Four grams of L-carnitine per day for three months: that is what eight healthy men took in a Swiss trial, without significantly increasing their muscle content. This result challenges L-carnitine's “fat burner” image. Here is its actual role, what studies show about weight and exercise, and the precautions to know.
Key takeaways
- The body makes L-carnitine from lysine and methionine; red meat is its richest dietary source.
- Swallowed L-carnitine accumulates poorly in muscle: it took 24 weeks and 160 g of carbohydrate per day to increase it by 21%.
- Weight: around 1.2 kg more lost than control groups in a meta-analysis of 37 trials, mainly in adults with overweight.
- No health claim is authorised for carnitine in the EU; around 3 g per day, digestive complaints and a fishy body odour are possible.
What is L-carnitine?
L-carnitine is a small molecule the body makes in the liver, kidneys and brain from two amino acids: lysine and methionine. Around 95% of the body's carnitine is found in the heart and muscles.
Like taurine, it is mainly found in animal products. Per 85 g of cooked food, beef steak provides 42 to 122mg and chicken breast 2 to 4mg; a large glass of whole milk (240 ml) contains 8mg. For a 75 kg person, an omnivorous diet supplies 24 to 145mg per day, a vegan diet around 1.2mg, and the body makes around 14mg.
According to the US National Institutes of Health (NIH), healthy adults do not need to consume it: the liver and kidneys produce enough. True primary deficiencies of genetic origin or secondary deficiencies associated with chronic kidney failure or certain long-term medicines require medical care.
What does L-carnitine do in the body?
It acts as a shuttle for fats. Long-chain fatty acids cannot enter mitochondria, the cells' power plants, on their own. Carnitine carries them in and returns to collect more. Inside, their breakdown through beta-oxidation supplies NADH to the respiratory chain. Carnitine also helps remove some toxic compounds from mitochondria. Hence an appealing idea: adding more would make fat melt away.
Does L-carnitine “burn fat”?
Not simply by swallowing more. Fat transport depends on the carnitine already present in muscle, and these stores are difficult to increase.
Muscle stores are difficult to fill
Supplements are poorly absorbed: 14 to 18% of the dose, compared with 63 to 75% of dietary carnitine, according to the NIH. Excess in the blood leaves in urine, and insulin promotes carnitine entry into muscle. In the Swiss trial, 2 g taken twice daily for 3 months raised muscle content from 4.10 to 4.79 µmol/g, a non-significant difference, with no effect on maximal oxygen consumption or power.
The carbohydrate trial: what it actually showed
A British team overcame this barrier by adding plenty of carbohydrate to raise insulin. For 24 weeks, 14 young men of normal weight took 80 g of carbohydrate twice daily, with or without 2 g of L-carnitine L-tartrate. In the carnitine group, muscle content increased by 21%.
During moderate exercise, this group used 55% less muscle glycogen, indicating greater fat use. During intense exercise, its muscle contained 44% less lactate, and its performance over 30 minutes improved by 11%, compared with no change in the other group. But 160 g of carbohydrate per day, around 640 kcal, for six months is not a weight-loss strategy.
L-carnitine and weight loss: what do meta-analyses say?
They find an effect, but a small one. A 2020 meta-analysis pooled 37 randomised trials with 2,292 participants: L-carnitine produced an average of 1.21 kg more weight loss than controls, and 2.08 kg of fat mass loss in trials measuring it. Waist circumference and body fat percentage did not change significantly. An earlier analysis of 9 trials with 911 participants cited by the NIH found 1.33 kg; the NIH calls for larger studies.
- Trials are small, with around sixty participants on average.
- The effect is mainly observed in adults with overweight or obesity.
- Restricted to good-quality trials, the analysis confirms only the effect on weight.
- Dose-response analysis placed the maximum effect at around 2 g per day.
This magnitude, around one kilogram, resembles that of probiotics studied for weight (see our article on Lactobacillus gasseri and weight). Such a difference replaces neither diet nor physical activity.
Exercise: does L-carnitine improve performance or recovery?
Results are mixed. A 2021 systematic review analysed 11 studies with 203 participants aged 18 to 46. At moderate intensity, none of the six relevant studies showed a gain. At high intensity, some observed a gain with 3 to 4 g taken 60 to 90 minutes before the event, or 2 to 2.72 g daily for 9 to 24 weeks. The authors emphasise the small samples.
