Critical Commentary

The findings of preliminary, uncontrolled studies suggest that oral carnitine supplementation improves sperm motility both in a quantitative and qualitative manner [51,52]. This is in agreement with recent literature which reported thatboth l-carnitine (LC) and l-acetyl-carnitine (LAC) improved sperm motility parameters, especially in groups with lower baseline motility [54, 55]. A metabolic alteration has been postulated. Defective function in one of the intracellular carnitine-dependent systems may lead to a reduction in fatty-acid oxidation, thereby impairing energy-dependent sperm functions such as motility. Carnitine supplementation provides additional substrate for sperm energy metabolism and motility.

Carnitine's action to improve sperm parameters may differ according to baseline semen characteristics. Lenzi et al. were unable to demonstrate a change in seminal plasma levels in the carnitine arm [53], whereby baseline levels may have possibly not allowed, although unproven, detection of small but physiologically important increases in carnitine levels. Hence, the lack of improvement in semen parameters may in fact be due to no increase in seminal plasma or sperm carnitine levels.

LC is thought to work in a dose-dependent manner, as 2 g/day was reported to elevate sperm concentration, while a 3 g/day yielded no improvement. The lack of significant variation in sperm concentration following therapy excludes a direct positive effect on the spermatogenic process. The effects of carnitine may be post-testicular, as many reports do not account for any improvement in morphology [53, 56]. Carnitine supplementation in combination therapy with the antioxidant cinnoxicam was seen to improve sperm concentration, motility, and morphology in patients with iOAT [54].

Importantly, no side effects were reported in any subjects receiving LC/LAC treatment. In some cases, sperm parameters failed to improve. However, many of the studies reported elevated pregnancy rates as a result of the intervention. Therefore, the mechanism by which long-term combined carnitine treatment may improve fertilization capacity may not be directly evident from microscopic analysis [55]. Large-scale randomized controlled and dose-finding trials are necessary to confirm carnitine's effects on sperm characteristics. In vitro studies may help to further elucidate the compound's mechanism of action.

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