What is L-carnitine?
Carnitine plays a vital role in providing energy. It transports long-chain fatty acids into the mitochondria, where they can be oxidized ('burned') to generate energy. In addition, carnitine transports the toxic compounds out of this cell organelle to prevent excessive accumulation. In view of these important functions, carnitine is increasingly detectable, particularly in tissues such as the skeletal and cardiac muscles, which use fatty acids as a food fuel.
Most people's bodies usually make enough carnitine to meet their body's needs. For genetic or medical reasons, some individuals (such as premature babies) can't produce enough of this, so carnitine is an essential nutrient for them.
Different forms of L-carnitine
L-carnitine is the traditional biologically active form of carnitine found in the body, in foods and in most nutritional supplements.
A few more types of carnitine is listed here:
- L-carnitine L-tartrate - due to its high absorption rate, it is one of the most common forms used in dietary supplements for athletes. It can help alleviate symptoms related to physical activity, such as sore muscles, and promote muscle recovery.
- Acetyl-L-Carnitine -also known as ALCAR, may be the most effective connection for the brain. Because this can cross the blood-brain barrier, this compound has been shown to be useful in improving the brain function as well as neurological diseases such as dementia and Alzheimer's.
- Propionyl-L-carnitine - this form is well suited for the treatment of symptoms related to blood circulation, such as peripheral vascular diseases or high blood pressure. The effectiveness may be due to the production of nitric oxide, which improves blood flow.
- D-carnitine - this inactive form can cause a lack of carnitine in the human body because it inhibits the absorption of other more useful forms.
For most people, acetyl-L-carnitine and L-carnitine tartrate appear to be the most effective forms of general use. However, you should always choose the form that is best suited for your personal needs and goals.
Newer studies have shown that the potential benefits of various forms of carnitine can be used in diseases such as heart and brain disorders.
L-carnitine has a beneficial effect on general health, since it can support mitochondrial function and contribute to the growth or improvement of mitochondrial health. Mitochondria play an essential role in preventing diseases, delaying aging and maintaining vitality and health.
The most important effect of L-carnitine is related to the energy production in the body cells. Within the cells, L-carnitine promotes the transportation of fatty acids into the mitochondria. Mitochondria are like motors that convert the fatty acids introduced by L-carnitine into usable energy.
Approximately 98% of the body's L-carnitine storage is stored in the muscles, the heart muscle and in the brain, with small amounts still being detectable in the liver and blood.
The human body needs various enzymes for the L-carnitine synthesis as well as the amino acids lysine and methionine, as well as iron, niacin, vitamin B6 and alpha-ketoglutarate. Sufficient vitamin C is also necessary for L-carnitine to be synthesized efficiently.
In addition to the L-carnitine produced by the body itself, small amounts can also be ingested in the form of products of animal origin such as meat or fish.
Vegans or people with certain genetic problems may not be able to produce sufficient amounts of carnitine themselves or ingest them from external sources. This means that it is a 'conditionally essential' nutrient.
The usual daily dose of L-carnitine is between 500 mg and 3000 mg (1000 milligrams = 1 gram). Although the dosage varies from trial to trial, here is an overview of the recommended use and dosage for each form:
- Acetyl-L-Carnitine - - This form is best used to support brain health and performance. The dosages vary between 500 mg and 2000 mg per day. You should start with 500 mg a day in the morning and increase it as needed. Acetyl-L-Carnitine can have a slightly stimulating effect, which is why it should not be taken in the afternoon or evening - depending on your personal opinions.
- L-Carnitine L-Tartrate - This form is the most effective for promoting exercise performance. The doses vary between 1000 mg and 3000 mg per day.
- Propionyl-L-Carnitine - This form is best used to improve blood flow in people with high blood pressure or related health problems. The dosages vary between 400 mg and 1000 mg per day.
After reviewing the research results, the researchers concluded that doses up to 2000 mg per day are safe for long-term use and are an effective dose for most forms of L-carnitine.
