L-carnitine

L-Carnitine

1. Identity: Chemical Names, Structure, and Forms

L-carnitine (systematic IUPAC name: (3R)-3-hydroxy-4-(trimethylazaniumyl)butanoate; also known as levocarnitine, β-hydroxy-γ-N-trimethylaminobutyric acid, or γ-trimethylamino-β-hydroxybutyric acid) is a naturally occurring, water-soluble quaternary ammonium compound. L-carnitine [(R)-3-hydroxy-4-trimethylaminobutyrate] is an essential compound in the intermediary metabolism of eukaryotes, involved in the transport of activated long-chain fatty acids and products of peroxisomal β-oxidation into the mitochondria for subsequent completion of β-oxidation. Only the L-isomer is physiological. The D-isomer (D-carnitine) is not biologically active and cannot substitute for L-carnitine in metabolic roles; DL-carnitine is toxic and has no beneficial effects.

It has been estimated that the total carnitine content in the human body is about 300 mg/kg, with about 95% stored intracellularly in the heart and skeletal muscle, and the remaining part in the liver, kidney, and plasma. The amount of circulating plasma carnitine accounts for only 0.5% of total body carnitine.

1.1 Common Supplement Forms

Carnitine is available in dietary supplements containing only carnitine or a combination of carnitine and other ingredients. The two main forms of carnitine in dietary supplements are L-carnitine and acetyl-L-carnitine (ALCAR), and amounts range from about 3 mg to 5,000 mg. Additional forms available in the market and studied in clinical research include:

• L-carnitine (free form / levocarnitine) — the basic, free-form compound used widely in supplements.
• Acetyl-L-carnitine (ALCAR) — an acetylated ester of L-carnitine with enhanced blood-brain barrier penetration; studied for neurological and cognitive applications.
• Propionyl-L-carnitine (PLC) — a propionyl ester; investigated primarily in peripheral arterial disease and cardiac ischemia.
• L-carnitine L-tartrate — a salt form commonly used in sports-nutrition products due to its high absorption rate.
• Glycine propionyl-L-carnitine (GPLC) — a combination studied in the context of exercise performance.

The chemical structures of L-carnitine (β-hydroxy-γ-N-trimethylaminobutyric acid), acetyl-L-carnitine (ALCAR), and propionyl-L-carnitine are all closely related. The biosynthetic cofactors required include iron (Fe²⁺), NAD⁺, vitamin B6, and vitamin C.

2. Natural Sources and Dietary Content

L-carnitine is a major nutritional substance found in animal food because endogenic synthesis is inadequate to meet metabolic demands. Many foods, especially animal foods, naturally contain carnitine. Red meat is a good source of carnitine. Poultry, fish, and dairy have some carnitine. Vegetables, fruits, and grains have very small amounts of carnitine.

Fish, meat, and dairy sources supply at least 80% of the desired L-carnitine. Among animal foods, red meats carry the highest concentrations. The primary source is red meat, providing up to 140–190 mg L-carnitine per 100 g uncooked meat (e.g., beef and venison). The best source of L-carnitine is red meat, which has 56 to 162 mg per four-ounce portion of cooked steak and ground beef. 170 grams of cooked pork contain 50 mg of carnitine. Chicken breast contains around 3–5 mg of L-carnitine per 100 g.

In omnivorous humans, 75% of the body's carnitine pool is derived from dietary intake. In contrast, strict vegetarians and vegans rely heavily on endogenous synthesis. The rate of L-carnitine biosynthesis in humans was studied in strict vegetarians (i.e., in people who consume very little dietary carnitine) and estimated to be 1.2 µmol/kg of body weight/day.

Healthy people's bodies can make all the carnitine they need. Therefore, carnitine is not an essential nutrient, and experts haven't set a recommended daily amount. There are currently no documented suggested carnitine reference values. In most circumstances, the predicted average carnitine necessities for an adult are 20–200 mg/day, which are fulfilled by food and endogenous production.

3. History and Discovery

3.1 Isolation and Structural Elucidation

Carnitine was first isolated in 1905 from extracts of meat by Russian chemists Vladimir Gulewitsch and Rudolf Krimberg, who identified it as a novel constituent of muscle tissue. Independently, German biochemist Ernst Kutscher also reported its presence in meat extracts that same year. The compound was named "carnitine" after the Latin word carnis, meaning flesh, due to its abundance in animal muscle.

