Supplements

L-Carnitine

Updated · 29 sources · Editorial policy

L-carnitine is the natural, biologically active form of carnitine, the compound that carries long-chain fatty acids into mitochondria to be burned for energy. As a prescription drug it is called levocarnitine (Carnitor). As a sports supplement it is sold mainly as plain L-carnitine or as L-carnitine L-tartrate (LCLT), and it is marketed for fat burning, endurance and recovery.

The most important fact for exercisers: muscle already holds most of the body’s carnitine, and oral doses barely change it. In controlled trials, muscle carnitine rose only after 12–25 weeks of daily dosing taken with an insulin-raising carbohydrate drink. The better-supported effect is a modest reduction in markers of muscle damage and soreness after hard exercise, mostly from small LCLT studies. No trial has shown an anabolic effect.

This page focuses on L-carnitine and LCLT as used by people who train. The Carnitine page is the library’s general overview of the same molecule and covers the prescription drug, acetyl-L-carnitine and propionyl-L-carnitine in more depth. This page summarizes what clinical trials, the FDA label and reviews report. It is for education only and is not medical advice.

Property Detail
Class Endogenous nutrient; dietary supplement; prescription drug as levocarnitine
Forms used in exercise studies L-carnitine (free base); L-carnitine L-tartrate (LCLT), a salt with tartaric acid
LCLT content 3 g of LCLT provides 2 g of L-carnitine (4.5 g provides 3 g)
Doses in cited exercise trials 1–3 g LCLT/day for 3–5 weeks (recovery); L-carnitine or LCLT with a carbohydrate drink, once or twice daily, for 12–25 weeks (muscle loading)
Oral bioavailability ≈15% for the drug product; 5–18% after 1–6 g oral doses
Half-life (label) Terminal elimination half-life 17.4 h after a 20 mg/kg IV dose
Muscle carnitine change About +20% after 12–25 weeks, only when taken with an insulin-raising carbohydrate drink
US status Dietary supplement; levocarnitine is an Rx drug; not a controlled substance
WADA status Not on the 2026 Prohibited List; IV infusions over 100 mL per 12 h are a prohibited method

What it is

L-carnitine is made in the body from lysine and is also eaten in meat. Its main job in muscle is to move long-chain fatty acids across the inner mitochondrial membrane so they can be oxidized. It also buffers excess acetyl groups, which helps the muscle switch between fat and carbohydrate as fuel (Stephens et al., J Physiol 2007). The full biology, food sources and the deficiency states it treats are covered on the Carnitine page.

L-carnitine vs L-carnitine L-tartrate

LCLT is L-carnitine bound to L-tartaric acid as a salt. It is the form used in most recovery trials. The difference matters mainly for dose: part of the weight of LCLT is tartrate.

Form What a dose contains Where it appears in research
L-carnitine (free base) All of the stated weight is L-carnitine Weight-loss trials, 9-week resistance-training trial, the prescription drug
LCLT 3 g LCLT = 2 g L-carnitine (Stefan et al., Nutrients 2021); 4.5 g LCLT = 3 g L-carnitine (Shannon et al., Am J Clin Nutr 2016) Recovery and muscle-damage trials; muscle-loading trials

Papers do not always state which weight they mean. Two papers from the same laboratory and design describe their dose as “2 g L-carnitine/day” (Volek et al. 2002) and “2 g LCLT per day” (Kraemer et al. 2003). When comparing studies or products, check whether a number refers to LCLT or to L-carnitine.

How it works in the body

Absorption

The Carnitor label reports about 15% absolute bioavailability for oral levocarnitine (Carnitor tablet label). A pharmacokinetics review found 5–18% for oral doses of 1–6 g, much less than the fraction absorbed from food. The kidneys reabsorb carnitine until plasma levels reach about 40–60 µmol/L, and excrete the excess (Evans and Fornasini, Clin Pharmacokinet 2003). Much of an unabsorbed dose is broken down by gut bacteria, which leads to TMAO (see side effects).

