L-carnitine is a compound produced in the body from lysine and methionine.^[2][3] It is also found in food, primarily in meat products,^[4][5] and can be taken as a supplement. L-carnitine can be acetylated to produce acetyl-L-carnitine (ALCAR), which is similar to L-carnitine but crosses the blood-brain barrier more efficiently.^[6] L-carnitine is best known for its role in helping to ”shuttle” long-chain fatty acids into mitochondria to produce energy.^[7][2]

In people with peripheral artery disease (PAD), L-carnitine and propionyl-L-carnitine can help reduce a symptom called intermittent claudication (leg pain during exercise) and improve exercise capacity.^[8][9][10] However, in healthy people, supplementation with L-carnitine does not appear to improve exercise capacity or performance.^[11][12] That said, L-carnitine may help reduce exercise-induced muscle soreness and muscle damage, particularly following resistance exercise.^[13]

Supplementation with L-carnitine or acetyl-L-carnitine may improve sperm quality in males with infertility.^[14][15] Similarly, L-carnitine may improve ovulation and rate of pregnancy in women with polycystic ovary syndrome.^[16][17] However, there are very few studies examining the effect of carnitine on fertility; further high-quality randomized controlled trials are needed.

Supplementation with L-carnitine and acetyl-L-carnitine can lower liver enzymes^[18][19][20] in people with nonalcoholic fatty liver disease (NAFLD). Furthermore, L-carnitine may also have minor beneficial effects on metabolic syndrome. For example, supplementation with L-carnitine can improve blood pressure,^[21][22] blood glucose,^[23] blood lipids (including triglycerides,^{[18][21][24][25]} total cholesterol,^[24][25][26] LDL-c,^[25][26] and HDL^[25]), and markers of oxidative stress and chronic inflammation.^[27] Some meta-analyses have also found a small (on the order of 1 kg) beneficial effect of supplementation with L-carnitine, acetyl-L-carnitine, or propionyl-L-carnitine on weight loss.^[28][29][30] However, because studies often also include weight loss drugs or lifestyle interventions (exercise and/or diet-induced weight loss),^{[31][28][29][30]} further high-quality studies are needed to isolate the effect of carnitine supplements on weight loss and other aspects of metabolic syndrome.

Some studies report a “fishy” odor emanating from breath and sweat following supplementation with L-carnitine, which is likely due to the formation of trimethylamine.^[32] Dry mouth and gastrointestinal discomfort have also been reported in some studies using L-carnitine,^[33] while gastrointestinal problems (including nausea and gastric pain) are a common adverse effect of propionyl-L-carnitine.^[9][10] However, several meta-analyses conclude that L-carnitine and its derivatives are generally safe and well‐tolerated.^[18][9][10] Furthermore, supplementation of up to 2000 milligrams (mg) per day is considered safe for human consumption.^[1] Some human studies have even used doses of L-carnitine as high as 6000 mg/day without serious side effects,^[34] but a thorough safety evaluation of such high doses is lacking.^[1]

It’s not entirely clear how L-carnitine works. In tissues like muscle, L-carnitine is known to help “shuttle” long-chain fatty acids into mitochondria to produce energy.^[7][2] Further evidence suggests that it may exert some of its potential beneficial effects by boosting antioxidant capacity, protecting cellular membranes from oxidative stress, reducing inflammation, and increasing nitric oxide levels.^[3][6][2][7] Therefore, L-carnitine may work via several mechanisms, and further research is needed for a full understanding.