L-Carnitine

What it is

The body makes L-carnitine on its own from two amino acids, so unlike many compounds in this reference, it already exists naturally in cells rather than being an entirely foreign molecule. Its central role is shuttling long chain fatty acids across the membrane of mitochondria, where the cell converts fat into usable energy through a process called beta-oxidation.

An intravenous form, marketed under the approved name Carnitor, is used for people whose bodies do not make or retain enough L-carnitine on their own, including some people on kidney dialysis. Separately from that approved use, researchers have studied other forms of L-carnitine for weight, heart related outcomes, and recovery after intense exercise, questions distinct from the approved deficiency use.

How it is studied

Much of the higher quality evidence on L-carnitine comes from pooled analyses, called meta-analyses, that combine results across many randomized controlled human trials, some involving thousands of participants in total. Outcomes tracked include body weight and fat mass, heart rhythm and chest pain events after a heart attack, and recovery markers after resistance exercise, mainly muscle enzyme levels in the blood measured over a few weeks of daily amounts.

What studies report

• A pooled analysis of 37 randomized trials with 2,292 participants reported that L-carnitine significantly lowered body weight, body mass index and fat mass compared with placebo, with the reduction described as modest. [1]
• That same pooled analysis reported a non-linear relationship between the amount taken and weight loss, with 2,000 milligrams a day estimated to give close to the largest effect seen. [1]
• A pooled analysis of 13 controlled trials in people recovering from a heart attack, covering 3,629 patients, reported lower rates of death from any cause, dangerous heart rhythm problems, and chest pain, compared with people who did not receive it. [2]
• A three week trial in resistance trained men reported that taking 2 grams of L-carnitine L-tartrate a day lowered blood markers of muscle tissue damage and reduced muscle disruption seen on imaging after a bout of high repetition squat exercise, compared with placebo. [3]

What is not known

The weight and fat mass changes reported in the first pooled analysis are small in absolute terms, and it is not clear how much of the heart related findings from the second analysis would apply outside people recovering from a recent heart attack, since carnitine's approved use is for a specific carnitine deficiency, not general heart disease prevention. Effect sizes vary noticeably from study to study, and the exercise recovery finding comes from young, trained men over three weeks, so it is not known whether the same benefit would appear in other groups or over longer periods.

Reported adverse findings

• None of the three analyses cited here reported a significant excess of side effects from L-carnitine, though the heart attack recovery analysis involved a population that is usually monitored closely regardless of treatment. [2]

Regulatory status

An intravenous form of L-carnitine, marketed as Carnitor, is approved by the United States Food and Drug Administration to treat a first-hand or downstream carnitine shortfall, including in some people on kidney dialysis. That approval covers a specific medical deficiency, not general weight, heart or exercise use, which remain separate research questions.