L-carnitine is not an essential nutrient. Therefore, healthy children and adults do not necessarily have to ingest carnitine through foods or supplements, since the liver and kidneys produce sufficient amounts from the amino acids lysine and methionine to meet the daily needs.
The US National Food and Nutrition Board (FNB) of the National Academies (formerly the National Academy of Sciences) carried out studies in 1989 to investigate the functions of carnitine and came to the conclusion that it was not an essential nutrient acts. For this reason, the FNB has not determined any reference values for nutrient intake or recommendations with regard to the recommended daily allowance (RDA) for carnitine.
Carnitine has been studied extensively because it is important for energy production and a well-tolerated and generally safe dietary supplement. In the context of scientific studies, researchers prefer the use of acetyl-L-carnitine because it is better absorbed in the small intestine than L-carnitine and because it crosses the blood-brain barrier more efficiently (meaning, it reaches the brain tissue).
L-carnitine in food
Animal products such as meat, fish, poultry and milk are the best carnitine suppliers. In general, it can be said that the redder the meat, the higher the carnitine content.
Dairy products mainly contain carnitine in their whey content.
Carnitine occurs in two forms, known as D and L, which are mirror images (isomers) of each other. Only L-carnitine is active in the body and represents the form that occurs in food.
The carnitine content per gram of food:
- Beef 170g - 160 mg
- Pork 170g - 50 mg
- Chicken 170g - 6 mg
- Whole milk, 235ml - 8 mg
- Cod cooked, 115g - 4 to 7 mg
- Chicken breast cooked, 115g - 3 to 5 mg
- Ice cream, 120 ml - 3 mg
- Cheddar cheese, 57g - 2 mg
- Whole grain bread, 2 slices - 0.2 mg
- Cooked asparagus, approx. 120g - 0.1 mg
Adults who eat a mixed diet, which includes red meat and other animal products, consume approximately 60 - 180 milligrams of carnitine a day.
Vegans consume significantly less (approximately 10 - 12 milligrams) because they avoid foods of animal origin. Most of the carnitine found in food (54 - 86%) is absorbed by the small intestine and enters the bloodstream there.
The kidneys store carnitine efficiently, so even diets that are low in carnitine have little influence on the overall carnitine content of the body. Instead of being metabolized, excess carnitine is excreted in the urine through the kidneys as needed to maintain a stable concentration in the blood.
There are two types of carnitine deficiency. Primary carnitine deficiency is a genetic disorder of the cellular carnitine delivery system that typically manifests around the age of five with symptoms of cardiomyopathy, skeletal muscle weakness and hypoglycemia.
Secondary carnitine deficiencies can occur due to certain diseases (e.g. chronic kidney failure) or causes (e.g. taking certain antibiotics) that reduce carnitine absorption or increase its excretion. There is a scientific consensus on the importance of prescription carnitine supplements to treat such deficiencies.
L-carnitine and the brain function
Studies in humans have shown that taking acetyl-L-carnitine daily could counteract the decline in brain function associated with Alzheimer's and other brain disorders. In addition to this, similar positive effects on the general brain function of older people who did not have Alzheimer's or other brain diseases could be found.
In certain cases, it can even help protect the brain from cell damage. As part of a study, alcoholics took 2000 mg of acetyl-L-carnitine daily for 90 days. As a result, measurements of their brain functions showed significant improvements in all areas.
More research is necessary to make a statement about the long-term effects on healthy people who do not have a brain disease or impaired brain functions.
L-carnitine and the male fertility
The carnitine content of semen is directly related to the sperm count and motility , suggesting that the compound may be beneficial in the treatment of male infertility. Several studies have shown that supplementation with carnitine (2000 to 3000 mg per day over a period of 3 - 4 months) can improve the sperm quality.
Furthermore, a randomized, double-blind, cross-over study found that treating 100 infertile men with a daily dose of 2000 mg carnitine over a 2-month period could improve both concentration and overall motility and movement of their sperm.
The positive results observed in the studies could be due to both an increased mitochondrial fatty acid oxidation (which provides the sperm with more energy) and a reduced cell death in the testicles.
