L-carnitine is a naturally occurring amino acid derivative that's often taken as a weight loss supplement.
It plays a crucial role in the production of energy by transporting fatty acids into your cells' mitochondria (1, 2, 3).
The mitochondria act as engines within your cells, burning these fats to create usable energy.
Your body can actually 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).
In addition to the L-carnitine produced in your body, you can also obtain small amounts from the diet by eating animal products like meat or fish (5).
Vegans or people with certain genetic issues may be unable to produce or obtain enough. This means that it's a "conditionally essential" nutrient (6).
L-carnitine is an amino acid derivative that can be produced in the body or obtained by eating animal flesh. It is also available as a supplement.
Different types of carnitine L-carnitine is the standard biologically active form of carnitine, which is found in your body, in foods and in most supplements.
Here are several other types of carnitine:
• D-Carnitine: This inactive form may cause a carnitine deficiency in the human 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 the brain. It may also be used to treat neurological conditions such as Alzheimer's disease.
• Propionyl-L-Carnitine: This form is well-suited for blood-flow-related issues such as peripheral vascular disease and high blood pressure. It may work via the production of nitric oxide, which improves blood flow (9, 10).
• L-Carnitine L-Tartrate: This is one of the most common forms found in sports supplements, due to its rapid absorption rate. It may help with exercise-related factors such as muscle soreness and recovery (11, 12, 13).
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.
Although L-carnitine is the standard form, you can also take acetyl-L-carnitine, propionyl-L-carnitine and L-carnitine L-tartrate.
L-carnitine's role in the body
L-carnitine's main role in the body has to do with mitochondrial function and energy production (3, 14, 15).
In cells, it helps transport fatty acids into the mitochondria, where they can be burned for energy.
About 98% of the body's L-carnitine stores are found in your muscles, along with trace amounts in the liver and blood (16, 17).
For general health, it benefits your mitochondrial function and may help increase
mitochondrial growth and health, which plays a key role in disease and healthy aging (18, 19, 20, 21).
Newer research has shown the potential benefits of different forms of carnitine, which may be used for conditions including heart and brain diseases (22, 23).
L-carnitine's primary role is to transport fatty acids into your cells to be processed or "burned" by your mitochondria for energy.
L-carnitine and weight loss
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.
However, the human body is extremely complex, and the results of both human and animal studies are mixed (24, 25, 26, 27).
In one study, 38 women were split into two groups. One group took an L-carnitine supplement, while the other did not. Both performed four exercise sessions per week for eight weeks.
The researchers found no difference in weight loss between the two groups, although five participants taking L-carnitine did experience nausea or diarrhea (24).
Another human study monitored L-carnitine's effect on the amount of fat that participants burned during a 90-minute stationary bicycle workout.
The researchers found that four weeks of taking supplements did not increase the amount of fat that participants burned (28).
However, one analysis of nine studies found that participants lost an average of 2.9 lbs (1.3 kg) more weight when they were taking L-carnitine. Most of these studies were on obese individuals or the elderly (29).
More research is needed to confirm the benefits of L-carnitine in a younger, more active population. It may aid in weight loss for obese individuals or the elderly, although a solid diet and exercise regimen must be in place first.
Although the cellular mechanism of L-carnitine makes it seem like it could benefit weight loss, the effects are small and research is mixed.
Effects on brain function
L-carnitine may have benefits for brain function.
Some animal studies suggest the acetyl form, acetyl-L-carnitine (ALCAR), can help prevent age-related mental decline and improve markers of learning (30, 31).
In human studies, taking acetyl-L-carnitine daily helped reverse the decline in brain function associated with Alzheimer's and other brain diseases (32, 33, 34).
It was also found to have similar benefits for general brain function in elderly individuals who did not have Alzheimer's or other brain conditions (35, 36, 37).
In specific cases, it may even help protect your brain from cell damage. In one study, alcoholics took 2 grams of acetyl-L-carnitine per day for 90 days. Afterward, they showed significant improvements in all measures of brain function (38).
More research is still required to investigate the long-term benefits in healthy individuals who are free from disease or problems with brain function.
L-carnitine, specifically acetyl-L-carnitine, can have beneficial effects on brain function and other related diseases.
Other health benefits
Here are a few more health benefits that have been linked to L-carnitine supplementation.
