L-Carnitine

L-carnitine is a compound produced in the body from lysine and methionine and is also found in food (primarily in meat products). It can be acetylated to produce acetyl-L-carnitine (ALCAR), which is similar to L-carnitine but crosses the blood-brain barrier more efficiently. L-carnitine is best known for its involvement in the mitochondrial oxidation of long-chain fatty acids.

L-carnitine seems to have high utility in liver diseases, where it reduces ammonia levels and improves the symptoms of hepatic encephalopathy and various markers of liver function. An improvement in sperm-quality has been found with supplementation at high doses, and improvements in male fertility have been noted in a small number of studies. L-carnitine seems to help individuals with polycystic ovary syndrome by reducing some of the symptoms, with one study reporting an increase in fertility.

Acetyl-L-carnitine (and possibly L-carnitine too, but studies have only used ALCAR) shows some efficacy in improving the symptoms of depression, though more research is needed to determine how useful it is. Although it seems to reduce fatigue in older adults with low muscular endurance, its effects in athletes during physical activity aren't particularly consistent, though research supports small improvements. L-carnitine may slightly limit muscle-damage during resistance exercise.

When it comes to fat-loss, individual studies have not shown very good results. Some studies have found minor fat loss, which is typically attributed to increased physical activity due to increased energy levels.

L-carnitine seems to have minor beneficial effects on blood pressure, blood glucose, insulin sensitivity, blood lipids, oxidative stress, and inflammation. Overall, it seems somewhat beneficial in metabolic syndrome.

A few studies have noted a benign adverse effect of “fishy” odor with L-carnitine supplementation, which is likely due to the formation of trimethylamine. Gastrointestinal discomfort has also been reported in some studies. One study has reported a narrowing of the carotid arteries of individuals with metabolic syndrome who supplemented with 2 grams of L-carnitine for six months.

It’s not yet entirely clear how L-carnitine works. That said, evidence suggests that it may exert some of its potential beneficial effects by supporting cellular energy production, boosting antioxidant capacity, protecting cellular membranes from oxidative stress, reducing inflammation, and increasing nitric oxide levels.