L-Carnitine

What L-carnitine is

L-carnitine is a compound that your body produces itself; the last precursor is gamma-butyrobetaine. In the diet, it comes mainly from red meat. As a supplement, L-carnitine is available as the pure base, as L-carnitine L-tartrate, which was used, for example, in a 24-week study on muscle metabolism, and as fumarate. It should be distinguished from acetyl-L-carnitine (ALCAR), which carries an acetyl group and has mainly been studied for mood, nerves and memory.

In the US, levocarnitine is approved as a medicine, for congenital carnitine deficiency and for dialysis patients. In the BK-Score, human evidence stands at 6 out of 10 points. This axis rates the state of knowledge, not the substance: there are many studies, but the effects for healthy people are small.

How it works

Carnitine shuttles long-chain fatty acids into the mitochondria, where they are burned. This gave rise to the idea of the fat burner: more carnitine, more fat burning. It is not quite that simple, because muscle accumulates additional carnitine only slowly.

That it is possible was shown in humans by Wall and colleagues in 2011: 14 young men took L-carnitine L-tartrate for 24 weeks together with carbohydrates that were meant to raise insulin levels. After 24 weeks, muscle carnitine content was 21 percent higher, and during light exercise the muscles used 55 percent less glycogen, which is consistent with greater fat burning. All effects appeared only after 24 weeks.

What the studies show

Muscle soreness: Yarizadh 2020

The meta-analysis evaluated 7 randomized controlled trials, selected from 604 hits. L-carnitine reduced muscle soreness at all five time points, directly after exercise and after 24, 48, 72 and 96 hours. CK, myoglobin and LDH were lower only after 24 hours, not later. According to the authors’ analysis, the effect appeared with strength training and in untrained people.

Weight: Talenezhad 2020

Across 37 randomized trials with 2,292 participants, body weight fell by 1.21 kg (95 % interval −1.73 to −0.68), BMI by 0.24 kg/m² and fat mass by 2.08 kg (−3.44 to −0.72). Waist circumference and body fat percentage did not change significantly. The effect was most pronounced in people with overweight; the dose-response curve reached its maximum at about 2,000 mg per day.

Muscle metabolism: Wall 2011

In a double-blind study, 14 healthy men took 80 g of carbohydrates twice daily for 24 weeks, half of them additionally 2 g of L-carnitine L-tartrate each time. Muscle carnitine content rose only in the carnitine group, by 21 percent. During light exercise, the muscles spared 55 percent of glycogen; during intense exercise, the lactate content in muscle was 44 percent lower. Work output in the test rose by 11 percent compared with baseline, while it stayed the same in the control group.

After a heart attack: DiNicolantonio 2013

The meta-analysis of 13 controlled trials with 3,629 patients after acute myocardial infarction found 27 percent lower all-cause mortality with L-carnitine (odds ratio 0.73; 0.54 to 0.99), 65 percent fewer ventricular arrhythmias and 40 percent less angina. Heart failure and repeat infarctions did not become less frequent. The authors themselves call for large randomized trials under current therapy.

TMAO: Koeth 2013 and 2019, Samulak 2019

Gut bacteria break down carnitine into trimethylamine, from which TMAO is formed in the body. In mice, this accelerated atherosclerosis; in 2,595 patients, high carnitine levels predicted cardiac events only when TMAO was also high (Koeth 2013). In 2019, the same group showed with labeled carnitine that the pathway runs via gamma-butyrobetaine and that meat eaters produce more than 20 times as much TMAO as vegetarians and vegans; the second step could be stimulated by a meat diet and regular carnitine intake. In healthy older women, TMAO rose tenfold with 24 weeks of carnitine, while inflammatory values, vascular markers and blood lipids did not change (Samulak 2019).

Where the data stop

• Fat burner for people of normal weight. The effect on weight averages just over one kilogram, most clearly in people with overweight; waist circumference and body fat percentage did not change significantly.
• Rapid effect in the muscle. More carnitine in the muscle was measurable in the study by Wall only after 24 weeks and together with carbohydrates.
• Performance and recovery as an official claim. In 2011, EFSA assessed claims on recovery, muscle repair and endurance; none of them is on the EU list of authorized claims.
• Long-term consequences of TMAO. That TMAO rises sharply with carnitine has been measured in humans. Whether this increases cardiovascular risk over years has not been studied; the vascular markers measured so far remained unchanged over 24 weeks.
• Heart protection under current therapy. The myocardial infarction meta-analysis is based on older trials; the authors themselves call for new large trials.

Status, approval and legal

L-carnitine is sold in Germany as an ingredient of food supplements and sports products. Health claims for it are not authorized in the EU: in 2011, EFSA assessed claims including faster recovery after exercise, muscle repair and endurance, and L-carnitine is not on the list under Regulation (EU) No 432/2012.

As a medicine, levocarnitine is approved in the US: for primary systemic carnitine deficiency, for inborn errors of metabolism with secondary carnitine deficiency and for the prevention and treatment of carnitine deficiency in dialysis patients.

Safety

The US prescribing information lists mild gastrointestinal complaints such as nausea, vomiting, abdominal cramps and diarrhea, as well as body odor; these often subsided at a lower dose. Seizures have also been described, in people with and without a seizure disorder. Anyone with a seizure disorder should therefore discuss carnitine with a physician.

The open question is TMAO: the rise is well documented, its significance over years is not. In the BK-Score, safety stands at 7 out of 10 points; this axis measures how well safety has been studied, not how safe the substance is. This text is information and does not replace medical advice.

Sources

• Yarizadh H et al., J Am Coll Nutr 2020 – meta-analysis, 7 randomized trials, muscle soreness and muscle damage markers
• Talenezhad N et al., Clin Nutr ESPEN 2020 – meta-analysis, 37 randomized trials, 2,292 participants, body weight
• Wall BT et al., J Physiol 2011 – double-blind study, 14 men, 24 weeks, muscle carnitine and exercise metabolism
• DiNicolantonio JJ et al., Mayo Clin Proc 2013 – meta-analysis, 13 controlled trials, 3,629 patients after myocardial infarction
• Koeth RA et al., Nat Med 2013 – carnitine, gut microbiota and TMAO, mouse experiments and observation of 2,595 patients
• Koeth RA et al., J Clin Invest 2019 – isotope study, carnitine to TMAO via gamma-butyrobetaine in meat eaters and vegetarians
• Samulak JJ et al., Ann Nutr Metab 2019 – 24 weeks of carnitine in healthy older women, TMAO and atherosclerosis markers
• EFSA NDA Panel, EFSA Journal 2011 – assessment of health claims for L-carnitine, including recovery, muscle repair and endurance
• FDA – prescribing information Carnitor (levocarnitine), tablets and oral solution, 2015
• Regulation (EU) No 432/2012 – list of permitted health claims made on foods