Some studies have demonstrated a potential benefit for reducing blood pressure and the inflammatory process associated with heart disease (23, 39).
In one study, participants took 2 grams of acetyl-L-carnitine per day. It reduced their systolic blood pressure, an important indicator of heart health and disease risk, by almost 10 points (23).
L-carnitine has also been shown to cause improvements in patients with severe heart disorders, such as coronary heart disease and chronic heart failure (40, 41).
One 12-month study found 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.
However, several studies have shown some mild benefits when participants have taken L-carnitine supplements in larger doses, or for longer periods (43, 44, 45).
L-carnitine's benefits may be indirect and take weeks or months to show. This differs from supplements like caffeine or creatine, which can directly enhance sports performance.
L-carnitine may benefit:
• Muscle oxygen supply: It may increase oxygen supply to the muscles (48).
• Stamina: It may increase blood flow and nitric oxide production, helping delay the "burn" and reduce fatigue (48).
• Muscle soreness: It may reduce muscle soreness after exercise (49).
• Red blood cell production: It may increase the production of red blood cells, which transport oxygen throughout your body and muscles (50, 51).
Type 2 diabetes and insulin sensitivity
L-carnitine has also been shown to reduce symptoms of type 2 diabetes and its associated risk factors (52, 53, 54).
In one human study of patients with type 2 diabetes, L-carnitine improved the blood sugar response to a high-carb meal. This blood sugar response is an important indicator of diabetes risk and overall health (55).
It may also combat diabetes by increasing a key enzyme called AMPK, which improves the body's ability to use carbs (56).
Research shows L-carnitine may have benefits for exercise performance and help treat health conditions like heart disease and type 2 diabetes.
Safety and side effects
Like most natural supplements, L-carnitine seems to be fairly safe and free of serious side effects when used sensibly and as directed.
One study examined L-carnitine's safety by giving participants 3 grams every day for 21 days. A comprehensive blood panel was conducted for each participant at the beginning and end of the study, and no negative effects were seen (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).
For most people, a dose of 2 grams or less per day seems to be relatively safe and free from serious side effects.
Doses of 2 grams or less per day seem to be well tolerated and safe for most people. Some people have reported nausea or other digestive side effects, but no serious issues have been found.
Top food sources of L-carnitine
The best sources of L-carnitine are:
• Beef: 81 mg per 3 oz (85 grams).
• Pork: 24 mg per 3 oz (85 grams).
• Fish: 5 mg per 3 oz (85 grams).
• Chicken: 3 mg per 3 oz (85 grams).
• Milk: 8 mg per 8 oz (227 ml).
Interestingly, food sources of L-carnitine actually 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 it's taken in supplement form (59).
As noted before, your body also has the ability to produce it naturally from the amino acids methionine and lysine if your stores are low.
For these reasons, taking L-carnitine supplements would only be required in special cases — for example, if you're using it to treat a disease or health condition.
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.
Should you take it as a supplement?
Your L-carnitine levels are influenced by how much you're eating and how much your body is producing.
For this reason, L-carnitine levels are often lower in vegetarians and vegans, since they restrict or avoid animal products (6, 60).
Therefore, it may be wise for vegetarians and vegans to take L-carnitine supplements. However, no studies have been performed on these specific populations.
The elderly may also benefit from L-carnitine supplements. Research shows your levels tend to decline as you age (61, 62).
In one study, 2 grams of L-carnitine reduced fatigue and increased muscle function in elderly people. Other research shows acetyl-L-carnitine may also help boost brain health and function as you age (62, 63).
Additionally, the risk of deficiency is higher for those with diseases like cirrhosis and kidney disease. Therefore, a supplement may be beneficial (1, 64, 65).
Specific populations may benefit from taking L-carnitine supplements. This includes the elderly and people who rarely or never eat meat and fish.
L-carnitine dosage recommendations
The standard dose of L-carnitine is 500–2,000 mg per day.
Although the 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.
Based on a review of the research, up to 2,000 mg (2 grams) per day seems safe for long-term use and an effective dose for most forms of L-carnitine.
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 will probably not help you lose a significant amount of weight.
Most of the research actually supports its use for health, brain function and disease prevention. Supplements may also benefit the elderly or vegetarians, who have lower levels.
Out of all the different forms, acetyl-L-carnitine and L-carnitine are the most popular and seem to be most effective.