L-Carnitine for Heart, Fertility, Exercise and Weight: Evidence and TMAO Questions
What research found on L-carnitine for the heart, male fertility, exercise and weight, plus the open question about TMAO, in plain language with the limits of each study.
Content last updated . This page has not been medically reviewed.
What carnitine is
Carnitine is made in the body from two amino acids, lysine and methionine. [1] It is not one of the amino acids used to build proteins, so it is an amino acid derivative rather than a protein building block. It helps move fats into the cell's energy factories. Red meats such as beef and lamb are among the richest food sources, and fish, poultry and milk also contain it. [1]What the research found
Each row names the question, what the review found, and how much weight to give it. "Strength" is our plain-language judgement of the evidence as described in the abstracts we read.| Question | What the source found | Strength |
|---|---|---|
| Weight loss in adults | A review of 9 trials (911 people) found those given carnitine lost about 1.3 kg more than comparison groups. [2] A larger review of 37 trials reported a modest drop in weight, BMI and fat mass, but no significant effect on waist circumference or body fat percent. [3] | Small average effect; clinical meaning unclear |
| After a heart attack | A review of 13 controlled trials found lower death rates with carnitine. [4] A second review found no clear extra benefit between different daily amounts for death, heart failure, angina or repeat heart attack. [5] | Mixed; not a substitute for medical care |
| Chronic heart failure | A review of 17 trials (1,625 people) found better heart-function measures, but no significant differences in deaths, the 6-minute walk or side effects. [6] | Some measures improved; outcomes unclear |
| Male infertility | One review found better sperm movement and shape but no effect on sperm concentration and no demonstrable effect on pregnancy rates. [7] Another found small improvements, no difference in pregnancy rates, and poor study quality. [8] A network analysis reported better sperm quality and pregnancy rates, strongest for carnitine combined with micronutrients. [9] | Conflicting; pregnancy benefit not established |
| Exercise performance | A review found improved high-intensity performance in some studies and no improvement for moderate-intensity exercise. [10] An older review said carnitine appears safe but does not seem to have a significant effect on fatigue or exercise performance. [1] | Mixed |
Trials differ in people studied, form of carnitine and length. The amounts used in studies are study amounts, not advice, and we do not give a personal amount here.
The TMAO question
Gut bacteria can turn carnitine into a substance called TMAO. In a 2013 study, carnitine from red meat produced TMAO and sped up artery plaque in mice, and people who eat meat made more TMAO than vegans or vegetarians after taking carnitine. [11] A systematic review of healthy people taking carnitine for at least 12 weeks found that fasting TMAO rose, and called for longer studies on the heart. [12]What is not settled: a 2021 review says studies with carnitine show higher TMAO, yet in some cases no change in blood fats, and that the link between dietary carnitine and long-term TMAO and heart risk needs more research. [13] An Australia and New Zealand food regulator assessment concluded there is insufficient information to say whether TMAO is a surrogate marker for another factor, and judged intake up to 3 g a day of no public health concern for the food uses it was assessing. [14] We state both views because neither is final.
Labels and claims
Look for the form (L-carnitine, acetyl-L-carnitine, L-carnitine L-tartrate), the amount per serving and added ingredients. For weight claims, see weight-loss supplements and fat burners. For the brain-focused form, see Acetyl-L-Carnitine research.Frequently asked questions
Does L-carnitine burn fat?
Reviews of randomized trials found modest average weight loss, about 1 to 1.3 kg more than comparison groups, and no significant change in waist size in the larger review. [2][3] That is not the same as proof of "fat burning" for any one person.
Is L-carnitine good for the heart?
Reviews after heart attack and in heart failure found some improvements in measures, but the heart-failure review found no significant difference in deaths. [4][6] It is not a substitute for treatment prescribed by a cardiologist.
Does L-carnitine help men conceive?
Reviews find small gains in sperm movement or shape, but disagree on pregnancy rates, and one concluded study quality was poor. [7][8][9] A fertility specialist can discuss options; see male fertility concerns.
Is TMAO from carnitine dangerous?
Carnitine supplements raise TMAO in some studies, and TMAO has been linked to artery plaque in animal work. [11][12] Whether this causes harm in people is still being researched. [13]
Who should avoid L-carnitine or ask first?
