L-Carnitine
What is L-Carnitine?
Carnitine is a non-essential nutrient that has several roles in intermediary metabolism. It can be manufactured in the liver and kidneys. Dietary sources are provided from animal foods. Carnitine that is ingested or synthesised is in the L-isoform. L-carnitine is stored primarily in the heart and skeletal muscle.
Proposed Benefits of Carnitine
It is claimed that carnitine supplementation might enhance fatty acid transport and oxidation and so potentially reduce body fat levels.
It has also been suggested that carnitine supplementation may benefit endurance athletes by increasing fat oxidation during exercise, which will spare muscle glycogen.
It is also proposed that acetyl carnitine might enhance high intensity exercise performance in situations that might otherwise be limited by excess lactate and hydrogen ion accumulation.
Mechanism of Action of Carnitine
Several mechanisms have been suggested to explain how carnitine may enhance exercise performance. These include enhanced fatty acid oxidation, altered glucose homeostasis, enhanced acetyl carnitine production, modifications of training response and altered muscle fatigue resistance.
One of the primary roles of carnitine is as a component of the enzymes carnitine palmityltransferase I, carnitine palmityltransferase II and carnitine-acylcarnitine translocase, which transport fatty acids across the mitochondrial membrane where they are oxidised to produce energy. Thus, carnitine acts as a shuttle to allow fat to enter the mitochondria to be oxidised (burned for energy). The role of carnitine in fatty acid oxidation suggests that carnitine supplementation might increase fat oxidation, making more ATP available to muscular exercise. This might also decrease the use of muscle glycogen as a fuel and thus delay the onset of fatigue.
Another role for carnitine is to act as sink for acetyl-CoA units which are produced during high intensity exercise. By converting acetyl CoA to acetyl carnitine and CoA, carnitine could help maintain CoA availability which could increase the flux through the Krebs cycle. Furthermore, by reducing levels of acetyl CoA (by generating acetylcarnitine), the oxidative metabolism of glucose may be increased because of the enhanced activity of the enzyme pyruvate dehydrogenase (PDH). This could result in lower lactate production which might enhance exercise performance in situations that might other be limited by high lactate levels and increased muscle acidity.
Research on Carnitine
The available clinical trials designed to assess the effect of carnitine do not as yet permit definitive conclusions to be made.
Although most studies show an increase in plasma concentrations following carnitine supplementation of (1-6g) it is unclear whether muscle carnitine concentrations are increased.
Increasing fat oxidation and improving endurance performance
The majority of studies have not shown metabolic enhancement or improved performance with supplementation of carnitine of 2-6g per day over periods of 7 days to 4 weeks (1-7).
Arenas et al have shown that in periods ranging 1-6 months, carnitine supplementation prevented a training-associated decrease in muscle carnitine content and also increased the activity of key oxidative enzymes in trained athletes (8-10). The physiological effects of this are as yet unknown.
Carnitine supplementation may benefit exercise performance in disease states such as chronic renal failure and peripheral vascular disease (11,12).
Acting as a sink for acetyl units
Research which has looked at carnitine's potential as a sink for acetyl units suggests that the enzyme PDH is maximally active within a few seconds of high intensity exercise and additional carnitine is unlikely to further stimulate this activity (13). Carnitine content of skeletal muscle does fall during exercise, as acetylcarnitine accumulates, but to date there is no data supporting the postulate that lowered carnitine levels are rate limiting.
Two earlier studies do show that acute supplementation with 2g of carnitine 1 hour before exercise reduced the exercise-induced increase in blood lactate levels and have increased time to exhaustion (14,15). It is difficult to know if 90 minutes provides sufficient time for carnitine to be absorbed through the gut and into the bloodstream and taken up by the muscle. Hultman et al have shown that it is unlikely that carnitine supplementation over a period of days to weeks will change muscle total carnitine content in humans (16). Further studies have failed to show an effect of carnitine supplementation on performance (6,17,18).
The effect of L-carnitine on body composition has not been well-studied in humans.
In summary, the studies of carnitine supplementation on exercise performance have produced varied results with the majority showing no effect. However, this lack of effect may be due to problems with study design. Future research with larger numbers of subjects will need to be carried out so that we can make more definitive conclusions.
