People use carnitine as an energy booster, antioxidant, and to support mood and cognition. Is the evidence there to support these uses? Read on to discover more about the health effects and side effects of carnitine.
What is Carnitine?
Carnitine is an amino acid found in nearly every cell of the body. It is a generic name for a variety of compounds such as L-carnitine, acetyl-L-carnitine, and propionyl-L-carnitine. The three are chemically similar and some of their benefits overlap. However, they have some distinct mechanisms of action and uses.
- Acetyl-L-carnitine is generally used to improve brain functioning and cognition .
- L-carnitine is used to increase energy levels and physical performance [2, 3].
- Propionyl-L-carnitine is used to boost circulation .
Scientists think that all forms of carnitine play a vital role in the production of energy. Carnitine is said to help turn body fat into energy. It might also get rid of toxic compounds from the mitochondria to prevent their buildup. Carnitine is produced by the liver and kidneys and stored in muscle, brain, and heart tissue – all of which use fatty acids as energy .
You’ll get carnitine from meat, fish, poultry, and milk. People with genetic disorders, chronic diseases, and those on a vegetarian diet can often benefit from supplemental carnitine. The same is true for underweight and premature babies. Other conditions that cause carnitine deficiency include angina (chest pain) and intermittent claudication (cramping leg pain from blocked arteries) .
The FDA has approved the use of L-carnitine (by mouth or intravenously) treating L-carnitine deficiency caused by certain genetic diseases, end-stage kidney disease, and other disorders. It’s known under the brand name Carnitor.
However, other medical uses of carnitine supplements have not been approved by the FDA. Supplements generally lack solid clinical research. Regulations set manufacturing standards for them but don’t guarantee that they’re safe or effective. Speak with your doctor before supplementing.
- Effective for L-carnitine deficiency
- Supports mitochondrial health and energy function
- Allegedly supports liver and brain health
- May improve PCOS in women and semen quality and fertility in men
- Many popular uses are not supported by evidence
- May contribute to heart disease by metabolizing TMAO
- Can cause nausea or stomach upset
- Long-term use may cause oxidative stress in the liver and blood
- Affects thyroid function
Health Benefits of Carnitine
L-Carnitine Deficiency (Genetic)
Oral and intravenous L-carnitine is effective for the treatment of primary and secondary L-carnitine deficiency due to inborn errors of metabolism (genetic).
Likely Effective for:
L-Carnitine Deficiency in Kidney Disease
Dialysis reduces carnitine levels in people with end-stage kidney disease. The FDA has approved intravenous (by IV) L-carnitine for treating and preventing L-carnitine deficiency in these patients.
Despite being FDA-approved for this indication, some studies have had mixed results. Evidence suggests that l-carnitine improves anemia and inflammation in people with this condition. However, it does not seem to affect the patients’ quality of life, muscle cramping, low blood pressure, breathing, or exercise performance .
Carnitine taken by mouth is not approved for carnitine deficiency in kidney patients undergoing dialysis.
Possibly Effective for:
Chest Pain from Heart Disease (Angina)
Evidence suggests that taking carnitine by mouth or intravenously may improve exercise tolerance in people with chronic stable angina. As an add-on to conventional therapy, l-carnitine may also reduce the frequency of chest pain (angina) in people with angina (microvascular type) [8, 9].
Scientists think carnitine and its derivatives may protect heart function by increasing glucose metabolism, increasing blood flow, correcting abnormal heart rhythms, and reducing oxidative stress. These mechanisms have yet to be confirmed in humans .
Taking L-carnitine by mouth or intravenously (by IV) seems to improve symptoms and increase exercise capacity in people with heart failure. One study suggests that a combination of l-carnitine with ubiquinol, a form of CoQ10, may also improve symptoms of heart failure (the combination product is called Carni Q-Gel, by Tishcon Corporation) [11, 12].
In a 3-year-long study on patients with heart failure, L-carnitine appeared to increase survival rates compared to the placebo .
Scientists are investigating the effects of a combination of taurine and L-carnitine on heart cells .
Heart Inflammation (Myocarditis)
Carnitine (D-L-carnitine) by mouth seems to reduce death rates from myocarditis associated with diphtheria, according to clinical trials [15, 16].
Overactive Thyroid (Hyperthyroidism)
Some clinical evidence suggests that l-carnitine may improve symptoms of high thyroid hormones, such as rapid or pounding heartbeat, nervousness, and weakness .
In one randomized trial, 2 and 4 grams per day of L-carnitine reversed hyperthyroid symptoms. In another study, L-carnitine relieved a “thyroid storm” .
Some scientists believe that l-carnitine may inhibit both triiodothyronine (T3) and thyroxine (T4) entry into the cell nuclei. This is relevant because thyroid hormone action is mainly mediated by specific nuclear receptors .
Infertility in Men
Despite some mixed findings, most clinical studies suggest that L-carnitine increases sperm count and sperm motility in men with infertility [19, 20, 21].
Some researchers have suggested that the amount of carnitine in semen is directly related to sperm count and mobility. Carnitine seems to provide more energy for sperm cells and it may reduce cell death in the testes, according to preliminary findings .
