L-carnitine

What is L-carnitine?

L-carnitine is a compound made in the body from the amino acids lysine and methionine, and it is also obtained from food, especially red meat and dairy. Its main job is to carry long-chain fatty acids into mitochondria, the parts of cells where fat is burned for energy. Because of that role, L-carnitine has long been marketed as a “fat burner” and an exercise performance aid.

Healthy adults usually make enough carnitine and get more from their diet. True deficiency is uncommon and is usually linked to genetic disorders, certain medications or kidney disease requiring dialysis.

What the research says

Because the body already makes carnitine, it was never obvious that extra would increase fat burning. Trials have produced mixed results, which is why pooled analyses are useful. A 2020 systematic review and meta-analysis in Clinical Nutrition ESPEN included 37 randomized controlled trials with 2,292 participants. On average, L-carnitine reduced body weight by about 1.2 kg, body mass index by about 0.24 points and fat mass by about 2 kg compared with placebo. There was no significant effect on waist circumference or body fat percentage. A dose-response analysis suggested the maximum effect on body weight at around 2,000 mg a day, and the benefit was seen mainly in adults with overweight or obesity.

The authors described the effect as modest. In practical terms, it is a small addition to the results of diet and exercise, not a stand-alone solution. We rate the evidence as limited for weight management in healthy people.

How it’s used in supplements

L-carnitine is sold as L-carnitine base, L-carnitine L-tartrate (common in sports products) and acetyl-L-carnitine (often marketed for brain health). Weight-related studies have generally used 1 to 3 grams a day for several weeks or months.

At those doses, side effects can include nausea, stomach upset, diarrhea and a “fishy” body odor. L-carnitine may reduce the effect of thyroid hormone and may increase the effect of the blood thinner warfarin, so people taking either should talk to their doctor first. Some research has also raised questions about whether high carnitine intake increases levels of TMAO, a compound produced by gut bacteria that has been studied in relation to heart health; the clinical meaning of this is still being investigated.

In Java Burn, L-carnitine is one of several ingredients in a packet that weighs only about 2.5 grams in total, and the amount is not disclosed. It is unlikely that a single packet contains the roughly 2,000 mg associated with the largest effects in research.