For recovery, a meta-analysis of 7 trials found less pronounced muscle soreness up to 96 hours after exercise, but reduced muscle-damage markers only at 24 hours and only in resistance training or untrained people. The link between intense exercise and free radicals is explained in our article on oxidative stress.
Tartrate, acetyl and propionyl: how do the forms differ?
Liquid, powder or capsule (see hard capsule, soft capsule or tablet), the presentation does not change the molecule. Compare the amount of L-carnitine actually supplied, not the salt's weight.
| Form | What it is | What is known |
|---|---|---|
| L-carnitine | Natural form, found in food | Studied for weight and exercise |
| L-carnitine L-tartrate | Salt containing around 68% L-carnitine | Trials on exercise and recovery |
| L-carnitine fumarate | Salt linked to fumaric acid | Rarely studied alone |
| Acetyl-L-carnitine | L-carnitine carrying an acetyl group | Fatigue and cognition in older people: limited evidence |
| Propionyl-L-carnitine | L-carnitine carrying a propionyl group | Walking-related leg pain: 1 positive trial of 3 |
| D-carnitine, DL-carnitine | “Mirror” form of L-carnitine | Avoid |
In peripheral arterial disease of the legs, propionyl-L-carnitine at 2 g per day for 4 to 6 months improved maximal walking time in only one of three trials. D-carnitine, L-carnitine's mirror image, should be avoided: a myasthenia-like muscle weakness syndrome was reported with the DL-carnitine mixture, but not with L-carnitine.
Acetyl-L-carnitine: fatigue and cognition in older people
In a double-blind Italian trial, 96 people aged 71 to 88 reporting fatigue received acetyl-L-carnitine, 2 g twice daily for 180 days according to reviews summarising it, or placebo. Their fatigue decreased and their MMSE cognitive score improved by 3.4 points versus 0.5 with placebo. These findings from a team responsible for several similar trials require confirmation; lasting fatigue warrants medical advice.
In dementia, the 2003 Cochrane review of 11 trials, all in Alzheimer's disease, found a benefit in doctors' overall assessments, possibly due to chance, but none on objective measures. No evidence justifies routine use.
TMAO: should you worry?
What is established: unabsorbed L-carnitine is converted by intestinal bacteria into trimethylamine, which the liver converts into TMAO. Omnivores produce more than vegetarians and vegans. In 2,595 patients referred for cardiac assessment, high blood carnitine was associated with more cardiovascular disease and events, but only when TMAO was also high.
What is not established: a cause-and-effect relationship in humans, since accelerated atherosclerosis has been shown only in mice. In a placebo-controlled trial in 18 women aged 65 to 70, 1.5 g of L-carnitine L-tartrate per day for 24 weeks increased TMAO tenfold without changing cholesterol. Levels returned to normal four months after stopping. Choline follows the same pathway.
Adverse effects and interactions: what precautions apply?
Around 3 g per day, L-carnitine can cause nausea, vomiting, abdominal cramps, diarrhoea and a fishy body odour. It can also cause muscle weakness in severe kidney failure and seizures in people prone to them.
- Vitamin K antagonists: two published cases from 1993 and 2004 describe an increased acenocoumarol effect. In the second, INR rose to 4.65 after ten weeks at 1 g per day, then stabilised on stopping.
- Thyroid hormones: L-carnitine inhibits their entry into cell nuclei. In 50 women taking high-dose levothyroxine, 2 or 4 g daily reduced symptoms of excess hormones. As a precaution, consult your doctor if you are treated for hypothyroidism.
- Antiepileptic medicines such as valproate, phenobarbital, phenytoin and carbamazepine, and long-term pivalate antibiotics, lower carnitine. Any supplementation is a medical decision.
Does L-carnitine have a health claim?
No. No health claim is authorised in the European Union for carnitine in any form. An L-carnitine supplement therefore cannot claim an effect on weight, energy, performance or recovery. The study findings presented here apply to a specific dose, population and duration.
Frequently asked questions
When should you take L-carnitine: before or after exercise?
In single-dose trials considered positive, 3 to 4 g were taken 60 to 90 minutes before intense exercise. The trial increasing muscle carnitine combined it with carbohydrate to use insulin's effect. No timing guarantees a result.
What daily dose of L-carnitine?
There is no recommended intake. Trials presented here used 1 to 4 g per day; for weight, the greatest difference from controls corresponded to around 2 g. According to the NIH, no upper limit has been established.
