Unless you suffer from certain medical problems, your kidneys and liver make enough carnitine from the amino acids methionine and lysine to meet your body's needs. Thus, there isn't a recommended dietary allowance for carnitine, and it isn't considered an essential nutrient. Check with your doctor before taking supplements containing L-carnitine or other forms of carnitine to make sure it's safe for you -- and to figure out a correct dosage.
According to a review article published in Mayo Clinic Proceedings in June 2013, L-carnitine may help prevent heart disease. People also sometimes take carnitine in hopes of improving their athletic performance, limiting the effects of aging, decreasing the adverse effects of chemotherapy, improving insulin sensitivity or slowing the progression of HIV, although the evidence for these benefits is still preliminary and conflicting, according to the Office of Dietary Supplements.
Where It's Found
Dairy products and red meat provide most dietary carnitine, but you also get some from poultry, fish, wheat, peanut butter, tempeh, avocados and asparagus. L-carnitine is the least expensive and most commonly available carnitine supplement, but acetyl-L-carnitine is the form often used in studies on brain disorders and aging, and propionyl-L-carnitine is used in heart disease studies. Avoid D-carnitine supplements, as they may interfere with L-carnitine absorption.
A study published in Nature Medicine in 2013 found that the metabolism of L-carnitine may increase the risk for heart disease by promoting atherosclerosis, or clogging of the arteries, in some people. This effect appears to be more common in meat-eaters than in vegetarians, but further research is necessary to clarify the effects of L-carnitine on heart disease risk.
Side Effects and Contraindications
High doses of L-carnitine may cause vomiting, nausea, stomach cramps, diarrhea, body odor, rash and an increased appetite. L-carnitine may interact with certain medications, including thyroid hormones, making them potentially less effective. L-carnitine supplements may increase the frequency of seizures in those with seizure disorders.