L-Carnitine: Essential Metabolic Compound with Therapeutic Applications
L-carnitine is a quaternary ammonium compound that functions as an essential carrier molecule for transporting long-chain fatty acids across mitochondrial membranes for energy production through β-oxidation. 1
Biochemical Properties and Functions
L-carnitine (levocarnitine) is a naturally occurring compound with several key metabolic functions:
- Primary role: Transports long-chain fatty acids from the cytosol into mitochondria for β-oxidation and energy generation 1
- Chemical structure: 3-carboxy-2(R)-hydroxy-N,N,N-trimethyl-1-propanaminium, inner salt 2
- Molecular formula: C7H15NO3 with molecular weight of 161.20 2
- Secondary functions:
Biosynthesis and Sources
Endogenous Production
- Synthesized primarily in the liver and kidneys using amino acids L-lysine and L-methionine as substrates 1, 5
- Requires vitamin B6, nicotinic acids, vitamin C, and folate as cofactors for biosynthesis 3
- Healthy individuals (including strict vegetarians) typically synthesize sufficient L-carnitine without requiring supplementation 1
Dietary Sources
- Highest concentrations found in red meat (beef, lamb) 1, 5
- Other good sources include fish, poultry, and milk 1
- Plant foods contain minimal or no carnitine 3
- Typical dietary intake in omnivores: 2-5 mg/kg/day (approximately 250 mg/day for a 70-kg adult) 1
Absorption and Distribution
- Absorbed in the small intestine via carrier-mediated transport and passive diffusion 1, 6
- Bioavailability varies significantly:
- Primarily stored in skeletal muscle (main reservoir) at concentrations approximately 200 times higher than plasma 3
- Not bound to plasma proteins 6
Assessment of Carnitine Status
Evaluation of carnitine status requires measurement of:
- Free carnitine levels
- Total carnitine levels
- Acyl-to-free carnitine ratio (normal ratio ≤0.25; deficiency indicated by ratio >0.4) 1
- Additional tests: blood triglycerides, liver function tests, glucose, lactate, ammonium, and urine ketones 1
Clinical Applications
While L-carnitine has been investigated for numerous conditions, evidence supports limited therapeutic applications:
- Dialysis patients: May improve symptoms like malaise, muscle weakness, intradialytic cramps and hypotension in selected maintenance dialysis patients, though evidence is insufficient for routine use 1
- Erythropoietin-resistant anemia: Most promising application in dialysis patients 1
- Neonatal care: Indicated for premature infants on total parenteral nutrition for >4 weeks 4
- Sepsis: No definitive recommendation due to limited evidence, though it may help address energy metabolism disruption 1
- Cardiovascular disease: May improve energy metabolism and myocardial contractility in certain heart failure models 7
Safety and Monitoring
- Generally considered safe at appropriate doses
- High doses (≥3g/day) may cause gastrointestinal side effects including nausea, vomiting, abdominal cramps, and diarrhea 4
- Not degraded in the body; primarily eliminated via renal excretion 6
Key Considerations for Clinical Use
- Supplementation should be targeted to specific populations with demonstrated need rather than used routinely
- Dosing should align with physiological requirements (typically 2-5 mg/kg/day) 1
- Monitoring of carnitine status through appropriate biomarkers is essential when supplementation is indicated
L-carnitine represents an important metabolic compound with specific therapeutic applications, particularly in conditions affecting energy metabolism. While healthy individuals typically synthesize adequate amounts, certain clinical scenarios may warrant supplementation based on careful assessment of carnitine status.