However, a recent randomized controlled study with 21 infertile men showed that a daily intake of 3000 mg of carnitine over a period of 24 weeks could not achieve an increase in sperm motility or the total number of motile sperm as compared to placebos.
To assess the potential benefits of carnitine for infertility treatment, more extensive and carefully designed studies are required.
L-carnitine and athletic performance
When it comes to the effects of L-carnitine on athletic performance, the results are rather mixed. However, several studies have shown some slight improvements in participants who had taken L-carnitine supplements in larger doses or over long periods of time.
Some athletes take carnitine to improve performance. However, after twenty years of research, there still hasn't been any consistent evidence that dietary supplements containing carnitine can actually improve the physical performance of healthy subjects. The tests were carried out with daily doses of 2000 to 6000 mg for periods between one and 28 days.
The total carnitine content of the body in men weighing 70 kilograms is approximately 20 grams, the majority of which is concentrated mainly in the skeletal muscles). For example, carnitine supplements do not appear to increase oxygen utilization or metabolism during exercise, nor do they contain carnitine in the muscles.
However, a study with 10 strength athletes showed that L-carnitine tartrate significantly reduced the regeneration time after performing 5 sets with 15-20 squats each. It also reduced muscle damage.
L-carnitine and the heart
Because the carnitine level in the failing heart muscle is low, additional amounts can counteract the toxic effects of free fatty acids and improve the carbohydrate metabolism.
In short-term studies, carnitine had both an oral and an injected anti-ischemic effect. In a double-blind, placebo-controlled, multi-center clinical study in Italy, 2,330 patients with an acute front wall infarction received either an L-carnitine preparation (9 g/day intravenously for 5 days, 4 g/day orally for 6 months) or a placebo. Treatment with L-carnitine significantly reduced the death rate 5 days after randomization, but did not significantly affect the risk of heart failure or death after 6 months.
The authors of a meta-analysis from 2013 combined the results of this study with those from 12 smaller studies . They concluded that treatment with L-carnitine in patients with acute myocardial infarction over a median follow-up period of 2 months reduced overall mortality by 27%, ventricular arrhythmias by 65% and angina by 40%, but not the risk of heart failure or reduced recurrence of a heart attack.
Some studies have shown potential benefits in lowering blood pressure and alleviating inflammatory processes associated with heart diseases.
As part of a study, the participants took 2000 mg of acetyl-L-carnitine a day. This led to a reduction in your systolic blood pressure - an important indicator of heart health and disease risk - by almost 10 points.
The study also found that L-carnitine can improve the condition of patients with severe heart conditions such as coronary heart disease or chronic heart failure.
During a 12-month study, a decrease in heart failure and death was observed among those taking L-carnitine supplements.
A 2013 study, which included both rodents and 2,595 people who underwent elective cardiac evaluation, found that L-carnitine is converted to trimethylamine N-oxide (TMAO) via gut microbiota. It is a proatherogenic substance that is associated with risks of cardiovascular disease.
Because of the differences in the composition of the intestinal bacteria, study participants whose diet contained meat produced more TMAO than vegans or vegetarians after consuming L-carnitine. The study also found dose-related connections between fasting plasma L-carnitine concentrations and the risk of coronary artery disease, peripheral artery disease and general cardiovascular disease but only in participants who had high TMAO concentrations.
The researchers found that these recognitions could partly explain the link between high red meat consumption (a rich source of carnitine) and an increased risk of cardiovascular disease. However, more research is needed to fully understand the effects of carnitine on cardiovascular health.
L-carnitine and peripheral artery diseases
Several studies have shown the effectiveness of carnitine as a dietary supplement in the treatment of cardiac ischemia (restriction of blood flow to the heart) and peripheral artery disease (the main symptom is poor circulation in the legs, known as intermittent claudication).
Claudicatio (window shopper's disease) was due to an insufficient supply of oxygen-rich blood in the legs and, due to incomplete utilization, led to an accumulation of acetylcarnitine in the muscles. Patients with peripheral arterial diseases who develop claudication have significant impairments when performing physical exercises and have difficulty walking even at short distances at slow speeds. The research showed that carnitine could improve the performance of the skeletal muscles in the leg.