The history of research into carnitine falls into four periods: first, the period of its simultaneous discovery as a constituent of vertebrate muscle by Gulewitsch and Krimberg and by Kutscher in 1905; then, the period in which its chemical structure was established (approximately 1927); and next, the delineation of its major physiological function (1935–1965). Finally, the discoveries of its biosynthetic pathway, transport mechanisms, and primary and secondary carnitine deficiency and syndromes occurred from 1961 to the present.

The chemical structure of carnitine was elucidated in 1927 by Japanese researchers Masao Tomita and Seizi Sendju, who determined it to be (3-carboxy-2-hydroxypropyl)trimethylammonium hydroxide inner salt, a quaternary ammonium compound.

3.2 Early Biological Role: "Vitamin BT"

Carnitine was shown to be essential for larval development of the mealworm Tenebrio molitor and was originally designated vitamin B_T based on this requirement. In 1952, L-carnitine was found to be essential for the growth of the mealworm, Tenebrio molitor. Without carnitine, the mealworms could not use fat stores when starved. This discovery positioned carnitine as a growth factor for certain organisms, though subsequent research revealed that mammals synthesize it endogenously.

Later, it was discovered that carnitine can be synthesized in mammals and is now considered to be a quasi-nutrient or conditionally essential nutrient, as neonates have reduced biosynthesis and rely on placental transfer of carnitine in utero and exogenous sources after birth.

3.3 Traditional and Historical Use

Unlike many botanical or herbal supplements, carnitine does not have a classical history of deliberate therapeutic use in traditional medicine systems. Its history is fundamentally one of scientific discovery rather than folk or ethnobotanical tradition. The compound was identified as a biochemical entity in 1905 and its physiological role — fatty acid transport into mitochondria — was only delineated between the 1930s and 1960s. Investigations of its metabolic role began in the 1940s, as a result of studies by Fraenkel of the nutritional requirements of insects. Friedman and Fraenkel discovered that carnitine was reversibly acetylated by acetyl-CoA in the presence of muscle homogenate. Medical and supplemental uses of carnitine preparations were therefore products of 20th-century biochemistry rather of traditional healing practices, and its use as a dietary supplement emerged primarily in the latter half of the 20th century following elucidation of its role in fatty acid metabolism.

4. Biosynthesis: Active Compounds and Mechanisms

4.1 Endogenous Synthesis

Humans can synthesize L-carnitine from the amino acids lysine and methionine in a multi-step process. The biosynthesis of carnitine is a multi-step process that converts the essential amino acid lysine into L-carnitine through a series of enzymatic reactions. This pathway primarily occurs in the liver and kidney and involves four key enzymes: Trimethyllysine Hydroxylase (TMLH), Hydroxytrimethyllysine Aldolase (HTMLA), Trimethylaminobutyraldehyde Dehydrogenase (TMABA-DH), and γ-Butyrobetaine Hydroxylase (BBOX).

Carnitine synthesis begins with methylation of lysine by S-adenosylmethionine acting as a methyl donor, resulting in the formation of ε-N-trimethyllysine. Trimethyllysine is enzymatically transformed into β-hydroxy-trimethyllysine. From the synthesized β-hydroxy-trimethyllysine, trimethylaminobutyl aldehyde is formed, and is then converted to γ-butyrobetaine. γ-Butyrobetaine hydroxylase then catalyses the stereospecific hydroxylation of γ-butyrobetaine to L-carnitine.

The four enzymes involved in endogenous L-carnitine biosynthesis are all ubiquitous except γ-butyrobetaine hydroxylase, which is absent from cardiac and skeletal muscle. L-carnitine is therefore primarily synthesized in the liver and transported via the bloodstream to cardiac and skeletal muscle, which rely on L-carnitine for fatty acid oxidation yet cannot synthesize it.

Biosynthesis requires lysine, methionine, vitamin C, iron, vitamin B6, and niacin as essential cofactors. Vitamin C (ascorbic acid) is particularly critical: γ-Butyrobetaine hydroxylase catalyses the last step in carnitine biosynthesis, the formation of L-carnitine from γ-butyrobetaine, a reaction dependent on Fe²⁺, α-ketoglutarate, ascorbate and oxygen.

In healthy people, carnitine homeostasis is maintained through endogenous biosynthesis of L-carnitine, absorption of carnitine from dietary sources, and reabsorption of carnitine by the kidneys.