Is L-carnitine harmful to the liver?
The NIH does not list liver injury among its adverse effects. Valproate, an antiepileptic medicine, can be toxic to the liver; this poisoning is treated in hospital, sometimes with injectable L-carnitine. With liver disease, consult your doctor.
Do vegetarians lack L-carnitine?
Their diet provides very little, but the body makes it. In a 12-week study, 1 g twice daily increased muscle carnitine by around 13% in 16 vegetarians, but not in 8 omnivores, with no effect on maximal oxygen consumption.
Does L-carnitine improve male fertility?
It is studied for sperm quality. A 2022 Cochrane review including 6 carnitine trials in 1,089 men observed better sperm motility and concentration at some time points, but no effect on pregnancies or births. Difficulty conceiving warrants a medical assessment of the couple.
L-carnitine or creatine: what is the difference?
They are distinct molecules. Creatine has an authorised claim: “Creatine increases physical performance in successive bursts of short-term, high intensity exercise”, for consumption of 3 g per day. L-carnitine has none.
Conclusion
L-carnitine is essential for transporting fats into mitochondria, but a healthy adult's body produces enough. As a supplement, it accumulates poorly in muscle. In trials, supplemented participants lost around one kilogram more than controls, and performance findings remain small and inconsistent. Training, in contrast, genuinely changes muscle metabolism, including its NAD+ (see how to increase NAD+). If you take anticoagulants or thyroid treatment, first seek advice from a health professional.
Sources
- NIH, Office of Dietary Supplements (2023). Carnitine: Fact Sheet for Health Professionals (updated 17 April 2023). ods.od.nih.gov
- Commission Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health. Official Journal of the European Union, L 136, 1-40 (consolidated version of 19 August 2024). EUR-Lex
- Wächter S et al. (2002). Long-term administration of L-carnitine to humans: effect on skeletal muscle carnitine content and physical performance. Clinica Chimica Acta, 318(1-2), 51-61. doi:10.1016/S0009-8981(01)00804-X
- Wall BT et al. (2011). Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans. Journal of Physiology, 589(4), 963-973. doi:10.1113/jphysiol.2010.201343
- Talenezhad N et al. (2020). Effects of L-carnitine supplementation on weight loss and body composition: a systematic review and meta-analysis of 37 randomized controlled clinical trials with dose-response analysis. Clinical Nutrition ESPEN, 37, 9-23. doi:10.1016/j.clnesp.2020.03.008
- Mielgo-Ayuso J et al. (2021). Effect of acute and chronic oral L-carnitine supplementation on exercise performance based on the exercise intensity: a systematic review. Nutrients, 13(12), 4359. doi:10.3390/nu13124359
- Yarizadh H et al. (2020). The effect of L-carnitine supplementation on exercise-induced muscle damage: a systematic review and meta-analysis of randomized clinical trials. Journal of the American College of Nutrition, 39(5), 457-468. doi:10.1080/07315724.2019.1661804
- Malaguarnera M et al. (2008). Acetyl L-carnitine (ALC) treatment in elderly patients with fatigue. Archives of Gerontology and Geriatrics, 46(2), 181-190. doi:10.1016/j.archger.2007.03.012
- Hudson S, Tabet N (2003). Acetyl-L-carnitine for dementia. Cochrane Database of Systematic Reviews, (2), CD003158. doi:10.1002/14651858.CD003158
- Bazzato G et al. (1981). Myasthenia-like syndrome after D,L- but not L-carnitine. Lancet, 317(8231), 1209. doi:10.1016/S0140-6736(81)92372-2
- Koeth RA et al. (2013). Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nature Medicine, 19(5), 576-585. doi:10.1038/nm.3145
- Samulak JJ et al. (2019). Plasma trimethylamine-N-oxide following cessation of L-carnitine supplementation in healthy aged women. Nutrients, 11(6), 1322. doi:10.3390/nu11061322
- Benvenga S et al. (2001). Usefulness of L-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial. Journal of Clinical Endocrinology and Metabolism, 86(8), 3579-3594. doi:10.1210/jcem.86.8.7747
- Bachmann HU, Hoffmann A (2004). Interaction of food supplement L-carnitine with oral anticoagulant acenocoumarol. Swiss Medical Weekly, 134(25-26), 385. doi:10.4414/smw.2004.10631
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