In a European multi-center clinical study, supplementation with L-carnitine (in the form of propionyl-L-carnitine at 2 g/day for 12 months) in patients with moderate to severe claudication related to the maximum distance covered and the perceived quality of life achieved a significant improvement compared to patients who received only placebos.
A similar multi-center study conducted in the United States and Russia found that administering the same daily dose and form of carnitine over a period of 6 months in patients with claudication, with impaired walking, had an improvement in the covered walking distance, the physical pain reduced and there was an improvement in the general health status as compared to the patients in the control group.
The authors of a systematic review and meta-analysis, which examined this and 12 other randomized clinical studies, concluded that propionyl-L-carnitine significantly increased the maximum distance covered in patients with claudication.
These results suggest that L-carnitine, if taken over a period of up to 1 year, could have positive effects on the cardiovascular system in patients with certain health factors. However, other studies have raised concerns about the effects of chronic carnitine exposure on the cardiovascular system.
L-carnitine and the aging process
It is believed that the decrease in the mitochondrial function contributes to the aging process. Carnitine could be involved, since the carnitine content in the tissue decreases with age, thereby reducing the integrity of the mitochondrial membrane. Studies in older rats have shown that supplementation with high doses of acetyl-L-carnitine and alpha-lipoic acid (an antioxidant) can reduce mitochondrial decay. The animals moved more and improved their memory performance.
However, there are currently no similar studies of this type carried out on humans. However, a meta-analysis of double-blind, placebo-controlled studies suggests that acetyl-L-carnitine supplements can improve memory performance and reduce mental decline in older adults with mild cognitive impairment and Alzheimer's. In these studies, the subjects were given 1500 to 3000 mg of acetyl-L-carnitine daily over a period of 3 - 12 months.
L-carnitine and type II diabetes
Insulin resistance, which plays an important role in the development of type II diabetes, could be related to a disruption of fatty acid oxidation in the muscle. This raises the question of whether mitochondrial dysfunction could be a factor that causes the disease to progress. Increased fat storage in fat-free tissue has become a marker for insulin resistance.
The results of a previous research indicated that intravenous supplementation with L-carnitine could improve insulin sensitivity in diabetics by reducing the fat content in the muscle and possibly lowering the blood sugar level due to a faster increase in fat oxidation in the cells.
A newer analysis of two multi-center clinical studies of subjects with type I or type II diabetes showed that oral treatment with acetyl-L-carnitine (3000 mg/day) over a period of one year provided significant relief from nerve pain as well as improved vibration perception in patients with diabetic neuropathy. Treatment was most effective in patients with type II diabetes with a short duration of illness.
It has also been shown that L-carnitine can reduce the symptoms of type II diabetes and the associated risk factors.
In a study in patients with type II diabetes, L-carnitine improved the blood sugar response to carbohydrate rich meals. This blood sugar reaction is an important indicator of the risk of diabetes and general health.
In addition, L-carnitine can fight diabetes by increasing an essential enzyme called AMPK, which improves the body's ability to utilize carbohydrates.
L-carnitine - HIV and AIDS
The human immunodeficiency virus (HIV) causes a decrease in the number of lymphocytes (a type of white blood cells), which leads to an acquired immune deficiency syndrome (AIDS). In HIV-infected people, fat often accumulates in some areas of the body while it is broken down in other areas, as well as their blood fat percentage (hyperlipidemia) and their insulin resistance, which together leads to a lipodystrophy syndrome.
This syndrome can indicate a mitochondrial toxicity caused by HIV infection and the antiretroviral medications used to treat it, which in turn can lead to a lack of carnitine, which affects mitochondrial fat metabolism.
Little is known about the molecular mechanisms that causes these processes. Preliminary research approaches provide conflicting results, but suggest that both intravenous and oral carnitine supplementation (in daily doses of 2000 to 6000 mg over several weeks or months) slow the death of lymphocytes in HIV-infected persons (which possibly restricts the progression of the HIV infection) , reduce neuropathy and could have a positive effect on blood lipid levels.
