Carnitine has many functions which make it an attractive supplement, probably most noteworthy is its ability to promote fat oxidation. A not so well publicized and understood role of carnitine is the regulation of testosterone. Prior work has shown that carnitine can help prevent declines in testosterone during stressful situations, and still other work has shown it may be involved in control of testosterone synthesis. In this recent study, a novel role of carnitine on the expression of the testosterone receptor in skeletal muscle was investigated. This receptor is called the androgen receptor. Ten healthy men were provided with either L-carnitine L-tartrate (equivalent to 2 g carnitine/day) or placebo for a 3 week period. The subjects then performed a bout of whole body intense resistance exercise with a carbohydrate-protein beverage or water provided immediately after exercise. Muscle biopsies were taken from the thigh before exercise and 1 hour after exercise to determine androgen receptor content. Three weeks of carnitine supplementation led to a significant increase in resting (pre-exercise) androgen receptor content compared to placebo. When a protein-carbohydrate beverage was consumed after resistance exercise there was a further increase in androgen receptor one hour after the workout. These results indicate that L-carnitine L-tartrate supplementation can favorably influence the capacity of skeletal muscle to take up and utilize testosterone from the circulation.

Kraemer WJ, Spiering BA, Volek JS, Ratamess NA, Sharman MJ, Rubin MR, French DN, Silvestre R, Hatfield DL, Van Heest JL, Vingren JL, Judelson DA, Deschenes MR, Maresh CM. Androgenic responses to resistance exercise: effects of feeding and L-carnitine. Med Sci Sports Exerc. 2006 Jul;38(7):1288-96.