L-Carnitine

An amino acid derivative the body makes, taken mainly for weight and exercise.

People take it for

Weight and body fat
Exercise and muscle
Tiredness and mood
Sperm quality
Cholesterol and blood sugar
Inflammation markers

A plain-English overview, not advice. Written from published reviews and trials and from what people say in public forums.

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What is L-Carnitine?

L-carnitine is a small compound built from amino acids that every mammal carries, and its job is ferrying long-chain fats across the inner wall of the mitochondria, the compartments of a cell where fuel is burned. The liver and kidneys make some of it and the rest arrives in food, mostly meat and dairy, so people who eat little of either take in less. That place at the door of the fat-burning machinery is what made it popular, and on a UK shelf it appears as plain L-carnitine, as L-carnitine tartrate and as acetyl-L-carnitine.

Why do people take L-Carnitine?

Weight and body fat

The thinking. Carnitine’s everyday job is carrying fat into the mitochondria to be burned, so the thought that taking more tips the body towards using fat is an old one. It is the commonest reason people in Britain buy it, usually alongside a change in eating or training.

What research has found. Four pooled analyses of randomised trials measured weight, body mass index and fat mass, the largest gathering eight earlier reviews and over sixteen thousand people, and found average falls of around a kilogram, mainly in adults carrying extra weight. This is one of the best-studied uses, and several reviews of trials broadly agree.

Exercise and muscle

The thinking. Muscle holds a large share of the body’s carnitine and makes none of its own. Reviews describe the free carnitine inside working mitochondria as what limits how fast fat can be burned once an effort gets hard, so the idea is that topping the store up keeps that route open.

What research has found. A review of eleven trials sorted the results by how hard the exercise was: high-intensity tests came out better, moderate efforts showed no difference, whether taken for weeks or an hour beforehand. A trial in older women measured no change in strength over 24 weeks. Studies here point in different directions, so this one is still being worked out; the dose, how long people took it and who took part all seem to matter.

Tiredness and mood

The thinking. Tiredness is one of the commonest things people hope a supplement will shift, and carnitine’s place in energy metabolism makes it an obvious candidate. The acetyl form crosses into the brain readily, and low levels of it have been described alongside low mood.

What research has found. A pooled analysis of twelve randomised trials in just under eight hundred adults measured depression scores and found them lower than with placebo, most clearly in older adults. Two small trials measured better mood and grip strength; a larger one, in adults having cancer care, found no difference in daily tiredness. A fair body of trials has looked at this, and most point the same way.

Sperm quality

The thinking. Carnitine is normally abundant in the fluid where sperm mature, and reviews describe it as a natural antioxidant that soaks up the reactive molecules known to damage sperm. The idea is that adding more steadies that environment, and men often take it while a couple is trying for a baby.

What research has found. Three pooled analyses measured sperm movement, concentration and shape, and mostly found better movement and a higher share of normally formed sperm; one also measured a rise in testosterone, while another found no change in concentration. Where trials recorded pregnancy rates there was no clear difference. Research here is still young: a few studies so far, mostly pointing the same way.

Cholesterol and blood sugar

The thinking. Carnitine sits where the body decides what to do with fat, so it is sometimes picked up after a blood test shows a high cholesterol or a high fasting sugar. Reviews note that carnitine levels run low in people with type 2 diabetes.

What research has found. Four pooled analyses measured blood fats and sugar: total and LDL cholesterol, triglycerides and lipoprotein(a) came out lower on average, most clearly above two grams a day, and fasting glucose fell a little in adults with type 2 diabetes. One found that fall only where carnitine was swallowed, not where it was given by drip. Only a few studies have measured this so far, though they mostly agree.

Inflammation markers

The thinking. Besides moving fat about, reviews describe carnitine as a free-radical scavenger, able to mop up some of the reactive molecules a cell throws off. The thinking is that less of that everyday wear shows as lower readings on inflammation markers.

