• L-carnitine is derived from the amino acids lysine and methionine within the human body but can also be ingested from animal products. A deficiency of carnitine is rare, even in vegans. Carnitine is stored within the heart and skeletal muscles, and has several roles.
• An obligatory component of the transfer process for fatty acids from the bloodstream into muscle mitochondria for use as fuel.
  • Buffers excess acetyl-CoA within the mitochondria in order to maintain the rate of fuel delivery from carbohydrate during prolonged endurance exercise and reduce lactate accumulation during high intensity exercise.
  • May play an anti-oxidant role to help muscle recovery and reduce muscle protein breakdown following intense exercise such as heavy training loads or repeated competition performance over short time periods.
• For carnitine supplementation to change fatty acid transport and buffering capability within the muscle mitochondria, it would be necessary to increase muscle carnitine concentration. However, very few studies have measured muscle carnitine concentrations as this is technically challenging to do.
  • There is no evidence for changes in muscle carnitine concentrations following short periods (4 weeks or less) of supplementation, nor in most longer term studies which do not co-ingest carbohydrate with the carnitine.
  • Two longer term studies undertaken in recreational athletes (1.4-3g L-carnitine daily for 12-24 weeks) showed increases in muscle carnitine concentration were possible provided a sufficient amount of carbohydrate (80g) was consumed with each dose of L-carnitine.^1,2 However, the metabolic and performance benefits remain uncertain, with one study showing an increased work output over 30 min all out performance test at 24 weeks supplementation (but no increase at 12 weeks¹) while Shannon et al.² found no impact on high intensity interval training adaptations.
  • A similar carnitine supplementation protocol (2g L-carnitine daily for 12 weeks) but without carbohydrate co-ingestion increased muscle carnitine concentration in vegetarians but not omnivores. No differences were found in energy metabolism or lactate levels during 1 hour of moderate intensity exercise in either group3 across the study.
  • Over the first 12 weeks of supplementation in the study by Wall et al.¹, body fat increased 1.8kg (perhaps as a result of an extra 160g carbohydrate per day) in the control group but did not change in the carnitine group. This may be partly explained by a higher energy expenditure during 30 min exercise at 50% VO_2max in the carnitine-supplemented group.⁴ However, since there were no body fat changes reported by Shannon et al.² using a similar supplementation protocol, it is difficult to know whether this was an impact of the carnitine supplementation or other factors over that extended period of time.
• Preliminary research on muscle pain, muscle disruption (via MRI) and blood markers of muscle damage have found favourable adaptations following 3 weeks L-carnitine supplementation.⁵
• One study of 9 weeks supplementation of 2g L-carnitine L-tartrate daily during resistance training found reductions in markers of oxidative stress and improvements in muscle strength but not muscle mass.⁶
• Carnitine is also a popular weight loss supplement due to its proposed role in facilitating fat oxidation. However, there is a lack of evidence supporting the efficacy of carnitine in further enhancing fat loss.

Carnitine may be useful in several clinical settings where carnitine deficiency may be induced, such as maintaining brain and muscle function in the elderly and in the treatment of some forms of cardiovascular disease. Vegetarians may be particularly responsive to carnitine supplementation, presumably because of lower dietary intake.