4.2 Primary Mechanism of Action: The Carnitine Shuttle

The defining biochemical function of L-carnitine is the facilitation of long-chain fatty acid transport across the inner mitochondrial membrane, a process known as the carnitine shuttle or carnitine-acylcarnitine translocase system. L-carnitine serves as a critical cofactor in energy metabolism by facilitating the transport of long-chain fatty acids into the mitochondria, where they undergo β-oxidation to produce adenosine triphosphate (ATP), the cell's primary energy currency, with particular importance in skeletal muscle, cardiac muscle, and during periods of fasting or exercise.

Carnitine (3-hydroxy-4-N-trimethylaminobutyrate) represents an amino acid derivative and a micronutrient that plays a key role in intermediary metabolism with the main function being the transport of long-chain fatty acids from the cytosol to the mitochondrial matrix where fatty acid β-oxidation occurs. Without adequate carnitine, long-chain acyl groups cannot enter the mitochondrial matrix for oxidation.

4.3 Secondary Mechanisms

Beyond its role in fatty acid transport, L-carnitine and its acylated derivatives participate in several other cellular processes:

• Acetyl-CoA/CoA buffering: Carnitine accepts acyl groups from excess acyl-CoA, regenerating free CoA and thereby preventing the inhibition of key enzymes by acyl-CoA accumulation. L-carnitine functions as a requisite mediator of acyl transport and accepts acyl groups from a variety of acyl-CoA derivatives in cells and tissues throughout the body.
• Removal of toxic acyl intermediates: In secondary carnitine deficiency, carnitine enhances excretion of toxic metabolites and generation of free CoA.
• Modulation of pyruvate metabolism: L-carnitine modulates metabolic pathways, including pyruvate dehydrogenase activity, proteolysis, and protein synthesis, while also having anti-inflammatory and antioxidant characteristics.
• Peroxisomal function: Products of peroxisomal β-oxidation (medium- and short-chain acylcarnitines) are also transported to the mitochondria via the carnitine shuttle for complete oxidation.

5. Absorption, Transport, and Excretion

Absorption of supplemental L-carnitine is about 14% to 18%, much less than that of dietary L-carnitine. The bioavailability of L-carnitine obtained through meals is approximately four times higher than that obtained from dietary supplements. Short-term supplementation with L-carnitine (2 to 4 g/day) has been studied, with supplemental bioavailability notably lower than from food sources.

Uptake of L-carnitine depends on an energy-dependent transport system against the concentration gradient. The organic cation transporters (OCTNs) regulate tissue distribution and intracellular homeostasis of L-carnitine and function both in its intestinal absorption and renal reabsorption. Hereditary or acquired defects in the transport mechanisms are the major cause of L-carnitine deficiency, leading to pathologies such as cardiomyopathy and skeletal muscle myopathy.

The kidneys play a central role in carnitine homeostasis through reabsorption. Carnitine does not undergo metabolic changes and is therefore eliminated as free carnitine in urine. However, a part of carnitine that is not absorbed at the level of the small intestine is completely degraded by bacteria in the large intestine to produce trimethylamine, a quaternary amine that, after enterocyte absorption, is oxidized in the liver by flavin-containing monooxygenase 3 to form trimethylamine-N-oxide (TMAO).

6. Carnitine Deficiency

6.1 Primary Carnitine Deficiency (PCD)

Two types of carnitine deficiency states exist. Primary carnitine deficiency is a genetic disorder of the cellular carnitine transporter system that causes a shortage of carnitine within cells. Primary carnitine deficiency usually presents during infancy or early childhood. It can result in epilepsy and encephalopathy in infants; seizures, irregular heartbeat, and breathing problems in adolescents and young adults; and myopathy, rhabdomyolysis, cardiomyopathy, or sudden death in older people.

Use of L-carnitine in primary carnitine deficiency restores plasma carnitine levels to nearly normal, but muscle carnitine levels rise slightly. Muscle function can be normalized in patients with carnitine deficiency when muscle carnitine levels remain less than 10% of controls. Cardiomyopathy often responds well to carnitine supplementation.

6.2 Secondary Carnitine Deficiency (SCD)

Secondary carnitine deficiency (SCD), unlike PCD, is caused by or occurs in combination with other conditions such as liver or renal disease, fatty acid metabolic abnormalities, or the use of therapeutic drugs like valproic acid or pivampicillin. SCD is found in individuals with renal tubular diseases, where carnitine excretion may be high, as well as hemodialysis patients. Inadequate synthesis of carnitine produces carnitine depletion in dialysis patients, resulting in carnitine depletion and a simultaneous relative rise in esterified carnitine in certain individuals.

