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Quick answer: Healthy adults make enough carnitine in the liver and kidneys, so most people do not need a supplement (NIH ODS). In trials, L-carnitine gave modest extra weight loss, about 1.2 to 1.3 kg more than placebo (Pooyandjoo et al., 2016; Talenezhad et al., 2020), and acetyl-L-carnitine did not improve memory tests in people with Alzheimer’s disease (Hudson and Tabet, 2003). Doses around 3 g a day can cause stomach upset and a fishy body odor.
In This Article
Most People Make Enough Carnitine
Carnitine is the general name for several related compounds, including L-carnitine, acetyl-L-carnitine and propionyl-L-carnitine. The body makes it in the liver, kidneys and brain from two amino acids, lysine and methionine. Healthy children and adults do not need carnitine from food or supplements, and in 1989 the Food and Nutrition Board concluded that it is not an essential nutrient (NIH ODS).
Red meat is the richest food source: 3 ounces of cooked beef steak provides about 42 to 122 mg, while vegetables, fruits and grains provide almost none. Needs can exceed what the body makes in some situations, such as premature birth or kidney problems, and some seizure medicines lower blood carnitine (NIH ODS).
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What the Trials Show for Each Form
L-carnitine for weight loss
A meta-analysis of 9 trials with 911 adults found that people taking carnitine lost 1.33 kg more than the control groups, and the effect got smaller the longer the trials lasted (Pooyandjoo et al., 2016). A larger meta-analysis of 37 trials with 2,292 people found 1.21 kg more weight loss and 2.08 kg more fat loss, but no change in waist size or body fat percentage. Only the effect on body weight held up in the high-quality trials, and the benefit leveled off at about 2,000 mg a day (Talenezhad et al., 2020).
The NIH Office of Dietary Supplements notes that weight was a secondary outcome in most studies and that the results are equivocal; in one 6-month trial, 2 g a day did not change weight in 94 adults with overweight and newly diagnosed type 2 diabetes.
L-carnitine for exercise
In a 24-week trial, 14 healthy men took 2 g of L-carnitine L-tartrate with 80 g of carbohydrate twice a day, while a control group took the carbohydrate alone. Muscle carnitine rose 21%, the men used 55% less muscle glycogen during easy cycling, and their work output in a performance test rose 11% from baseline, with no change in the control group (Wall et al., 2011). The carbohydrate was given to raise insulin, which helps move carnitine into muscle, and it adds 160 g of carbohydrate a day.
Without the carbohydrate, results were weaker: in 24 men who took 1 g twice a day for 12 weeks, carnitine did not change VO2 max, blood lactate or performance, and muscle carnitine rose about 13% only in the vegetarians (NIH ODS). A 2021 review of 11 studies found some gains in high-intensity tests with 3 to 4 g taken 60 to 90 minutes before or 2 to 2.72 g a day for 9 to 24 weeks, but no improvement in moderate-intensity exercise (Mielgo-Ayuso et al., 2021).
Acetyl-L-carnitine (ALCAR) for memory
A Cochrane review of 11 trials, all in people with Alzheimer’s disease, found no benefit of acetyl-L-carnitine on memory and thinking tests, dementia severity or daily function. Doctors’ overall ratings favored it at 12 and 24 weeks but not at 52 weeks, and the reviewers said this result may be due to chance (Hudson and Tabet, 2003). An older meta-analysis of trials in mild cognitive impairment and early Alzheimer’s disease, using 1.5 to 3.0 g a day for 3 to 12 months, found a small advantage over placebo (Montgomery et al., 2003). Neither review studied healthy people.
Glycine propionyl-L-carnitine (GPLC)
GPLC is sold for blood flow and performance, but the trials are few and small. In 32 adults doing aerobic training, 1 or 3 g a day for 8 weeks did not change performance or muscle carnitine (Smith et al., 2008). In 45 men who lift weights, 3.0 or 4.5 g a day for 4 weeks lowered power in later sprints by 2% to 7% compared with their starting tests (Jacobs and Goldstein, 2010). GPLC raised blood nitrate and nitrite, a marker of nitric oxide, in 11 of 15 men, but that study did not measure heart health or performance (Bloomer et al., 2007).
