l-carnitine

L-carnitine is a nutrient that the body produces on its own (in the liver and kidneys) and also obtains from food, with red meat being a particularly rich source; according to the NIH Office of Dietary Supplements, it plays a role in converting food into usable energy. Because healthy children and adults synthesize all the carnitine they need, the NIH does not classify it as an essential nutrient, and no recommended daily amount has been established for it. The two forms most commonly sold as supplements are L-carnitine and acetyl-L-carnitine, and the NIH notes that the body absorbs carnitine from food far more efficiently than from supplements.

According to the NIH, researchers are still investigating whether carnitine supplements meaningfully affect health, and so far the results have been largely mixed or inconclusive. Studies covering heart disease, peripheral artery disease, athletic performance, weight loss, osteoarthritis, insulin resistance and diabetes, and dementia have produced inconsistent findings, and the NIH repeatedly emphasizes that more research is needed; in the weight-loss studies where any effect was seen, the difference was small, on the order of a few pounds. The most consistent signal is that carnitine may improve sperm health in men with infertility, though the NIH notes this does not appear to raise pregnancy rates in their female partners. There is no evidence that healthy people who already make enough carnitine benefit from taking it as a supplement.

No recommended daily amount has been set, because a healthy body makes all the carnitine it needs and a typical omnivorous diet supplies roughly 24 to 145 mg per day. Clinical studies have generally used about 1 to 4 grams per day (some cardiovascular trials used more), while much higher doses (around 20 to 200 mg/kg/day) are reserved for diagnosed carnitine deficiency managed under medical supervision. Because doses of roughly 3 grams or more per day can cause side effects, talk with your doctor or pharmacist before taking carnitine, especially if you have a health condition or take other medications.

The NIH states that carnitine obtained from foods and beverages is safe and that no upper limit has been established. However, taking about 3 grams or more per day as a supplement can cause adverse effects; in people with chronic kidney disease it may bring on muscle weakness, and in people with a seizure disorder it may increase seizure risk. Separately, the NIH notes that gut bacteria can metabolize carnitine into TMAO, a substance that some studies have associated with higher cardiovascular risk; this effect appears more pronounced in people who eat meat than in vegetarians, and the NIH says the implications are not yet well understood and require further research.

At doses of roughly 3 grams or more per day, carnitine supplements may cause digestive upset (such as nausea, vomiting, stomach cramps, and diarrhea) as well as a fishy body odor. At higher amounts, the NIH notes that carnitine can also cause muscle weakness in people with chronic kidney disease and may raise seizure risk in people who have a seizure disorder.

Some medications can lower the body's carnitine levels. The NIH notes that long-term use of certain antibiotics, such as pivampicillin (used to help prevent urinary tract infections), can reduce carnitine in the body, and that anticonvulsant medications such as phenobarbital and valproic acid can lower blood levels of carnitine. Let your doctor, pharmacist, and other health care providers know about all supplements and medicines you take so they can check for possible interactions.