Carnitine supplementation in total parenteral nutrition (TPN) prevents secondary carnitine deficiency in preterm newborns. Carnitine supplementation in fatty acid oxidation disorders and other organic acidurias is used to correct carnitine deficiency and to allow removal of toxic intermediates. The other goal of therapy is to restore CoA levels. However, carnitine therapy for long-chain fatty acid oxidation defects has become questionable because it promotes formation of long-chain acylcarnitines that may cause arrhythmogenesis and membrane dysfunction.

7. Scientific Evidence by Area of Use

7.1 Cardiovascular Disease and Acute Myocardial Infarction

Several studies have examined supplemental carnitine in the management of cardiac ischemia (restriction of blood flow to the heart) and peripheral arterial disease. Because levels of carnitine are low in the failing heart muscle, supplemental amounts might be beneficial to the organ by counteracting the toxic effects of free fatty acids and improving carbohydrate metabolism. In short-term studies, carnitine has demonstrated anti-ischemic properties when given orally and by injection.

A key systematic review and meta-analysis addressed secondary prevention after acute myocardial infarction (AMI): the objective was to evaluate the effects of L-carnitine compared with placebo or control on morbidity and mortality in the setting of acute myocardial infarction, through a systematic review and meta-analysis of 13 controlled trials (N=3,629) to determine the effects of L-carnitine vs. placebo or control on mortality, ventricular arrhythmias (VAs), angina, heart failure, and reinfarction. Compared with placebo or control, L-carnitine was associated with a 27% reduction in all-cause mortality, a 65% reduction in VAs, and a 40% reduction in anginal symptoms in patients experiencing an acute myocardial infarction. Further study with large randomized controlled trials of this inexpensive and safe therapy in the modern era was noted as warranted.

Chronic Heart Failure (CHF): 17 RCTs with 1,625 CHF patients were included in one meta-analysis. L-carnitine treatment in CHF was associated with considerable improvement in overall efficacy, left ventricular ejection fraction (LVEF) (WMD: 4.14%, P = 0.01), stroke volume (SV) (WMD: 8.21 ml, P = 0.01), cardiac output (CO) (WMD: 0.88 L/min, P < 0.01), and E/A ratio (WMD: 0.23, P < 0.01). A separate meta-analysis for dilated cardiomyopathy found: a total of 23 RCTs conducted in China with 1,455 DCM patients were included. L-carnitine therapy was associated with considerable improvement in overall efficacy (RR = 1.28, P < 0.0001), left ventricular ejection fraction (LVEF) (MD = 6.16%, P < 0.0001), and cardiac output (CO) (MD = 0.88 L/min, P < 0.0001) compared to the control group.

Evidence strength: The meta-analyses are positive but contain important limitations, including heterogeneity in dosing protocols and patient populations, a predominance of smaller trials (including Chinese trials with variable blinding quality), and no definitive large-scale modern RCT comparable to contemporary cardiovascular trials. Results should be interpreted with appropriate caution.

7.2 Peripheral Arterial Disease and Intermittent Claudication

Supplementation with L-carnitine and propionyl-L-carnitine may improve metabolism and exercise performance of ischemic muscles. Preliminary studies show that L-carnitine improves treadmill performance, while propionyl-L-carnitine improves walking distance. The body of evidence in this area is considered preliminary, with more consistent results observed for propionyl-L-carnitine than for L-carnitine in intermittent claudication specifically.

7.3 Kidney Disease and Hemodialysis

Through normalizing the lowered carnitine palmitoyl transferase function in red cells, L-carnitine can help uremic patients with a variety of complications, including muscle symptoms, impaired exercise and functional capacities, cardiac complications, and erythropoietin-resistant anemia.

Other studies have reported that L-carnitine stabilises the erythrocyte membrane structure in mature erythrocytes, prolongs erythrocyte survival, stimulates erythropoiesis, and improves response to erythropoietin through its anti-inflammatory effect. L-carnitine has therefore been evaluated as an adjuvant to erythropoiesis-stimulating agents (ESAs) for treating anemia. Carnitine supplementation may alleviate a number of symptoms from dialysis-related carnitine deficiency, thus contributing to improved quality of life of end-stage kidney disease (ESKD) patients.

Evidence strength: Evidence in hemodialysis patients is moderate. Carnitine levels are reliably depleted in dialysis patients, and supplementation can normalize levels, but robust evidence for hard clinical endpoints (mortality, hospitalization) is less consistent.

