L-Carnitine Explained Simply

1. What Is L-Carnitine?

L-carnitine is a substance that the body produces itself. It is found mainly in skeletal and cardiac muscle as well as in liver and kidney. The body needs it to convert fat into energy. Researchers study it primarily in athletes, in cardiovascular diseases, in unfulfilled desire to have children, in kidney diseases and in disorders of memory and nerves.

2. Where Is It Found, and What Is It Made From?

L-carnitine is found mainly in animal foods such as meat and dairy products. Plant foods contain only little of it. For dietary supplements, it is produced artificially, either by fermentation or synthetically. Studies examine various forms, for example free L-carnitine, acetyl-L-carnitine, L-carnitine L-tartrate and propionyl-L-carnitine. It is taken as a tablet or as an infusion.

3. What Does L-Carnitine Do? What Research Shows

Metabolism and Blood Lipids

Several large reviews show that L-carnitine can improve blood sugar and blood lipids in people with diabetes, impaired glucose tolerance or obesity 4572428. In women with polycystic ovary syndrome, blood sugar and blood lipids also decreased 13. In people with metabolic syndrome, however, cholesterol and narrowing of the carotid artery increased more with L-carnitine than with placebo (sham treatment) 37, and in dialysis patients, LDL cholesterol and blood lipids increased with L-carnitine tablets 58. With combination products containing other substances, the effect cannot be attributed to L-carnitine alone 5041.

Heart and Circulation

A review found that after heart surgery, the heart's pumping power improved and atrial fibrillation occurred less frequently with L-carnitine 16. In dialysis patients, pumping power was also better 20. In coronary heart disease, inflammatory markers decreased more than with placebo (sham treatment) 45; reviews also found a lower inflammatory marker in the blood in other groups 433. However, a small study after bypass surgery showed no advantage 53. The results apply only to the respective form and preparation studied.

Dialysis and Kidney Disease

In dialysis patients, carnitine levels rose with both intravenous and oral administration 405455. In carnitine deficiency, muscle mass and hand grip strength were maintained with intravenous administration, whereas they declined without treatment 43. Whether L-carnitine improves the effect of erythropoietin, a medication for anemia, has not been answered consistently: one review and one preliminary study found a better response 1156, another study and another review found no difference 409. The results come predominantly from intravenous administration and cannot be readily transferred to oral intake in other population groups.

Cognition and Nervous System

In diabetic neuropathy, a combination product of acetyl-L-carnitine and other substances improved symptoms and nerve conduction values 42. In breast cancer patients receiving paclitaxel, oral acetyl-L-carnitine reduced neuropathic complaints 52. In hepatic encephalopathy, oral acetyl-L-carnitine improved ammonia levels and the number connection test in a review 18. In healthy young adults, a review found no effect on reaction time, alertness and memory after short-term intake; however, the study situation does not yet allow a reliable conclusion 32.

Fertility

In men with impaired fertility, reviews found better sperm motility 82530. In women with polycystic ovary syndrome, pregnancy and ovulation rates were higher with L-carnitine 12. In IVF patients with previous failure, oral L-carnitine was associated with more oocyte maturation and live births 59. A study in women with polycystic ovary syndrome undergoing artificial fertilization, however, found no differences in oocyte maturation, embryo quality and pregnancy rates 38.

Tolerability and Safety

Several controlled studies (with a comparison group) found no differences in adverse events 19353657. In hepatic encephalopathy, they were rare and minor 18. A review in dialysis patients and a study in pemphigus vulgaris reported no side effects 951. Less frequent vomiting occurred in a study with oral acetyl-L-carnitine in breast cancer patients 52. However, a large study in women receiving taxane chemotherapy found that acetyl-L-carnitine did not prevent nerve damage and nerve complaints were later more severe than with placebo 34. Overall, the evidence is low, the studies are small and heterogeneous (very different).

Other Investigations

Review articles and several studies with placebo comparison found lower inflammation values in the blood with L-carnitine 2293139464749 and less oxidative stress, meaning less burden on the cells from aggressive oxygen compounds 2294748. After exercise, the muscles recovered better with L-carnitine L-tartrate than with placebo 44. Systematic reviews (ordered summary of all studies) on poisoning 22 and on skin treatment 23 also include combinations with other substances that do not allow conclusions about L-carnitine alone. The overall strength of evidence is high. However, the heterogeneity of forms, doses and populations limits generalization.

4. How Was L-Carnitine Taken in the Studies?

In the studies, L-carnitine was predominantly taken as a tablet or capsule. Free L-carnitine (levocarnitine), acetyl-L-carnitine and L-carnitine L-tartrate were used. In dialysis patients, levocarnitine was given partly as an infusion after dialysis, partly as a tablet. Individual investigations examined combination products with other substances. From such mixtures, no effect of L-carnitine alone can be derived.

5. Is L-Carnitine Safe? Side Effects in the Studies

The results always apply only to the exact form and preparation studied. In dialysis patients, an infusion was often used; these results cannot be readily transferred to the tablet in other population groups. If a preparation contains other substances besides L-carnitine, no effect of the individual substance can be derived. Since L-carnitine is endogenous and taken in through the diet, the baseline status of the participants is also crucial for interpreting a finding. The studies are often small and short, and long-term studies on safety in healthy individuals are still pending.

All details with tables and sources: full expert page on L-Carnitine