Why muscle loading needs insulin

Muscle takes up carnitine through a sodium-dependent transporter (OCTN2), and plasma carnitine rising on its own does not push more into muscle. The Nottingham group showed that insulin is the missing signal:

  • IV infusion, 8 men. Five hours of IV L-carnitine raised muscle total carnitine from 22.0 to 24.7 mmol/kg dry muscle only when insulin was clamped at a high level. At fasting insulin, muscle carnitine did not change (Stephens et al., FASEB J 2006).
  • Oral dosing with sugar. Men taking 3 g/day of L-carnitine excreted less carnitine in urine when they also drank carbohydrate solutions (24-hour excretion 155 vs 212 mg in the acute study). The authors concluded insulin increased whole-body carnitine retention (Stephens et al., J Appl Physiol 2007).
  • Protein did not substitute. After 4.5 g LCLT, an 80 g carbohydrate drink increased carnitine uptake across the forearm. A drink of 40 g carbohydrate plus 40 g protein raised insulin similarly but blunted uptake (Shannon et al. 2016).

Long-term loading trials

Trial Participants Daily regimen Duration Muscle total carnitine
Wächter et al., Clin Chim Acta 2002 8 healthy men L-carnitine 2 g twice daily, no carbohydrate 3 months No significant change
Wall et al., J Physiol 2011 14 healthy men 2 g LCLT + 80 g carbohydrate twice daily vs carbohydrate alone 24 weeks +21%
Stephens et al., J Physiol 2013 12 men 1.36 g L-carnitine + 80 g carbohydrate twice daily vs carbohydrate alone 12 weeks +20%
Chee et al., Aging Cell 2021 14 men aged about 69 4.5 g LCLT in a 44.4 g carbohydrate, 13.8 g protein drink vs placebo drink 25 weeks +20%

Every positive loading trial paired carnitine with a carbohydrate drink (44–160 g of carbohydrate a day) for months, and each enrolled 14 or fewer people. The Chee drink contained 13.8 g of protein and still raised muscle carnitine, so the acute protein finding above may not rule out mixed drinks over the long term. Short courses of a few weeks, which is the length of most recovery studies, are not expected to change muscle carnitine content.

Regulatory status

  • US: L-carnitine and LCLT are sold as dietary supplements. Levocarnitine is also an FDA-approved prescription drug for carnitine deficiency (Carnitor, approved 1985; Drugs@FDA, NDA 018948). It is not a controlled substance. The approved indications and labeled doses are listed on the Carnitine page.
  • WADA: carnitine is not named on the list. The list prohibits “intravenous infusions and/or injections of more than a total of 100 mL per 12-hour period,” except those legitimately received in hospital treatment, surgery or clinical diagnostic investigations. This applies to IV carnitine like any other substance (WADA 2026 Prohibited List, M2.2).

What the research shows

Fuel use and performance after muscle loading

In the 24-week Wall trial, the LCLT plus carbohydrate group used 55% less muscle glycogen during cycling at 50% VO₂max (maximal oxygen uptake) and had 44% lower muscle lactate at 80% VO₂max than the carbohydrate-only group. Work output in a 30-minute performance trial rose 11% from baseline, with no change in the control group (Wall et al. 2011).

In 14 older men who also did twice-weekly cycling, 25 weeks of LCLT in a carbohydrate-protein drink raised whole-body fat oxidation during moderate cycling by 20%, mainly from fat stored inside muscle. Insulin sensitivity did not change (Chee et al. 2021).

These are mechanistic results from small trials with demanding regimens. They do not show that a typical supplement dose taken without carbohydrate increases fat burning during exercise.

Performance without loading

A 2021 systematic review of 11 placebo-controlled studies found that 3–4 g of L-carnitine taken 60–90 minutes before testing, or 2–2.72 g/day for 9–24 weeks, improved some high-intensity measures such as Wingate peak power and leg-press repetitions. No acute or chronic regimen improved moderate-intensity performance (Mielgo-Ayuso et al., Nutrients 2021). The review pooled no numbers, and the included studies were small.

