Carnitine in muscle, serum, and urine of nonprofessional athletes: Effects of physical exercise, training, and L‐carnitine administration

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Abstract
Efficient utilization of fatty acids to sustain prolonged physical efforts is thought to be dependent on the carnitine shuttle of muscle. A study has been carried out in 24 athletes (13 long‐distance runners and 11 sprinters). These subjects received placebo or L‐carnitine (1 g/orally b.i.d.) during a 6‐month period of training. In endurance athletes, training induced lowering of total and free muscle carnitine. Increase of esterified muscle carnitine was also observed. Post‐exertional overflow of acetylcarnitine and long chain acylcarnitine, as well as reduction of the free fraction was also noticed in the blood. Fasting plasma carnitine levels, however, were not affected in carnitine‐treated atheletes at rest. These changes were likely related with the significantly increased urinary excretion of esterified and total carnitine which occurred after physical exercise. In the sprinters only, a decrease in free and total carnitine of muscle was detected after training. Both these potentially unfavourable effects were prevented by oral administration of L‐carnitine. Our data suggest that training in endurance athletes, and to a lesser extent, in sprinters, is associated with a decrease in free and total carnitine of muscle, due to an increased overflow of short‐chain carnitine esters in urine.