7.4 Type 2 Diabetes and Insulin Resistance

Insulin resistance, which plays an important role in the development of type 2 diabetes, may be associated with a defect in fatty-acid oxidation in muscle. This raises the question as to whether mitochondrial dysfunction might be a factor in the development of the disease. Increased storage of fat in lean tissues has become a marker for insulin resistance. Early research suggests that supplementation with L-carnitine intravenously may improve insulin sensitivity in diabetics by decreasing fat levels in muscle and may lower glucose levels in the blood by more promptly increasing its oxidation in cells.

Regarding body weight: in a clinical trial in 258 patients with uncontrolled type 2 diabetes, 2 g/day L-carnitine plus the pharmaceutical ingredient orlistat (360 mg/day) for 1 year significantly increased weight loss compared to orlistat alone. However, 2 g/day L-carnitine alone for 6 months did not affect weight loss in 94 men and women who were overweight and had newly diagnosed type 2 diabetes.

A meta-analysis on body mass in type 2 diabetes found: a total of 10 RCT studies, 1,239 T2DM patients were included for analysis. The study found that L-carnitine could reduce the body mass of T2DM patients. Based on only one study each for acetyl-L-carnitine and propionyl-L-carnitine, no significant effects were found for those forms. To achieve a plateau of efficacy (80% E_max), 2 g/day L-carnitine was required for at least 2 weeks.

Evidence strength: Preliminary to moderate; evidence in diabetes is predominantly for metabolic parameters rather than hard outcomes. Most trials are short-term and of modest size.

7.5 Weight Loss

A 2016 systematic review and meta-analysis combined the results from nine carnitine supplementation clinical trials in adults that assessed weight loss, with a total of 911 participants. In eight trials, the daily carnitine doses ranged from 1.8 to 4 g/day L-carnitine or levocarnitine for 30 to 360 days; in one trial, the dose of L-carnitine was 15 mg/kg/day for 182 days. One 6-month study found no effect on weight loss, but another study found that those who took the weight-loss drug orlistat plus carnitine supplements for 1 year lost an average of 4 pounds more than those who only took orlistat. Other studies also found that people who took carnitine supplements lost more weight than people who didn't take them, but the difference was small — only a few pounds. Larger studies are needed to better understand the effects of carnitine supplements on weight loss.

Evidence strength: Weak to modest. Effects on weight loss are small and inconsistent when carnitine is used alone. Studies have primarily evaluated carnitine's effects on lipid levels, cardiovascular disease, and type 2 diabetes; weight loss has been a secondary outcome in most studies.

7.6 Male Fertility and Sperm Quality

Carnitines are naturally occurring antioxidants in mammals and are normally abundant in the epididymal luminal fluid of men. The epididymis, where sperm mature and acquire motility, contains very high concentrations of L-carnitine, suggesting an important role in sperm energy metabolism.

Multiple systematic reviews and meta-analyses have assessed L-carnitine in idiopathic male infertility. A meta-analysis of nine RCTs found: comparing L-carnitine and/or L-acetyl-carnitine therapy to placebo showed significant improvement in pregnancy rate (OR = 4.10, P < 0.0001), total sperm motility (WMD = 7.43, P = 0.04), forward sperm motility (WMD = 11.83, P = 0.04) and atypical sperm cells (WMD = −5.72, P < 0.00001). However, no significant difference was found in sperm concentration or semen volume.

A 2025 meta-analysis across 14 RCTs involving 1,453 men confirmed: LC supplementation significantly increased the sperm concentration (MD = 6.85), total motility (MD = 10.41%), and normal morphology (MD = 1.78%). LAC supplementation improved total motility (MD = 17.03%) and forward motility (MD = 13.5%). The same analysis reported an apparent effective dose range: an effective dose range for LC supplementation appears to be 1–3 g/day, LAC at 0.5–2 g/day for at least 3–6 months.

A network meta-analysis across 23 RCTs with 1,917 patients compared multiple antioxidants: L-carnitine, L-carnitine+L-acetylcarnitine, coenzyme-Q10, ω-3 fatty acid, and selenium were more efficacious than placebo in sperm quality parameters. L-carnitine was ranked first in sperm motility and sperm morphology (WMD 6.52% [95% CI: 2.55% to 10.05%], WMD 4.96% [0.20% to 9.73%]).

However, a 2020 systematic review cautioned: analysis showed that carnitines significantly improve total sperm motility, progressive sperm motility and sperm morphology, but without effect on sperm concentration. The data indicate that carnitine does not significantly improve pregnancy rates in infertile couples with male infertility, despite improvements in sperm motility and morphology. However, natural conception was not a primary outcome in most studies, indeed most did not follow-up until pregnancy. Therefore, more evidence is required to study the effects of carnitines on pregnancy outcomes.

