L-carnitine is one of the world's most popular sports supplements — and one of the most misunderstood. Here's what the research actually shows.
What Is L-Carnitine?
L-carnitine is synthesised in the liver and kidneys from the amino acids lysine and methionine. Its primary role is transporting long-chain fatty acids across the inner mitochondrial membrane, where they're oxidised (burned) for ATP energy. Without sufficient carnitine, fatty acid transport is impaired.
The Mechanism — and Its Limits
The logic: if carnitine is the "shuttle" transporting fat into cells to burn, more carnitine should mean more fat burning. The reality: healthy adults synthesise sufficient carnitine, and supplemental doses don't automatically increase fat oxidation unless stores are genuinely depleted — more common in vegetarians, vegans, and older adults who get little dietary carnitine.
What the Meta-Analysis Shows
A 2016 meta-analysis in Obesity Reviews (9 RCTs, 911 participants) found L-carnitine supplementation produced a modest but statistically significant additional weight loss of approximately 1.33 kg (2.9 lbs) versus placebo. Effects were strongest in older adults and those with metabolic conditions. Benefits are most pronounced when combined with exercise.
Best Forms
- L-carnitine L-tartrate: Best bioavailability; best for exercise performance and recovery
- Acetyl-L-carnitine (ALCAR): Crosses the blood-brain barrier; best for cognitive benefits alongside fat metabolism
- Glycine propionyl-L-carnitine (GPLC): Best for circulation and cardiovascular health
Dosage
500–2,000mg per day. Take with a carbohydrate-containing meal (insulin spike helps uptake into muscle). 30–60 minutes before exercise for performance benefits. Red meat and dairy are richest dietary sources — vegetarians tend to have lower baseline levels and may see greater supplementation benefit.