Rating of Efficacy for Carnitine
Research on L-carnitine supplementation has produced mixed results. While most studies show no performance benefit, a few studies do demonstrate an erogenic potential.
5/10
References
1. Barnett C., Costill D.L. and Vukovich M.D. Effect of L-carnitine supplementation on muscle and blood carnitine content and lactate accumulation during high intensity sprint cycling. Int J Sport Nutr. 4:280-288, 1994.
2. Colombani P., Wenk C. and Kunz I. Effects of L-carnitine supplementation on physical performance and energy metabolism of endurance-trained athletes: a double-blind crossover field study. Eur J Appl Physiol. 73:434-439, 1996.
3. Decombaz J., Deriaz O., Acheson K., Gmuender B. and Jequier E. Effect of L-carnitine on submaximal exercise after depletion of muscle glycogen. Med Sci Sports Exerc. 25:733-740, 1993.
4. Oyono-Enguelle S., Freund H. and Ott C. Prolonged submaximal exercise and L-carnitine in humans. Eur J Appl Physiol. 58:53-61, 1988.
5. Soop M., Bjorkman O., Cederblad G., Hagenfeldt L. and Wahren J. Influence of carnitine supplementation on muscle substrate and carnitine metabolism during exercise. J Appl Physiol. 64:2394-2399, 1998.
6. Trappe S.W., Costill D.L., Goodpaster M.D. and Fink W.J. The effects of L-carnitine supplementation on performance during interval swimming. In J Sport Med. 15:181-185, 1994.
7. Vukovich M.D., Costill D.L. and Fink W.J. Carnitine supplementation: effect on muscle carnitine and glycogen content during exercise. Med Sci Sports Exerc. 26:1122-1129, 1994.
8. Arenas J., Ricoy J.R. and Encinas A.R. Carnitine in muscle, serum, and urine of nonprofessional athletes: effects of physical exercise, training, and L-carnitine administration. Muscle Nerve. 14:598-604, 1991.
9. Arenas J., Huertas R., Campos Y., Diaz E., Villalon J.M. and Vilas E. Effects of L-carnitine on the pyruvate dehydrogenase complex and carnitine palmitoly transferase activities in muscle of endurance athletes. FASEB. 341:91-93, 1994.
10. Huertas R., Campos Y., Diaz E. and Arenas J. Respiratory chain enzymes in muscle of endurance athletes: effect of L-carnitine. Biochem Biophys Res Commun. 188:102-107, 1992.
11. Ahmad S., Robertson H.T. and Golper T.A. Multicenter trial of L-carnitine in maintenance hemodialysis patients II. Clinical and biochemical effects. Kidney Int. 38:912-918, 1990.
12. Brevetti G., Chiariello M. and Ferulano G. Increases in walking distance in patients with peripheral vascular disease with L-carnitine: a double-blind, cross-over study. Circulation. 77:767-773, 1988.
13. Heinonen O.J. Carnitine and physical exercise. Sports Med. 22:109-132, 1996.
14. Siliprandi N., Di Lisa F. and Pieralisi G. Metabolic changes induced by maximal exercise in human subjects following L-carnitine administration. Biochimica et Biophysica Acta. 1034:17-21, 1990.
15. Vecchiet L., Di Lisa F. and Pieralisi G. Influence of L-carnitine administration on maximal physical exercise. Eur J Appl Physiol. 61:486-490, 1990.
16. Hultman E., Cederblad G. and Harper P. Carnitine administration as a tool to modify energy metabolism during exercise. Eur J Appl Physiol. 6, 1991.
17. Greig C., Finch K.M., Jones D.A., Cooper M., Sargeant A.J. and Forte C.A. The effect of oral supplementation with L-carnitine on maximum and submaximum exercise capacity. Eur J Appl Physiol. 56:457-460, 1987.
18. Wyss V., Ganzit G.P. and Rienzi A. Effects of L-carnitine administration on VO2max and the aerobic-anaerobic threshold in normoxia and acute hypoxia. Eur J Appl Physiol. 60:1-6, 1990.
|
...Health
is the art of
Living...
