Carnitine is a conditionally essential amino acid (although some considerit a vitamin). More specifically, carnitine is produced in the liver and kidney from various precursors. In particular, the amino acids lysine and methionine are required, as well as iron and vitamins B6, PP and C. The amount of endogenous carnitine that the body is able to produce (approx. 20mg/day in sedentary individuals) is insufficient for normal requirements, especially considering that 15 to 50mg/day is normally excreted through the urine. It is only possible to fully meet one’s carnitine requirements through food intake. In particular, meat and dairy products are rich in it. It is known that almost all carnitine in the body (20-25g) is located in the muscles, i.e. where it is most needed.

Function

  As you probably remember from the part of the book devoted to vitamins, the main biological role of carnitine is the transport of fats in the blood stream to the peripheral cells so that they can be oxidised. The theoretical consequence is that increased carnitine intake, through supplementation, results in increased fat oxidation. This would have a positive impact on metabolic performance based on aerobiosis (i.e. those in which the physical load is extensive, with an emphasis on endurance). If the theoretical assumption underlying carnitine supplementation is correct, the athlete should have a higher requirement for this amino acid than the sedentary person. In this regard, studies are conflicting: when measuring the carnitine levels of some footballers, it was actually found that their stores were relatively low compared to those of sedentary people. It would be inferred from this that the latter would have a lower requirement than athletes, but some authors dispute this logical consequence.

Negative notes

  It appears that carnitine can inhibit T3 and T4 production by the thyroid gland, with obvious negative consequences on cellular metabolic activity. Carnitine therefore also has the effect of hindering the slimming process where this is desired. On the other hand, it also has a slimming function (for details, see the section on supplements for body weight management), so the two opposing effects could cancel each otherout. It is also possible that one of the two effects will prevail over the other and each athlete will respond to carnitine supplementation in an original way. Another downside of carnitine is its poor bioavailability: only 5-15% of all carnitine taken in with food or supplementation is absorbed in the gastrointestinal tract. It is possible to increase the amount by combining carnitine with a significant amount of carbohydrates in the same meal (it is not possible to give precise indications). Even so, however, the amount of the amino acid that is absorbed remains small. Finally, the efficacy of carnitine supplementation varies greatly from person to person, so some people may benefit greatly from it, while others may not. The reason is unclear, but researchers suspect that there is a link to the amount of carnitine in the body before supplementation begins. If this hypothesis were correct, carnitine supplementation would only be advisable for those who, on the one hand, have low carnitine stores and, on the other hand, have difficulty in improving extensive endurance performance.

Form and posology

  The main forms in which carnitine supplements are available on the market are tablets and soluble powder. A daily intake of 2g produces, on average, (only) an 8% increase in the body’s carnitine stores over two weeks.