acetyl-l-carnitine

Acetyl-L-carnitine is one of the two main forms of carnitine sold as a dietary supplement (the other is L-carnitine). Carnitine is a nutrient the body uses to help turn fats into energy by shuttling fatty acids into the mitochondria, the cells' energy compartments. According to the NIH Office of Dietary Supplements, healthy children and adults make all the carnitine they need, so it is classed as "conditionally essential" rather than a required nutrient, and the body absorbs carnitine from food (mainly red meat, with smaller amounts in poultry, fish, and dairy) far better than it absorbs carnitine from supplements.

Evidence is mixed and incomplete for every use, and acetyl-L-carnitine is not proven to treat, cure, or prevent any condition. For dementia and cognitive decline the research is genuinely inconclusive: a Cochrane review cited by NIH found that the routine clinical use of acetyl-L-carnitine for the signs and symptoms of dementia was not justified, and the NIH says its effects on Alzheimer's disease remain unclear and under active study. A few small or older trials reported short-term gains, but more recent studies were less positive. For mood, the support is thin and indirect: Memorial Sloan Kettering notes one trial in elderly patients with dysthymic disorder where it appeared comparable to fluoxetine, which is far from establishing a benefit. For diabetic peripheral neuropathy, MSK notes acetyl-L-carnitine "may help reduce" it based on a small number of trials, so any benefit is preliminary. For male fertility, a pooled analysis cited by NIH found supplemental carnitine improved sperm motility by about 8% and morphology by about 5% but did not change sperm concentration, and a Cochrane review found no effect on live-birth or pregnancy rates. Importantly, MSK reports that acetyl-L-carnitine can actually increase chemotherapy-induced peripheral neuropathy (an effect that persisted at 2-year follow-up), and current guidelines do not recommend it for preventing that complication.

There is no recommended daily amount for carnitine because the body makes its own, and the NIH notes that supplement amounts vary widely (roughly 3 mg to 5,000 mg per product). Research studies have generally used acetyl-L-carnitine or L-carnitine at doses up to about 2 to 4 grams per day, but these are study doses, not official recommendations or a safe-use guideline. Because acetyl-L-carnitine may not be appropriate for everyone and can interact with medications, talk to your doctor or pharmacist before starting it and to decide whether any dose is right for you.

There is no established tolerable upper intake level for carnitine. The NIH reports that doses of roughly 3 grams a day or more can cause side effects (see below). People with kidney problems (uremia) or a seizure disorder should be especially cautious, because high amounts can cause muscle weakness in people with uremia and can increase the risk of seizures in people prone to them. Cancer patients should be particularly careful: Memorial Sloan Kettering warns that acetyl-L-carnitine can increase nerve pain caused by chemotherapy. As with any supplement, anyone who is pregnant, nursing, taking medication, or managing a medical condition should check with a clinician first.

According to the NIH, doses of about 3 grams a day or more can cause side effects that are mostly mild and digestive — nausea, vomiting, abdominal cramps, and diarrhea — along with a characteristic "fishy" body odor. The fishy odor is linked to gut bacteria converting carnitine into a compound called TMAO (trimethylamine N-oxide); some research associates higher TMAO with increased cardiovascular risk, though the NIH notes the implications are not well understood and need more study. Less common effects include muscle weakness in people with kidney problems (uremia) and seizures in people with a seizure disorder.

Carnitine may add to the effect of blood thinners and increase bleeding risk, so Memorial Sloan Kettering advises talking to your provider if you take anticoagulants such as warfarin. Carnitine has also been studied as a peripheral antagonist of thyroid hormone, meaning it could counteract thyroid hormone activity, so it may be relevant if you are on thyroid treatment. Separately, the NIH notes that certain medications can lower the body's carnitine levels, including some pivalate-containing antibiotics and anticonvulsants such as valproic acid and phenobarbital. Because supplement-drug interactions can be serious, tell your doctor or pharmacist about acetyl-L-carnitine before combining it with any prescription medication.