Doses Used in the Studies
| Form | Dose studied | Main finding | Caveat |
|---|---|---|---|
| L-carnitine (weight) | 1.8 to 4 g a day in most trials | About 1.2 to 1.3 kg more weight loss than placebo | Effect faded over time; no change in waist or body fat percentage |
| L-carnitine L-tartrate (exercise) | 2 g twice a day with 80 g of carbohydrate, 24 weeks | Muscle carnitine up 21%, work output up 11% | One small trial; needs 160 g of carbohydrate a day |
| Acetyl-L-carnitine (memory) | 1.5 to 3.0 g a day | No benefit on memory tests in Alzheimer’s disease | The reviews did not include healthy people |
| GPLC (performance) | 1 to 4.5 g a day | No benefit in an 8-week trial; less sprint power at 3 to 4.5 g | Few small trials |
Carnitine has no official upper limit, but doses of about 3 g a day can cause side effects (NIH ODS). Sources: Pooyandjoo et al., 2016; Talenezhad et al., 2020; Wall et al., 2011; Hudson and Tabet, 2003; Smith et al., 2008.
Why Stacking Carnitine Forms Is a Bad Idea
Some carnitine guides suggest taking 2 to 4 g of L-carnitine, 1 to 2 g of acetyl-L-carnitine and 1 to 3 g of GPLC together, which adds up to as much as 9 g a day. Side effects already appear at about 3 g a day (NIH ODS), and none of the trials described above tested the three forms together.
If you decide to try carnitine, pick one form for one goal, stay at the doses used in trials and stop if you get stomach problems or a fishy body odor. Carnitine also raises TMAO, a compound made by gut bacteria that has been linked to heart disease (Koeth et al., 2013), so more is not better.
How to Read a Carnitine Label
- Check the form. The two main forms in supplements are L-carnitine and acetyl-L-carnitine, and amounts range from about 3 mg to 5,000 mg (NIH ODS). Match the form to the trials for your goal.
- Check the total daily dose, adding up every product you take. Side effects appear at about 3 g a day (NIH ODS).
- Look for an independent quality seal such as NSF Certified for Sport, Informed-Choice or the Banned Substances Control Group, especially if you are drug-tested (NIH ODS).
- Be wary of claims such as “burns fat” or “boosts brain power”: the trials above found modest or no effects.
Side Effects, TMAO and Drug Interactions
Doses of about 3 g a day can cause nausea, vomiting, abdominal cramps, diarrhea and a fishy body odor. Carnitine can also cause muscle weakness in people with uremia, a complication of kidney failure, and seizures in people with seizure disorders (NIH ODS).
TMAO and heart health
Gut bacteria turn carnitine into TMAO. In mice, carnitine supplements raised TMAO and sped up atherosclerosis. In 2,595 people having heart checkups, higher blood carnitine predicted heart disease and heart attacks, strokes or deaths, but only when TMAO was also high, and people who eat meat made more TMAO from carnitine than vegans and vegetarians (Koeth et al., 2013). These are animal and observational findings, not proof that supplements cause heart disease.
In a 6-month trial in 157 adults with metabolic syndrome and existing carotid plaque, 2 g of L-carnitine a day did not change total plaque volume, but narrowing of the carotid arteries increased 9.3% more than with placebo. Total and LDL cholesterol also rose more, although those differences did not hold up after the authors corrected for testing many outcomes. The trial was funded by the Heart and Stroke Foundation of Canada and academic sources (Johri et al., 2022).
Medicines that interact with carnitine
- Pivalate-conjugated antibiotics, such as pivampicillin, can deplete carnitine with long-term use (NIH ODS).