People with thyroid disease, kidney disease or dialysis, heart disease, or who take prescription medicines should ask a clinician first. [15][14] Pregnancy and children were not covered in the sources we read.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety, interactions and who should ask first
- Side effects. The same regulator assessment found no adverse effects attributable to intakes up to 3 g a day in the studies it reviewed; above that, nausea, stomach upset and a fishy body or urine odor were the only treatment-related effects seen. [14] This is a regulator's review of study data, not a personal limit.
- Thyroid medicine. A 2001 trial describes carnitine as a naturally occurring peripheral antagonist of thyroid hormone action. [15] If you take thyroid medicine or have thyroid disease, ask your prescriber or pharmacist first.
- Kidney disease and dialysis. The regulator assessment notes that people on hemodialysis can have depleted carnitine stores and larger rises in blood carnitine. [14] Any use should be directed by the kidney care team.
- Heart conditions. Carnitine is not a replacement for heart treatment. Ask your cardiologist before adding it.
- Pregnancy, children, liver disease, seizure disorders, blood thinners, and surgery. Specific warnings for these were not described in the sources we read, so we cannot say it is safe. Ask a clinician, and see Supplements before surgery and Supplement safety resources.
Sudden chest pain, trouble breathing, or fainting need emergency care (911 in the US). Carnitine will not help with these.
References
- Pekala et al., L-carnitine: metabolic functions and meaning in humans life. Curr Drug Metab 2011 (abstract) Read 2026-10-23.
- Pooyandjoo et al., The effect of (L-)carnitine on weight loss in adults: systematic review and meta-analysis. Obes Rev 2016 (abstract) Read 2026-10-23.
- Talenezhad et al., Effects of L-carnitine supplementation on weight loss and body composition: systematic review and meta-analysis of 37 RCTs. Clin Nutr ESPEN 2020 (abstract) Read 2026-10-23.
- DiNicolantonio et al., L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis. Mayo Clin Proc 2013 (abstract) Read 2026-10-23.
- Shang et al., Effective dosing of L-carnitine in the secondary prevention of cardiovascular disease: systematic review and meta-analysis. BMC Cardiovasc Disord 2014 (abstract) Read 2026-10-23.
- Song et al., Efficacy and Safety of L-Carnitine Treatment for Chronic Heart Failure: meta-analysis of randomized controlled trials. Biomed Res Int 2017 (abstract) Read 2026-10-23.
- Khaw et al., L-carnitine and L-acetylcarnitine supplementation for idiopathic male infertility. Reprod Fertil 2020 (abstract) Read 2026-10-23.
- Sharma et al., Carnitine, coenzyme Q10 and selenium on pregnancy and semen parameters in couples with idiopathic male infertility: systematic review and meta-analysis. Urology 2022 (abstract) Read 2026-10-23.
- Zafar et al., Effectiveness of Nutritional Therapies in Male Factor Infertility Treatment: systematic review and network meta-analysis. Drugs 2023 (abstract) Read 2026-10-23.
- Mielgo-Ayuso et al., Effect of Acute and Chronic Oral L-Carnitine Supplementation on Exercise Performance Based on the Exercise Intensity: a systematic review. Nutrients 2021 (abstract) Read 2026-10-23.
- Koeth et al., Intestinal microbiota metabolism of L-carnitine, a nutrient in red meat, promotes atherosclerosis. Nat Med 2013 (abstract) Read 2026-10-23.
- Sawicka et al., The bright and the dark sides of L-carnitine supplementation: a systematic review. J Int Soc Sports Nutr 2020 (abstract) Read 2026-10-23.
- Thomas and Fernandez, Trimethylamine N-Oxide (TMAO), Diet and Cardiovascular Disease. Curr Atheroscler Rep 2021 (abstract) Read 2026-10-23.
- Food Standards Australia New Zealand, Application A1102 L-carnitine in food: Supporting document 1, risk and technical assessment (publication year not shown in the text we read) Read 2026-10-23.
- Benvenga et al., Usefulness of L-carnitine in iatrogenic hyperthyroidism: randomized, double-blind, placebo-controlled trial. J Clin Endocrinol Metab 2001 (abstract) Read 2026-10-23.