Individual trials illustrate the mixed picture:

Trial Design Regimen Main result
Koozehchian et al., J Exerc Nutrition Biochem 2018 RCT, 23 resistance-trained men, 9 weeks of training L-carnitine 2 g/day vs placebo Bench press volume and Wingate power rose within the carnitine group; the group-by-time interaction for bench press was not significant; no change in body weight, fat mass or fat-free mass vs placebo
Devrim-Lanpir et al., Nutrients 2025 Crossover, 20 trained men 3 g LCLT 90 min before a CrossFit workout 202.4 vs 204.5 repetitions with placebo (p = 0.810); no difference in perceived exertion

Recovery and muscle damage (LCLT)

This is the most consistent area of research, and almost all of it uses LCLT. In each trial, participants took the supplement for several weeks before a damaging bout of squats or lunges.

Trial Participants Regimen Findings vs placebo
Volek et al., Am J Physiol Endocrinol Metab 2002 10 resistance-trained men, crossover 2 g L-carnitine/day as LCLT, 3 weeks Lower myoglobin, creatine kinase and purine-breakdown markers; MRI muscle disruption 41–45% of the placebo area
Spiering et al., J Strength Cond Res 2007 8 men, repeated-measures 0, 1 or 2 g LCLT/day, 3 weeks Both 1 g and 2 g lowered hypoxanthine, xanthine oxidase, myoglobin and soreness
Ho et al., Metabolism 2010 9 men and 9 women (mean ages 45 and 52), crossover 2 g L-carnitine/day as LCLT, 3 weeks and 3 days Lower muscle-damage, free-radical and soreness markers; strength and power not affected
Stefan et al., Nutrients 2021 80 adults aged 21–65 randomized, 73 completed 3 g LCLT/day, 5 weeks Better perceived recovery and soreness (p = 0.021); lower creatine kinase (p = 0.016); smaller declines in strength and power; no change in body composition

A meta-analysis of 7 RCTs found lower soreness over 0–96 hours and lower damage markers at 24 hours, mainly in resistance-exercise studies and untrained people; details are on the Carnitine page (Yarizadh et al., J Am Coll Nutr 2020).

Limits of this evidence:

  • The trials are small (8–80 people) and short.
  • Most outcomes are blood markers and soreness scores. Only Stefan 2021 found a benefit on strength and power, and Ho 2010 found none.
  • Much of the work comes from one research group or from the LCLT manufacturer. The Stefan trial was funded by Lonza, which makes the Carnipure LCLT used in it and in Ho 2010, and two of its authors were Lonza employees. A 2018 review of carnitine and recovery was sponsored by Lonza (Fielding et al., Nutrients 2018).

Hormones and muscle growth

L-carnitine is sometimes promoted as having anabolic or hormonal effects. The trials do not support this:

  • Hormones. After 3 weeks of LCLT, the hormone response to squats was measured in 10 trained men. LCLT raised IGF-binding protein 3 but had no other direct effect on testosterone, growth hormone or IGF-1 (Kraemer et al., J Strength Cond Res 2003). A related study reported more androgen receptors in muscle, but not more muscle or strength; it is discussed on the Carnitine page.
  • Older women, no training. 1,500 mg/day of LCLT for 24 weeks raised plasma carnitine but did not change body composition, muscle strength, IGF-1 or inflammatory markers (Sawicka et al., Nutrients 2018).
  • Older women, resistance training. In a 24-week training study in women aged 60–75, 1 g LCLT plus 3 g leucine daily did not increase strength or MRI muscle volume more than leucine alone or no supplement (Sawicka et al., J Nutr Health Aging 2022).