- Valproic acid and other seizure medicines, including phenobarbital, phenytoin and carbamazepine, lower blood carnitine (NIH ODS). Do not start carnitine on your own if you take them; ask your doctor.
Common Questions About Carnitine
Does L-carnitine help you lose weight?
A little. Across 37 trials, people taking it lost 1.21 kg more than with placebo, but waist size and body fat percentage did not change (Talenezhad et al., 2020), and the effect shrank the longer the trials lasted (Pooyandjoo et al., 2016).
Do I need carnitine if I eat little or no meat?
Usually not. The liver and kidneys make enough for healthy adults. In a 12-week trial, vegetarians who took carnitine raised their muscle carnitine but did not perform better (NIH ODS).
Does acetyl-L-carnitine improve memory?
Not on objective tests in people with Alzheimer’s disease, according to a Cochrane review of 11 trials (Hudson and Tabet, 2003). The reviews did not include healthy people.
Is L-carnitine bad for your heart?
It is not settled. Gut bacteria turn carnitine into TMAO, which is linked to heart disease (Koeth et al., 2013), and in a trial in people with metabolic syndrome, 2 g a day was linked to faster narrowing of the carotid arteries (Johri et al., 2022). If you have heart disease or metabolic syndrome, ask your doctor first.
How much carnitine is too much?
There is no official upper limit, but about 3 g a day can cause nausea, vomiting, cramps, diarrhea and a fishy body odor (NIH ODS).
The Bottom Line on Carnitine
Most people do not need carnitine (NIH ODS). L-carnitine gives at most a small extra weight loss (Talenezhad et al., 2020), the clearest exercise result needed large amounts of carbohydrate (Wall et al., 2011), and acetyl-L-carnitine has not shown memory benefits on objective tests (Hudson and Tabet, 2003). The possible link with TMAO and faster artery narrowing is a reason for caution (Johri et al., 2022).
- Do not expect carnitine to burn fat on its own.
- If you try it, use one form at a time and stay at about 2 g a day or less.
- Stop if you get stomach problems or a fishy body odor.
- Ask your doctor first if you have heart disease, metabolic syndrome, kidney failure or seizures.
Scientific References
- Pooyandjoo et al., 2016. The effect of (L-)carnitine on weight loss in adults: a systematic review and meta-analysis of randomized controlled trials Obes Rev.
- Talenezhad 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 Clin Nutr ESPEN.
- Wall et al., 2011. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans J Physiol.
- Mielgo-Ayuso et al., 2021. Effect of Acute and Chronic Oral l-Carnitine Supplementation on Exercise Performance Based on the Exercise Intensity: A Systematic Review Nutrients.
- Hudson and Tabet, 2003. Acetyl-L-carnitine for dementia Cochrane Database Syst Rev.
- Montgomery et al., 2003. Meta-analysis of double blind randomized controlled clinical trials of acetyl-L-carnitine versus placebo in the treatment of mild cognitive impairment and mild Alzheimer’s disease Int Clin Psychopharmacol.
- Smith et al., 2008. Effect of glycine propionyl-L-carnitine on aerobic and anaerobic exercise performance Int J Sport Nutr Exerc Metab.
- Jacobs and Goldstein, 2010. Long-term glycine propionyl-l-carnitine supplemention and paradoxical effects on repeated anaerobic sprint performance J Int Soc Sports Nutr.
- Bloomer et al., 2007. Glycine propionyl-L-carnitine increases plasma nitrate/nitrite in resistance trained men J Int Soc Sports Nutr.
- Koeth et al., 2013. Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis Nat Med.
- Johri et al., 2022. Progression of atherosclerosis with carnitine supplementation: a randomized controlled trial in the metabolic syndrome Nutr Metab (Lond).
- NIH Office of Dietary Supplements. Carnitine: Fact Sheet for Health Professionals. Updated April 17, 2023.
- NIH Office of Dietary Supplements. Dietary Supplements for Exercise and Athletic Performance: Fact Sheet for Health Professionals. Updated April 1, 2024.
