L-carnitine is a naturally occurring amino acid derivative that's often taken as a supplement.
It is used for weight loss and may have an impact on brain function.
However, popular claims about supplements don’t always match up with the science.
This article examines the potential risks and benefits of L-carnitine supplements and explains how this nutrient functions in your body.
L-carnitine is a nutrient and dietary supplement.
The mitochondria act as engines within your cells, burning these fats to create usable energy.
Your body can produce L-carnitine out of the amino acids lysine and methionine.
For your body to produce it in sufficient amounts, you also need plenty of vitamin C (4).
L-carnitine is the standard biologically active form of carnitine, which is found in your body, foods and most supplements.
Here are several other types of carnitine:
- D-carnitine: This inactive form may cause a carnitine deficiency in your body by inhibiting the absorption of other, more useful forms (7, 8).
- Acetyl-L-carnitine: Often called ALCAR, this is possibly the most effective form for your brain. Studies suggest that it may benefit people with neurodegenerative diseases (9).
- Propionyl-L-carnitine: This form is well-suited for circulatory issues, such as peripheral vascular disease and high blood pressure. It may boost production of nitric oxide, which improves blood flow (10, 11).
- L-carnitine L-tartrate: This is commonly added to sports supplements due to its rapid absorption rate. It may aid muscle soreness and recovery in exercise (12, 13, 14).
For most people, acetyl-L-carnitine and L-carnitine seem to be the most effective for general use. However, you should always pick the form that's best for your personal needs and goals.
Role in Your Body
In cells, it helps transport fatty acids into the mitochondria, where they can be burned for energy.
Summary L-carnitine is an amino acid derivative that transports fatty acids into your cells to be processed for energy. It is made by your body and also available as a supplement.
In theory, using L-carnitine as a weight loss supplement makes sense.
Since L-carnitine helps move more fatty acids into your cells to be burned for energy, you might think this would increase your ability to burn fat and lose weight.
In an eight-week study in 38 women who exercised four times per week, there was no difference in weight loss between those who took L-carnitine and those who didn’t (24).
What’s more, five of the participants taking L-carnitine experienced nausea or diarrhea (24).
Another human study monitored L-carnitine's effect on fat burning during a 90-minute stationary bicycle workout. Four weeks of taking supplements did not increase fat burning (28).
However, one analysis of nine studies — mostly in obese individuals or older adults — found that people lost an average of 2.9 pounds (1.3 kg) more weight while taking L-carnitine (29).
More research is needed to confirm the benefits of L-carnitine in a younger, more active population.
While it may aid in weight loss for obese individuals or older adults, a thorough diet and exercise regimen must be in place first.
Summary Although the cellular mechanism of L-carnitine suggests it could benefit weight loss, its effects — if present at all — are small.
L-carnitine may benefit brain function.
In specific cases, this form may even protect your brain from cell damage.
In a 90-day study, people with alcohol addictions who took 2 grams of acetyl-L-carnitine per day experienced significant improvements in all measures of brain function (38).
More research is needed on long-term benefits for healthy individuals.
Summary L-carnitine — specifically acetyl-L-carnitine — can have beneficial effects on brain function in various diseases.
A few more health benefits have been linked to L-carnitine supplements.
In one study, 2 grams of acetyl-L-carnitine per day resulted in an almost 10-point drop in systolic blood pressure — the top number of a blood pressure reading and an important indicator of heart health and disease risk (23).
One 12-month study observed a reduction in heart failure and deaths among participants who took L-carnitine supplements (42).
The evidence is mixed when it comes to L-carnitine's effects on sports performance.
L-carnitine may benefit:
- Recovery: May improve exercise recovery (46, 47).
- Muscle oxygen supply: May increase oxygen supply to your muscles (48).
- Stamina: May increase blood flow and nitric oxide production, helping delay discomfort and reduce fatigue (48).
- Muscle soreness: May reduce muscle soreness after exercise (49).
- Red blood cell production: May increase the production of red blood cells, which transport oxygen throughout your body and muscles (50, 51).
Type 2 Diabetes
It may also combat diabetes by increasing a key enzyme called AMPK, which improves your body's ability to use carbs (56).
Summary Research suggests that L-carnitine may aid exercise performance and treat health conditions like heart disease and type 2 diabetes.
For most people, 2 grams or less per day is relatively safe and free from any serious side effects.
In one study, people who took 3 grams every day for 21 days experienced no negative effects (57).
In one review of L-carnitine's safety, doses of approximately 2 grams per day appeared to be safe for long-term use. However, there were some mild side effects, including nausea and stomach discomfort (24, 58).
However, L-carnitine supplements may raise your blood levels of trimethylamine-N-oxide (TMAO) over time. High levels of TMAO are linked to an increased risk of atherosclerosis — a disease which clogs your arteries (59, 60).
More studies on the safety of L-carnitine supplements are needed.
Summary Doses of 2 grams or less per day seem to be well tolerated and safe for most people. Tentative evidence suggests L-carnitine supplements might increase your risk of atherosclerosis.
The best sources of L-carnitine are (4):
- Beef: 81 mg per 3 ounces (85 grams)
- Pork: 24 mg per 3 ounces (85 grams)
- Fish: 5 mg per 3 ounces (85 grams)
- Chicken: 3 mg per 3 ounces (85 grams)
- Milk: 8 mg per 8 ounces (227 ml)
Interestingly, food sources of L-carnitine have a greater absorption rate than supplements.
According to one study, 57–84% of L-carnitine is absorbed when it's consumed from food, compared to only 14–18% when taken as a supplement (61).
As noted before, your body can also produce this substance naturally from the amino acids methionine and lysine if your stores are low.
For these reasons, L-carnitine supplements are only necessary in special cases, such as disease treatment.
Summary The main dietary sources of L-carnitine are meat, fish and some other animal products, such as milk. A healthy individual can also produce sufficient amounts within the body.
Your L-carnitine levels are influenced by how much you're eating and how much your body is producing.
Therefore, vegetarians and vegans may want to consider L-carnitine supplements. However, no studies have confirmed the benefits of carnitine supplements in these specific populations.
In one study, 2 grams of L-carnitine reduced fatigue and increased muscle function in older adults. Other research reveals that acetyl-L-carnitine may also help boost brain health and function as you age (64, 65).
As with any supplement, you should speak with your doctor before taking L-carnitine.
Summary Specific populations may benefit from L-carnitine supplements. This includes older adults and people who rarely or never eat meat and fish.
The standard dose of L-carnitine is 500–2,000 mg per day.
Although dosage varies from study to study, here is an overview of the use and dose for each form:
- Acetyl-L-carnitine: This form is best for brain health and function. Doses vary from 600–2,500 mg per day.
- L-carnitine L-tartrate: This form is most effective for exercise performance. Doses vary from 1,000–4,000 mg per day.
- Propionyl-L-carnitine: This form is best for improving blood flow in those with high blood pressure or related health conditions. Doses vary from 400–1,000 mg per day.
Up to 2,000 mg (2 grams) per day seems safe and effective in the long term.
Summary Although the recommended dose varies, around 500–2,000 mg (0.5–2 grams) seems to be both safe and effective.
L-carnitine is best known as a fat burner — but the overall research is mixed. It is unlikely to cause significant weight loss.
However, studies support its use for health, brain function and disease prevention. Supplements may also benefit those with lower levels, such as older adults, vegans and vegetarians.
Of the different forms, acetyl-L-carnitine and L-carnitine are the most popular and seem to be most effective.