A study of 100 infertile men revealed that the intake of carnitine supplements increased sperm motility .
Infertile men with varicose veins in the scrotum (varicocele) took carnitine for 6 months (250 mg, 4x a day). Results showed increased sperm count, mobility, and concentration .
Scientists are also exploring the antioxidant effects of carnitine on sperm in cells and in animals .
Clomiphene is the typical treatment for infertility in women with PCOS. However, some women do not respond to this medication alone. Clinical research suggests that adding L-carnitine from day 3 of the cycle until the day of pregnancy testing to a clomiphene treatment cycles increases ovulation and pregnancy rates compared with placebo in these women .
Valproic Acid Side Effects
Valproic acid is an anticonvulsant drug that can reduce the body’s carnitine levels, potentially resulting in high ammonia levels, and brain or liver damage. L-carnitine supplementation in such individuals may prevent or reduce high ammonia levels (hyperammonemia) and the severity of liver and brain damage, according to some clinical trials .
Insufficient Evidence for:
The following purported benefits are only supported by limited, low-quality clinical studies.
There is insufficient evidence to support the use of carnitine for any of the below-listed uses.
Remember to speak with a doctor before taking carnitine supplements. Carnitine should never be used as a replacement for approved medical therapies.
L-carnitine is often marketed as a weight loss aid and performance enhancer, but there is insufficient evidence to back up these purported benefits.
The evidence so far has been mixed and inconsistent. According to one analysis of clinical trials, L-carnitine may reduce weight by about 1.3 kg and decrease body mass index by about 0.5 kg/m2 compared to control in both nonobese and obese adults .
One trial suggests it may help reduce weight in overweight people with diabetes when used as an add on to the anti-obesity drug orlistat .
In another study of overweight women diagnosed with polycystic ovary syndrome, carnitine supplements reduced their weight, body mass index, and waist and hip size (circumference) .
However, L-carnitine does not seem to affect weight loss in overweight or obese people without diabetes or PCOS. The effects of carnitine on weight loss also appear to weaken with time. Larger trials on more diverse populations are needed .
Body Composition and Performance Enhancement
According to one unproven hypothesis, younger people have a sufficient amount of carnitine in the body, unlike older people. In line with this, carnitine has been researched for improving body composition and muscle mass in older people. The results have been mixed.
The existing studies suggest that L-carnitine may be beneficial in weak patients over 75 years of age but that it likely isn’t beneficial in healthy older women [30, 31, 32].
Others claim that carnitine may improve exercise endurance in all ages by increasing muscle carnitine levels, but data are lacking to support this claim .
Some evidence suggests that maximal exercise in trained athletes is linked with lower L-carnitine blood levels .
Initial studies have suggested that dietary carnitine stimulates the breakdown of fats into energy, reduces the amount of lactic acid produced during exercise, and speeds up recovery from exercise stress. Further studies failed to show any benefit of L-carnitine on athletic performance or endurance in male athletes or healthy men [35, 36].
Although it’s uncertain what caused these conflicting results, all these clinical trials suffered from flaws such as small sample sizes and short duration. Larger, longer, higher-quality trials are needed to determine the effects of L-carnitine on athletic performance.
Fatigue From Disease and Aging
Fatigue is common in cancer patients after treatments from chemotherapy, radiation therapy, and poor nutrition. Some scientists believe that low carnitine levels can contribute to this fatigue.
Nonetheless, there is not enough evidence to rate the effectiveness of L-carnitine on fatigue from disease and aging.
In a study of elderly subjects, acetyl-L-carnitine treatment reduced physical and mental fatigue, reduced muscle pain, and improved cognitive function .
In one study, 250 milligrams of carnitine 3 times a day improved fatigue and quality of life in terminal cancer patients .
One group of scientists are investigating the effects of acetyl-l-carnitine and lipoic acid on metabolic function in old rats. This combination has not been researched in humans .
In Dialysis Patients
Patients with kidney disease undergoing dialysis often have anemia with deformed red blood cells. The deformity prevents the blood cells from carrying enough oxygen to the body’s tissues, causing muscle weakness and fatigue .
A study following dialysis patients found that carnitine supplements reduced red blood cell deformity and increased overall red blood cell count in 3 months .
Another study on dialysis patients found that carnitine injections helped sustain higher levels of oxygen use (improved endurance), such as during exercise and also reduced overall fatigue .
Evidence is lacking to determine the effects of carnitine on mood.
Initial studies suggested that acetyl-L-carnitine has some antidepressant potential in elderly patients with age-associated depression .
A study found that supplementing the diets of fibromyalgia patients with acetyl-L-carnitine improved depressive symptoms and reduced muscle pain .
More human studies are needed.
Scientists are also investigating the effects of acetyl-L-carnitine on mice with depression .
It’s uncertain how L-carnitine affects cognition since clinical trials are limited.
Some scientists proposed that carnitine levels gradually decline as Alzheimer’s progresses, suggesting that patients with Alzheimer’s could potentially benefit from acetyl-L-carnitine treatment .
