What is it and what does it do?
L-carnitine (or acetyl L-carnitine) is a non-essential amino acid that is famed for its role in fat metabolism. Without L-Carnitine our body wouldn’t be able to use fat as fuel and we would be completely dependent on our limited glycogen stores and protein breakdown for energy, which is far from ideal in endurance or high intensity exercise. Without fat metabolism we fatigue much quicker. In prolonged exercise fat stores are broken down for energy production, turning triglycerides in fatty acids that our muscle cells mitochondria are able to turn into energy to perform muscle contractions.
It has been established that muscle free carnitine availability may be limiting to fat oxidation during high intensity submaximal exercise, such as cycling or running (Stephens at al 2007). Increasing muscle total carnitine content in resting healthy humans (by providing it alongside high GI carbohydrate) reduces muscle glycolysis, increases glycogen storage (by preserving muscle glycogen) and is accompanied by an apparent increase in fat oxidation.
Why use it?
Even though L-carnitine is produced by the body naturally, particularly if your diet contains plenty of meat, its’ availability during exercise may be limited so providing an external source maybe beneficial. Taking l-carnitine alongside high GI carbohydrate such as maltodextrin seems to help drive the uptake into muscle cells (Wall et al 2011).
The Wall (2011) study specifically looked at cycling performance and l-carnitine supplementation. One group of athletes consume 2 grams of L-carnitine along with 80 grams of a high-glycaemic carbohydrate first thing in the morning and four hours later for 24 weeks. Another group only took the carbohydrate.
The researchers found that during low-intensity cycling, the subjects taking L-carnitine burned 55 percent less muscle glycogen while increasing the body’s ability to burn fat by 55 percent. During high-intensity cycling, the subjects taking L-carnitine had lower levels of lactic acid and higher levels of creatine phosphate, one of the primary building blocks of ATP. All this combined should delay fatigue and make an athlete less dependent on carbohydrate intake, which could be particularly useful if the availability of fuelling is limited or you are an athlete who struggles with gels and bars in events up to middle distance.
L-carnitine supplementation has also been linked to an improvement in recovery and reduced muscle soreness. Whilst not all evidence strongly indicates this it does make sense if you consider the reduction in lactic acid produced as discussed above.
Wall, B. T., Stephens, F. B., Constantin‐Teodosiu, D., Marimuthu, K., Macdonald, I. A., & 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), 963-973
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 Journal of Physiology 1; 581(Pt 2): 431–444