Evidence strength: Moderate. Multiple RCTs and meta-analyses consistently demonstrate improvements in sperm motility and morphology parameters. Evidence for pregnancy rates is less consistent, partly due to study design limitations.

7.7 Athletic Performance and Exercise Recovery

Interest in the potential of L-carnitine supplementation to improve athletic performance is related to its important roles in energy metabolism. Among supplements, L-carnitine became popular following rumors that it helped the Italian national soccer team to win the world championship in 1982, and it is often portrayed as a "fat burner", supposedly by increasing the aerobic contribution to exercise by increasing fat oxidation and muscle mass and reducing fat mass.

The core challenge has been raising muscle carnitine content through oral supplementation. Despite the hypothesis, 20 years of research has provided no compelling evidence that carnitine supplementation can improve athletic performance. Several factors account for the lack of a positive effect: muscle carnitine content is tightly regulated and levels are not easily increased with supplementation, and data on the optimal relationship between muscle carnitine content and muscle metabolic function are not available.

A number of small, poorly controlled studies have reported that either acute (dose given one hour before exercise bout) or short-term (two to three weeks) supplementation with L-carnitine (2 to 4 g/day) supported energy production, cardiorespiratory fitness, and endurance capacity during physical exercise. However, in a double-blind, placebo-controlled trial in 32 healthy adults, propionyl-L-carnitine (1 g/day or 3 g/day) for eight weeks did not improve aerobic or anaerobic exercise performance.

Research has shown that co-ingestion of carnitine with carbohydrates to induce hyperinsulinemia can increase muscle carnitine content. A study in older individuals found that L-carnitine supplementation resulted in a 20% increase in muscle total carnitine content (20.1 ± 1.2 to 23.9 ± 1.7 mmol/kg/dm; P < 0.01) and a 20% increase in total fat oxidation (181.1 ± 15.0 to 220.4 ± 19.6 J/kg lean body mass/min; P < 0.01), predominantly due to increased intramyocellular lipid (IMCL) utilization. These changes were associated with increased expression of genes involved in fat metabolism (ACAT1, DGKD & PLIN2; P < 0.05).

Regarding post-exercise recovery specifically, later studies point to the positive impact of dietary supplementation with L-carnitine on the recovery process after exercise. It is demonstrated that L-carnitine alleviates muscle injury and reduces markers of cellular damage and free radical formation accompanied by attenuation of muscle soreness. The supplementation-based increase in serum and muscle L-carnitine contents is suggested to enhance blood flow and oxygen supply to the muscle tissue via improved endothelial function thereby reducing hypoxia-induced cellular and biochemical disruptions.

Evidence strength: Weak to mixed for performance enhancement in healthy, well-nourished athletes. There is limited but suggestive evidence for enhanced fat oxidation under specific conditions (co-ingestion with carbohydrates, older individuals). Evidence for post-exercise recovery is more consistent but drawn mostly from smaller trials.

7.8 Neurological Conditions: Cognition, Alzheimer's Disease, and Peripheral Neuropathy

There is some low-quality evidence to suggest that supplemental L-carnitine or ALCAR may be beneficial as adjuncts to standard medical therapy of depression, Alzheimer's disease, and hepatic encephalopathy.

The metabolomic profiling of acylcarnitine molecules showed variations in serum concentrations of subjects along the continuum from cognitively healthy to affected by Alzheimer's disease. Changes in the blood concentrations of specific acylcarnitines in subjects with either subjective memory complaints, mild cognitive impairment, or Alzheimer's disease, compared to cognitively healthy peers may reflect changes in the transport of fatty acids into the mitochondria and/or impairments in energy production.

For diabetic peripheral neuropathy (DPN), acetyl-L-carnitine (ALCAR) may help reduce the severity of chemotherapy-induced peripheral neuropathy. High-quality evidence is needed to evaluate whether ALCAR may benefit the treatment of peripheral neuropathies associated with diabetes or caused by antiretroviral therapy. A Cochrane systematic review identified this as the first systematic review specifically for DPN and acknowledged limited but promising preliminary data from small studies.

Evidence strength: Weak to preliminary for cognitive outcomes in Alzheimer's disease and neuropathy. The evidence base largely consists of small trials with short follow-up periods; high-quality evidence is lacking.