What research has found. Two pooled analyses, the larger covering 48 trials and over three thousand adults, measured C-reactive protein, interleukin-6 and TNF-alpha and found them lower on average than with placebo, most clearly beyond twelve weeks. Three smaller trials, in adults with narrowed heart arteries and in intensive care, pointed the same way. Several reviews of trials have measured this, and they broadly agree.

What does L-Carnitine do in the body?

Fat cannot get into the mitochondria on its own. Long-chain fatty acids are latched onto carnitine, carried across the inner membrane and released inside, where they are broken down for energy. Cells draw carnitine from the blood through a transporter built for the job, and the kidneys reclaim most of what passes through.

Carnitine does a second job on the way out. When a cell gathers more activated fat and other acyl groups than it can use, carnitine binds them into acylcarnitines, which can leave the cell, keeping free coenzyme A available for the rest of its chemistry. How much of a swallowed dose reaches muscle is limited, which is why the trials that raised muscle carnitine gave it with carbohydrate.

What do people say about taking L-Carnitine?

These are anecdotes people have shared, not evidence.

What people like

Four of these six threads come from one long-term fatigue community, so the energy reports may reflect that group as much as the supplement.
Less tiredness and clearer thinking is the commonest good report, and several say they noticed it within the first few days.
A few have taken it for a year or more, one in six-week cycles, describing steadier energy and thinking over that time.

Common complaints

Disturbed sleep and a wired, overstimulated feeling is much the commonest complaint, and taking it first thing did not always help.
Several describe the opposite of what they hoped for: more tiredness, or feeling dazed for a day or two afterwards.
Loose stools and stomach upset come up repeatedly, usually on an empty stomach or once the daily amount goes above a gram.
In the two exercise and weight threads, the report is of nothing happening at all, after a fortnight and after six months.

How people take it

Amounts range widely, from a fraction of a capsule to about two grams a day; 500 mg before a session in the exercise threads.
Splitting the day’s amount in two and taking it with a meal is the usual answer given to loose stools.
Morning dosing is the common choice, made to protect sleep, and CoQ10, vitamin B12, magnesium and potassium are the usual companions.

Read from these public discussions:

Problems with Acetyl L-Carnitine? (Phoenix Rising forums, 2012)
Carnitine = more fatigue (Phoenix Rising forums, 2014)
Acetyl l carnitine side effects (Phoenix Rising forums, 2015)
L-carnitine is working for me (Phoenix Rising forums, 2018)
L-carnitine (Cyclingnews forum, 2015)
L-Carnitine (Cathe fitness forums, 2004)

What forms of L-Carnitine are there?

Form	What’s different	L-carnitine by weight	Per serving, on products we list
L-Carnitine	Plain L-carnitine, the form the body itself carries; sold as capsules, tablets and liquid.	about 100.0 %	500 mg
L-Carnitine tartrate	L-carnitine paired with tartaric acid as a salt, so part of each gram’s weight is the acid.	about 68.2 %	not in our price tables yet
Acetyl-L-carnitine (ALCAR)	L-carnitine carrying an acetyl group, one of the acylcarnitines the body forms as it shifts fats about.	about 100.0 %	500 mg

What else is worth knowing about L-Carnitine?

Gut bacteria turn some swallowed carnitine into a compound called TMAO. A review of trials in healthy adults on one to four grams a day found fasting TMAO rose, with no change in the inflammation or oxidation markers measured alongside, and asked for longer studies of the heart.
One trial gave three grams a day of the acetyl form to women having taxane chemotherapy: their nerve symptom scores came out worse than on placebo at 24 weeks, and worse still two years on.
Trials used 250 mg to four grams a day, most often one to three grams, for a few weeks up to six months. Labels give the amount per serving, which is often more than one capsule.
Who took part varies: adults carrying extra weight, adults with type 2 diabetes or narrowed heart arteries, men with fertility problems, and older adults. Where a review counted why people stopped, the reasons were headaches, facial tingling and stomach upsets, at rates close to placebo.

If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.

The research behind this page (32 studies)

How much does L-Carnitine cost?

That depends on the form and the pack, so we work out the price of the same dose of every L-Carnitine product we can find and rank them.

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