In one small study, acetyl-L-carnitine improved memory in older people and in Alzheimer patients. The authors suggested it might slow the progression of the disease, but proper trials would need to determine its mechanism and effectiveness .
Theoretically, consuming carnitine helps the body produce acetylcholine. According to some hypotheses, this neurotransmitter declines as memory loss advances .
Thus, carnitine might hypothetically increase energy production in brain cells that are starting to “slow down” and trigger memory loss. However, this is just a scientific hypothesis that has not been verified in clinical trials .
Insulin resistance has been linked with obesity and increased levels of fat in lean tissue.
Some researchers suggest that carnitine may improve insulin sensitivity by affecting fat oxidation in lean tissue and muscles. However, data are lacking to back up their theory .
In a study of overweight women with polycystic ovary syndrome, carnitine supplements lowered glucose and insulin blood levels and reduced insulin resistance .
Far more research is needed.
One hypothesis says that sleep-disordered breathing is associated with heart damage and altered heart carnitine metabolism. Although carnitine levels are low in heart tissue in those with chronic heart failure, carnitine blood levels may be increased because of “leakage” from damaged heart cells and altered carnitine metabolism .
Thus, some scientists think that carnitine supplementation can help patients suffering from obstructive sleep apnea, a condition where breathing repeatedly stops and starts during sleep. Carnitine helped improve airways for breathing and improved overall sleep quality in patients in one small study .
In a study of elderly subjects, acetyl-L-carnitine treatment improved sleep quality .
However, a study of fibromyalgia patients found no improvement in sleep disturbances by L-carnitine .
Further clinical trials are needed to rate the effectiveness of L-carnitine for sleep disorders.
Alertness in Narcolepsy
Acylcarnitine levels are abnormally low in narcolepsy patients. L-carnitine supplements may improve daytime alertness in these patients. L-carnitine treatment increased acylcarnitine and reduced blood lipids. Large-scale trials are required .
HIV patients may develop complications such as muscle, fat, and nerve degeneration. Carnitine supplements are being researched as a potential add-on strategy. In HIV positive individuals L-carnitine and acetyl-L-carnitine, as an add-on to conventional therapy, increased CD4 counts, reduced heart tissue damage, and reduced the number of triglycerides (fat) in the blood .
According to one scientific theory, L-carnitine may block the enzyme acidic sphingomyelinase, which prevents sphingomyelin breakdown in cells .
However, an initial study on patients with HIV-associated fat tissue degeneration did not find any evidence for using L-carnitine to reverse the effects of fat tissue degeneration. This study found that L-carnitine lowered cholesterol levels in the blood, but not the levels of triglycerides .
Large-scale trials are needed before we can draw any solid conclusions.
Lacking Evidence (Animal and Cellular Studies)
No clinical evidence supports the use of carnitine for any of the conditions listed in this section.
Below is a summary of the existing animal and cell-based research, which should guide further investigational efforts. However, the studies listed below should not be interpreted as supportive of any health benefit.
In a rat model of chronic kidney disease, the administration of carnitine improved kidney function. Carnitine lowered the levels of creatinine and BUN in the blood, reduced kidney tissue damage and abnormal kidney enlargement .
In a mouse model of osteoporosis, L-carnitine and isovaleryl-L-carnitine stimulated bone formation, improving bone turnover, bone density, bone size, and bone structure .
Another study found that dietary L-carnitine might improve the bone mineral content and bone mineral density in a female mouse model of postmenopausal bone loss. Results indicated slowed bone loss and improved bone structure .
Some scientists hypothesize that carnitine might promote bone mineralization, in part by inhibiting thyroid hormone action .
The effects of carnitine on bone health in humans are unknown.
In mouse studies of drug-induced seizure, pre-treating the mice with L-carnitine suppressed seizures dose-dependently [57, 58].
A rat model of hyperthyroidism induced by L-thyroxine injections suggested that L-carnitine might have dose-dependent protective effects against liver damage .
L-carnitine suppresses the onset of neuromuscular degeneration and increases the lifespan of mice with familial amyotrophic lateral sclerosis (ALS) .
It’s unknown how carnitine affects seizures, liver health, and ALS in humans.
L-Carnitine Side Effects & Precautions
In general, the adverse effects of oral or intravenous L-carnitine are mild and rare. It has been reported to cause vomiting, diarrhea, a fishy body odor, abdominal cramps, and nausea. More severe and rarer side effects are seizures and muscle weakness in people prone to heart disease .
Pregnant women should avoid L-carnitine; insufficient data are available.
Preclinical Safety Data
These side effects have not been confirmed in humans.
Some research suggests that carnitine is metabolized into TMAO, which changes cholesterol metabolism and may promote plaque build-up in the arteries. Theoretically, this leads to the degeneration of artery walls, restricted blood circulation, and blood clots. However, no human studies suggest that L-carnitine worsens cardiovascular health .
Carnitine appears to block both triiodothyronine (T3) and thyroxine (T4) entry into the cell. Long-term use of L-carnitine accelerated the production of reactive oxygen species in the liver and blood in mice .
L-carnitine may disturb kidney function by affecting ion transporters in mice .