7.9 Cancer-Related Fatigue and Cachexia

There is little evidence that L-carnitine supplementation improves cancer-related fatigue, low fertility, or overall physical health. Muscle wasting or atrophy, often known as reduction in skeletal muscle mass, is a frequent hallmark of various chronic disorders, including infectious diseases and cancer. Muscle wasting is also known as cachexia in chronic conditions, and it is commonly attended with adipose tissue loss. Carnitine supplementation has been studied in this context but evidence remains limited and no strong clinical recommendations are established.

7.10 Primary Carnitine Deficiency (Established Medical Use)

The sole area where L-carnitine supplementation has well-established, high-quality evidence and regulatory approval is in the treatment of primary carnitine deficiency. At high doses, L-carnitine corrects severe carnitine depletion and associated metabolic abnormalities observed in primary carnitine deficiency and enables the production of ketone bodies during fasting. Levocarnitine (L-carnitine) is approved as a pharmaceutical drug in the United States and in many other countries for primary and certain secondary carnitine deficiency states.

8. Body Systems and Health Areas Associated with L-Carnitine

• Mitochondrial/energy metabolism: Central role in fatty acid β-oxidation; affects ATP generation in all tissues with high energy demands.
• Cardiovascular system: L-carnitine is concentrated in cardiac muscle; associated with cardiac function, post-MI recovery, and management of heart failure.
• Skeletal muscle: Decreased concentrations of carnitine in skeletal muscle result in impairment of muscle function. This effect has led to the belief that carnitine supplementation can improve skeletal muscle function and athletic performance in healthy individuals.
• Kidney: The kidney is a primary site of carnitine biosynthesis and reabsorption; carnitine deficiency is common in dialysis patients.
• Nervous system: Acetyl-L-carnitine readily crosses the blood-brain barrier and is studied in neurodegenerative and neuropathic conditions.
• Male reproductive system: Carnitine is highly concentrated in the epididymis and is essential for sperm maturation and motility.
• Metabolic/endocrine system: Carnitine participates in insulin sensitivity and glucose metabolism, linking it to metabolic syndrome and type 2 diabetes research.

9. Dosage Forms and Dosages Reported in Studies

Carnitine is available in dietary supplements containing only carnitine or a combination of carnitine and other ingredients. The two main forms of carnitine in dietary supplements are L-carnitine and acetyl-L-carnitine, and amounts range from about 3 mg to 5,000 mg.

Dosages used across the principal areas of clinical research, as reported in published studies, are as follows:

• Cardiovascular / post-AMI: One multicenter clinical trial used carnitine given intravenously for five days, then 6 grams/day orally for one year.
• Chronic heart failure (general): Doses across the 17-RCT meta-analysis varied but were typically in the range of 1–3 g/day orally.
• Weight loss and type 2 diabetes: In eight trials in the 2016 meta-analysis, the daily carnitine doses ranged from 1.8 to 4 g/day L-carnitine or levocarnitine for 30 to 360 days; in one trial, the dose of L-carnitine was 15 mg/kg/day for 182 days.
• Type 2 diabetes body mass: To achieve a plateau of efficacy (80% E_max), 2 g/day L-carnitine was required for at least 2 weeks.
• Male infertility: Based on current evidence, an effective dose range for LC supplementation appears to be 1–3 g/day, LAC at 0.5–2 g/day for at least 3–6 months.
• Athletic performance / exercise: Studies used either acute doses (given one hour before exercise) or short-term (two to three weeks) supplementation with L-carnitine (2 to 4 g/day).
• Acetyl-L-carnitine (ALCAR) for neuropathy: Studies have used 3 g/day administered orally, typically for at least 52 weeks in diabetic neuropathy contexts.
• Mitochondrial disorders: A small, randomized, double-blind, controlled, crossover study evaluated the effects of 3 g/day oral carnitine supplementation on exercise tolerance in 12 patients with mitochondrial myopathy over 8 weeks of carnitine treatment and 8 weeks of placebo treatment separated by a 4-week washout period.
• Linus Pauling Institute recommendation (acetyl-L-carnitine): The Linus Pauling Institute recommends acetyl-L-carnitine at a daily dose of 0.5 to 1 g, noting that supplemental L-carnitine (doses, 0.6–7.0 g) is less efficiently absorbed compared to smaller amounts in food.

10. Safety Considerations and Drug Interactions

10.1 General Tolerability

Carnitine supplements are well tolerated and generally safe at doses up to about 4 g/day, although they can cause nausea, vomiting, abdominal cramps, diarrhea, and a fishy body odor. At doses of approximately 3 g/day, carnitine supplements can cause nausea, vomiting, abdominal cramps, diarrhea, and a fishy body odor.

