Aaron Deere is a sports nutritionist, functional medicine consultant and advanced personal trainer. He is based in London.
Carnitine is a derivative of the amino acids lysine and methionine and is found in highest concentrations in red meat, with 100g containing approximately 95mg. Increased intake has been proposed to increase fat oxidation, delay muscle glycogen depletion, and increase endurance exercise performance[1,2].
The primary function of carnitine is to act as a co-factor in the enzymatic system involved in conjugating fatty acids for transport for mitochondrial beta-oxidation[3,4]. In layman terms this simply means it transports fat to cells where it can be used as fuel. The fat-oxidation properties and the potential application to body composition is often the reason for the use of carnitine as a dietary supplement, but does the scientific evidence support these claims?
Muscle carnitine concentration
The fat-burning effect of carnitine supplementation is based on three progressive assumptions. The first is that muscle carnitine concentration is too low to facilitate optimal function of carnitine acyltransferases and support elevated rates of fat oxidation. Therefore, ingestion of carnitine would increase the concentration of muscle carnitine, resulting in increased carnitine levels that increase rates of fatty acid oxidation during exercise.
A research review addressed these assumptions and arrived at the following conclusions. glyocBioavailablilty of oral carnitine supplementation was <16-18% at 1-2g, with approximately 50% of the dose lost via urine within 24 hours, with single intravenous doses of carnitine having little effect on muscle carnitine levels. Muscle is only sensitive to carnitine levels when levels are less than 25-50% of normal, therefore supplementation would unlikely improve function.
A 24-week study highlighted increases of 10% in muscle carnitine versus placebo, but there was no quantifiable improvement in performance or body composition were seen. A later review highlighted an increase in muscle carnitine levels with supplementation in a hyperinsulinaemic state, a parameter that was previously untested. A subsequent review expanded on the hyperinsulinaemic state theory and showed that when carnitine supplementation was added to 80g of carbohydrates for a period of 24 weeks that total muscle carnitine was increased by 21%; and that there was a glycogen-sparing effect with the switch to fat as the preferred fuel.
Pros versus cons
The body of evidence on the effect of carnitine supplementation on body composition has returned equivocal results. Any proposed benefits on body composition would rely on an increased level of muscle carnitine. It is possible to raise muscle carnitine levels, but it requires a long duration of supplementation – 24 weeks – and the combination of 80g of carbohydrates. If trying to get as lean as possible, these additional carbs could likely have a negative effect on body composition and mitigate any gains from the increase in muscle carnitine levels.
1,4 SALDANHA AOKI M, RODRIGUEZ AMARAL ALMEIDA A, L, NAVARRO F, BICUDO PEREIRA COSTA-ROSA L, F, PEREIRA BACURAU R, F. 2004. Carnitine Supplementation Fails to Maximize Fat Mass Loss Induced by Endurance Training in Rats. Annals of Nutrition & Metabolism;48:90-94
2, 9 WALL, B.T., STEPHENS, F.B., CONSTANTIN-TEODOSIU, D., MARIMUTHU, K., MACDONALD, I.A. and GREENHAFF, P.L., 2011. Chronic oral ingestion of l-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. The Journal of physiology, 589(4), pp. 963-973.
3 REBOUCHE CJ. Carnitine. In: SHILS ME, SHIKE M, ROSS AC, CABALLERO B, COUSINS RJ, eds. Modern Nutrition in Health and Disease. 10th ed. Philadelphia: Lippincott, Williams & Wilkins; 2006:537-544.
5 KARLIC, H. and LOHNINGER, A., 2004. Supplementation of L-carnitine in athletes: does it make sense? Nutrition, 20(7), pp. 709-715.
6 BRASS, E.P., 2004. Carnitine and sports medicine: use or abuse? Annals of the New York Academy of Sciences, 1033(1), pp. 67-78.
7 ARENAS, J., RICOY, J., ENCINAS, A., POLA, P., D’IDDIO, S., ZEVIANI, M., DIDONATO, S. and CORSI, M., 1991. Carnitine in muscle, serum, and urine of nonprofessional athletes: Effects of physical exercise, training, and L-carnitine administration. Muscle & nerve, 14(7), pp. 598-604.
8 STEPHENS, F.B., CONSTANTIN-TEODOSIU, D. and GREENHAFF, P.L., 2007. New insights concerning the role of carnitine in the regulation of fuel metabolism in skeletal muscle. The Journal of physiology, 581(Pt 2), pp. 431-444.