Supplements and energy drinks that contain L-carnitine claim to be metabolism boosters that can help you shed weight and increase your athletic performance. However, there may not be enough scientific data to back up these claims. Read on to find out why.

The word “carnitine” comes from “carnis,” the Latin word for flesh, which is also the root of the word “carnivore.”

Your liver and kidneys make L-carnitine from two amino acids, lysine and methionine, so it isn’t required in your diet.

L-carnitine is stored in your skeletal muscles, brain, and heart. In men, L-carnitine is also stored in sperm.

This compound plays an important role in your body, escorting long-chain fatty acids to your cells’ mitochondria, where they produce energy. L-carnitine also ushers toxic byproducts of energy production out of your cells, so they don’t build up.

The Office of Dietary Supplements at the National Institutes of Health states that healthy adults and children don’t need to take L-carnitine supplements except for specific medical or genetic conditions.

According to the Office of Dietary Supplements, studies confirm that cancer patients who took between 250 milligrams (mg) to 4 grams (g) of L-carnitine per day experienced less fatigue. Research also shows that L-carnitine may:

  • improve insulin sensitivity and nerve pain in people with type 2 diabetes
  • increase fertility in men
  • lower mortality risk in those with ischemic myocardial infarction, or heart attack
  • result in mild cognitive improvements in older people

Twenty years of research can’t consistently support the claims about L-carnitine’s fat-burning capacity or its marketed ability to spur peak athletic performance.

L-carnitine is also found in food, mostly in red meat. A 4-ounce steak contains between 56 and 162 mg. A 4-ounce hamburger delivers between 87 and 99 mg of L-carnitine. The redder the meat, the higher its L-carnitine concentration. Smaller amounts of L-carnitine are also present in fish, poultry, and dairy products.

L-carnitine supplements at 3 g per day have potential side effects such as nausea and vomiting, cramps, and diarrhea. L-carnitine supplements can even wreck your social life, because taking 3 g a day can produce a fishy body odor.

People with kidney disease or seizure disorders should exercise particular caution with L-carnitine supplements or energy drinks that contain it, like Monster Energy. That’s because L-carnitine can cause muscle weakness in people with kidney disease, and seizures in people who have a seizure disorder. However, some studies show that L-carnitine may be beneficial for those on a specific type of dialysis.

The most serious complication associated with L-carnitine is its link to an elevated risk of heart disease. Cleveland Clinic researchers found that a metabolite of L-carnitine drives up your chances of developing atherosclerosis, or hardening of the arteries.

L-carnitine itself isn’t the problem. It’s what happens when it meets the bacteria in your intestines.

When you tuck into a steak or swallow an L-carnitine capsule, the bacteria in your gut produce a chemical called trimethylamine. Then your liver converts trimethylamine into trimethylamine N-oxide, or TMAO. This byproduct gums up your arteries.

Non-meat-eaters appear to have different gut bacteria and don’t experience the same TMAO spike if they do eat meat or take an L-carnitine pill. Researchers still don’t know which gut bacteria trigger TMAO production after eating meat or taking L-carnitine.

Further studies to identify these bacteria will reveal more about the interchange between what we eat, our intestinal flora, and the potential to develop cardiovascular disease.

The evidence about L-carnitine, TMAO, and heart disease prompted Mehmet Oz, MD, to backtrack on his prior support for L-carnitine supplements. Once a proponent, Oz rescinded his claim that L-carnitine could help you lose fat. He issued a warning not to take L-carnitine.

Who might need L-carnitine supplements?

A preterm infant who can’t make enough L-carnitine may need them. A rare genetic defect called primary carnitine deficiency may also require taking these supplements.

Those who experience nerve pain due to neuropathy or diabetes, have cognitive decline, are going through cancer treatment, or become deficient in L-carnitine for specific medical reasons may benefit from supplementation, though they should only do so under a doctor’s care.

For everyone else, it’s best to give L-carnitine a wide berth. If you’re a fan of energy drinks that contain L-carnitine, know that you’re unlikely to get a metabolic advantage.

The occasional steak won’t do you in, but you’re best off limiting your consumption of red meat to the occasional few bites.