Body composition

A meta-analysis of 37 RCTs found L-carnitine changed body weight by −1.21 kg (95% CI −1.73 to −0.68) and fat mass by −2.08 kg vs control, but not waist circumference or body fat percentage. Only the body weight result held in high-quality trials (Talenezhad et al., Clin Nutr ESPEN 2020). More detail is on the Carnitine page. In trained or exercising people, the controlled trials above (Koozehchian 2018, 9 weeks; Stefan 2021, 5 weeks) found no change in fat mass or lean mass.

One loading trial is often cited for fat loss. Over 12 weeks, men drinking 80 g of carbohydrate twice daily without carnitine gained 1.9 kg of body mass and 1.8 kg of fat. The group that took 1.36 g L-carnitine with the same drink did not gain weight, and their energy expenditure rose 6% (Stephens et al. 2013). This shows prevention of weight gain from an added 160 g of daily carbohydrate in 6 men per group, not fat loss.

Side effects and risks

Tolerability

The label lists nausea, vomiting, abdominal cramps, diarrhea and a body odor with oral carnitine, which often improve at lower doses (Carnitor tablet label). In the 5-week Stefan trial, mild adverse events (cramping, headache, nausea, upset stomach, swallowing problems) were reported at similar rates on LCLT and placebo: 42 vs 40 events, with none serious (Stefan et al. 2021). In the CrossFit trial, 10% of participants reported difficulty sleeping after LCLT (Devrim-Lanpir et al. 2025).

TMAO

Gut bacteria convert unabsorbed carnitine first to γ-butyrobetaine and then to trimethylamine, which the liver turns into trimethylamine N-oxide (TMAO). In mice, carnitine-rich diets raised TMAO and sped up atherosclerosis. In 2,595 people undergoing heart evaluation, higher plasma carnitine predicted heart disease risk only in those who also had high TMAO (Koeth et al., Nat Med 2013).

Supplement users are relevant to this in three ways:

  • Habitual intake changes the gut. Omnivores made more than 20 times more labeled TMAO from an oral carnitine dose than vegans and vegetarians. Taking L-carnitine daily for 2 months or more induced the bacterial step that produces trimethylamine (Koeth et al., J Clin Invest 2019). Several authors disclosed patents related to cardiovascular diagnostics, and some were employees of a consumer-products company.
  • LCLT raises TMAO in trials. 1 g/day of LCLT with leucine raised plasma TMAO over 24 weeks in older women (Sawicka et al. 2022). 1,500 mg/day of LCLT did the same, without changes in oxidative stress markers (Olek et al., Oxid Med Cell Longev 2019).
  • The rise reverses. TMAO returned to baseline within 4 months of stopping LCLT (Samulak et al., Nutrients 2019).

No trial has tested whether carnitine supplements cause heart attacks or strokes. The TMAO concern rests on associations and animal data, and the long-term significance of a supplement-driven TMAO rise is unknown.

Who should be cautious

The prescription label notes seizures in people with and without a seizure history, a rise in INR (a clotting-time measure) in people taking warfarin, and buildup of trimethylamine and TMAO with long-term high oral doses in severe kidney impairment or dialysis (Carnitor tablet label). These warnings apply to the same molecule sold as a supplement. Full details are on the Carnitine page.

Key takeaways

  • Muscle carnitine is hard to raise. Trials raised it by about 20% only with 12–25 weeks of daily dosing plus a carbohydrate drink. Plain L-carnitine for 3 months did not change it.
  • Check the dose basis. 3 g of LCLT provides 2 g of L-carnitine, and studies are not always clear which weight they report.
  • Recovery is the best-studied benefit. Small, mostly industry-linked LCLT trials found lower muscle-damage markers and soreness after hard exercise; effects on strength and power are inconsistent.
  • No anabolic or fat-loss effect in trainees. LCLT did not raise testosterone or add muscle, and trials in exercising people found no change in fat or lean mass.
  • It raises TMAO. Daily LCLT increases plasma TMAO, which reverses after stopping; whether this matters for heart health is unknown. Label warnings on seizures, warfarin and kidney disease also apply.

Sources

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