L-Carnitine is most often used for
L-carnitine is a compound produced in the body from lysine and methionine. It is also found in food, primarily in meat products, 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. L-carnitine is best known for its role in helping to ”shuttle” long-chain fatty acids into mitochondria to produce energy.
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. However, in healthy people, supplementation with L-carnitine does not appear to improve exercise capacity or performance. That said, L-carnitine may help reduce exercise-induced muscle soreness and muscle damage, particularly following resistance exercise.
Supplementation with L-carnitine or acetyl-L-carnitine may improve sperm quality in males with infertility. Similarly, L-carnitine may improve ovulation and rate of pregnancy in women with polycystic ovary syndrome. 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 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, blood glucose, blood lipids (including triglycerides, total cholesterol, LDL-c, and HDL), and markers of oxidative stress and chronic inflammation. 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. However, because studies often also include weight loss drugs or lifestyle interventions (exercise and/or diet-induced weight loss), 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. Dry mouth and gastrointestinal discomfort have also been reported in some studies using L-carnitine, while gastrointestinal problems (including nausea and gastric pain) are a common adverse effect of propionyl-L-carnitine. However, several meta-analyses conclude that L-carnitine and its derivatives are generally safe and well‐tolerated. Furthermore, supplementation of up to 2000 milligrams (mg) per day is considered safe for human consumption. Some human studies have even used doses of L-carnitine as high as 6000 mg/day without serious side effects, but a thorough safety evaluation of such high doses is lacking.
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. 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. Therefore, L-carnitine may work via several mechanisms, and further research is needed for a full understanding.
- Glycine Propionyl-L-Carnitine
- Carnosine (the product of beta-alanine)
The standard dose for L-carnitine is between 500–2000 milligrams per day (mg/day).
Supplementation with up to 2000 mg/day of L-carnitine is considered safe for humans. There are various other forms of carnitine supplementation available: The equivalent dosage is up to about 2700 mg/day for acetyl-L-carnitine and up to about 2900 mg/day for propionyl L-carnitine.