L-carnitine and renal insufficiency (kidney failure)
The carnitine homeostasis (balance within the body) can be significantly impaired in people with a kidney disease due to several factors. These include the reduced synthesis and increased excretion of the compound through the kidneys, the reduced food intake due to the low appetite and the reduced consumption of foods of animal origin.
Many patients with a terminal renal insufficiency, especially those undergoing hemodialysis treatment, suffer from a carnitine deficiency. The carnitine levels are low in both the blood and muscles, which can promote anemia, muscle weakness, fatigue, changes in lipid levels and heart disease. Numerous studies suggest that high-dose (often injected) carnitine supplements may improve some or all of these symptoms in patients undergoing a long-term hemodialysis treatment, but the studies have mostly been performed on a small number of patients and have not been double-blind studies.
A recently performed meta-analysis of these studies concluded that carnitine supplements may help treat anemia but not improve lipid levels, and that their effects on physical performance or cardiac activity stabilization have not been clearly demonstrated.
L-carnitine and cancer
Fatigue and a poor nutrition are common in cancer patients after chemotherapy or radiation treatment. They may also suffer from a lack of carnitine. In one study, treatment with carnitine preparations (4000 mg/day over a period of one week) improved the symptoms of fatigue and normalized blood carnitine levels in most subjects who had undergone chemotherapy.
In a further study in patients with terminal cancer, the patients treated with carnitine (doses from 250 milligrams to 3000 milligrams/day) reported an improvement in the symptoms of fatigue as well as improved mood and sleep quality. In both studies, most of the study participants had a carnitine deficiency before they started taking the carnitine supplements.
Who needs L-carnitine?
The level of L-carnitine depends on the amount of food consumed and the amount of carnitine produced by the body. As a result, L-carnitine levels are often lower in vegetarians and vegans because they restrict or avoid products of animal origin.
Therefore, it may be useful for vegetarians and vegans to take L-carnitine supplements. However, no studies have yet been carried out with these special population groups.
Older people can also benefit from L-carnitine supplements. Research has shown that carnitine levels decrease with age.
A study showed that taking 2000 mg L-carnitine in older people can reduce fatigue and increase muscle function. Other research has shown that acetyl-L-carnitine can also improve brain health and performance in older people.
The risk of a lack of carnitine is also increased in patients with diseases such as cirrhosis or kidney problems. As a result, carnitine supplementation can be useful in some cases.
L-carnitine side effects
Like most natural dietary supplements, L-carnitine is fairly safe and free of serious side effects if it is taken in a reasonable dose as recommended.
Within the frame of a study, the safety of L-carnitine was checked by administering 3000 mg daily to the participants over a period of 21 days. At the beginning and at the end of the study, a comprehensive blood count was drawn up for each participant, with no negative effects being observed.
According to a report on the safety of L-carnitine, doses of around 2000 mg per day appear to be safe for long-term use. While some subjects reported side effects such as nausea or other digestive problems, there were no serious problems.
L-carnitine and interactions with medication
Carnitine interacts with pivalate-conjugated antibiotics such as Pivampicillin, which are used in the long-term prevention of urinary tract infections. Long-term treatment with these antibiotics increases the excretion of pivaloyl-carnitine, which can lead to a lack of carnitine. Although the carnitine level can drop so far that the fatty acid oxidation is inhibited, no cases of an illness due to a lack of carnitine have yet been described.
The antiepileptic drug valproic acid interacts in many ways with the carnitine metabolism, which can result in a deficiency of L-carnitine. The areas affected are cellular uptake, biosynthesis, renal excretion and intracellular carnitine pool.   
L-carnitine preparations as food supplements
L-carnitine tartrate, acetyl-L-carnitine and propionyl-L-carnitine are available as dietary supplements without a prescription. Carnitine is often advertised as a helpful means of losing weight, improving exercise performance and increasing well-being. It has also been approved by the United States Food and Drug Administration (FDA) as a medication to treat primary and certain secondary carnitine deficiency symptoms.