour prior to exercise up to several months. The equivocal nature of the literature regarding substrate metabolism and performance, which may be due to differences in study design, and often poor experimental control, makes it difficult to fully assess the impact of supplementation on substrate metabolism.

PURPOSE:

To determine the effects of supplementation duration, with L-Carnitine L-tartrate (LC), on substrate metabolism during steady state exercise, under well controlled experimental conditions.

METHODS:

6 endurance-trained males were studied in a randomized double blind crossover fashion. After familiarisation, subjects attended the laboratory on 3 occasions, 2-weeks apart, 3-hours following the ingestion of a standardised pre-exercise meal. Each subject was given a set of treatments that included a chronic dose (3g LC/day for 2 weeks), an acute dose (3g glucose for 13d and 3g LC, 3 hours before exercise), and placebo (3g glucose/day for 2 weeks). Subjects performed 60 minutes of submaximal cycle ergometer exercise (60% VO2max) on each occasion. Dietary intake and physical activity was replicated for two days prior to each trial. Blood was drawn at rest and during exercise, and expired gas analysis used to determine substrate utilisation.

RESULTS:

No significant main effect was observed for RER (p = 0.13). Comparison of RER revealed a near significant effect (p = 0.05) between placebo and chronic trials, but not between placebo and acute trials (p = 0.78). No significant main effect of trial was observed for rate of carbohydrate oxidation (p = 0.13), although there was a difference between placebo and chronic trials (p = 0.02) but not placebo and acute trials (p = 0.70). A higher total carbohydrate oxidation over 60 minutes was observed following chronic supplementation compared with placebo (p = 0.02; mean (SD) of 93.8(17.3)g and 78.2(23.3)g, respectively). There were no significant effects on rate of, or total, fat oxidation, or on cardiorespiratory or haematological responses between trials.

CONCLUSION:

Under these experimental conditions, pre-treatment with LC tended to increase carbohydrate oxidation, rather than fat oxidation, during steady state exercise, and this effect was only observed following chronic supplementation. Lonza Ltd, Basel, Switzerland provided the L-Carnitine L-tartrate used in this study.