Rarer side effects include muscle weakness in patients with uremia and seizures in those with seizure disorders. High amounts can also cause muscle weakness in people with chronic kidney disease and can increase the risk of seizures in people who have a seizure disorder.

A review of evidence on the safety of oral carnitine supplementation concluded, based on clinical studies of the supplement's use for weight loss and energy balance, that carnitine intakes below 2,000 mg/day appear to be safe. However, the evidence is insufficient to ascertain carnitine's long-term safety.

10.2 The TMAO Pathway: An Active Safety Concern

A significant and actively researched safety concern involves the metabolic pathway from dietary L-carnitine to trimethylamine-N-oxide (TMAO). L-carnitine is metabolised to trimethylamine in the gut by the gut microflora and thence to trimethylamine-N-oxide (TMAO) in the liver. TMAO is a molecule which promotes atherogenesis through its interaction with macrophages and lipid metabolism.

Some research indicates that intestinal bacteria metabolize carnitine to form trimethylamine N-oxide (TMAO), a substance that might increase the risk of cardiovascular disease. This effect appears to be more pronounced in people who consume meat than in vegans or vegetarians. The implications of this effect are not well understood and require more research.

TMAO, produced through the gut-microbial metabolism of L-carnitine and subsequent liver oxidation, is associated with cardiovascular risks including atherosclerosis, heart attacks, and stroke. It contributes to cholesterol deposition, vascular dysfunction, and platelet aggregation. Omnivorous diets, rich in L-carnitine, are associated with higher TMAO levels compared to plant-based diets, which are linked to lower cardiovascular disease risks.

A study in patients with organic acidemias indicated that carnitine supplementation (approximately 100 mg/kg) results in significant elevations of plasma trimethylamine N-oxide (TMAO) despite dietary meat restrictions. This compound has been linked to a higher risk of cardiovascular disease. However, the implications of these findings are not well understood and require more research.

The relationship between TMAO and cardiovascular risk is complex. In a prospective cohort study, L-carnitine was associated with higher risk of prevalent coronary artery disease, peripheral artery disease, and overall cardiovascular disease, possibly mediated by its intestinal metabolite TMAO. However, observational studies are open to residual confounding, such as by socioeconomic position and health status, and cannot distinguish whether L-carnitine is a biomarker or a causal factor, making them difficult to use as a guide to interventions.

10.3 Drug Interactions

Anticonvulsants: Treatment with the anticonvulsants valproic acid, phenobarbital, phenytoin, and carbamazepine reduces blood levels of carnitine. In addition, the use of valproic acid with or without other anticonvulsants may cause hepatotoxicity and increase plasma ammonia concentrations, leading to encephalopathy. Intravenous L-carnitine administration might help treat valproic acid toxicity in children and adults, although the optimal regimen has not been identified.

Pivalate-containing antibiotics: Long-term use of some antibiotics, such as pivampicillin, used to prevent urinary tract infections, can lower the amount of carnitine in the body.

Antiretroviral therapy: Secondary carnitine deficiency is associated with zidovudine medication. Nucleoside reverse transcriptase inhibitors used in HIV treatment can deplete carnitine levels over time.

Long-chain fatty acid oxidation disorders: Carnitine therapy for long-chain fatty acid oxidation defects has become questionable because it promotes formation of long-chain acylcarnitines that may cause arrhythmogenesis and membrane dysfunction.

10.4 Upper Limit and Regulatory Status

Carnitine in foods and beverages is safe. Therefore, carnitine doesn't have an upper limit established by regulatory authorities for food. The Food and Nutrition Board of the National Academies has not issued a recommended dietary requirement for L-carnitine because it is not considered an essential nutrient for many people. Levocarnitine is additionally classified as a pharmaceutical drug when used to treat diagnosed carnitine deficiency states.

References

• NIH Office of Dietary Supplements — Carnitine: Fact Sheet for Health Professionals
• NIH Office of Dietary Supplements — Carnitine: Fact Sheet for Consumers
• NIH Office of Dietary Supplements — Carnitine: The Science Behind a Conditionally Essential Nutrient (Conference Summary)
• NIH Office of Dietary Supplements — Dietary Supplements for Weight Loss (Health Professional Fact Sheet)
• NIH Office of Dietary Supplements — Dietary Supplements for Primary Mitochondrial Disorders (Health Professional Fact Sheet)
• Linus Pauling Institute, Oregon State University — L-Carnitine (Micronutrient Information Center)
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