Pharmacokinetics of L-carnitine.
Level 5 - mechanism / opinion, no new human data
Narrative review synthesizing pharmacokinetic concepts and studies without systematic review methodology.
PubMed 12908852 · doi:10.2165/00003088-200342110-00002
What was done
This narrative review synthesized human pharmacokinetic data on endogenous and exogenous L-carnitine, focusing on absorption, distribution, carrier-mediated transport, and renal elimination in healthy states and clinical conditions such as primary/secondary deficiency and end-stage renal disease undergoing hemodialysis.
What was found
- Absorption: Dietary L-carnitine has high bioavailability (up to 75%), but absolute bioavailability drops to 5–18% following oral pharmacological doses of 1–6 g. - Distribution: Initial distribution volume after intravenous administration is approximately 0.2–0.3 L/kg (matching extracellular fluid volume); L-carnitine does not bind plasma proteins, exhibits very slow erythrocyte uptake, and equilibrates slowly across three compartments, with skeletal and cardiac muscle in the slowest pool. - Renal handling: Under baseline conditions, renal clearance is 1–3 mL/min due to 98–99% tubular reabsorption. The reabsorption threshold is 40–60 µmol/L (close to endogenous plasma levels), causing renal clearance to increase toward glomerular filtration rate (GFR) at higher plasma concentrations.
Why it matters
Understanding the low bioavailability of oral pharmacological doses and the saturable renal reabsorption threshold explains why carnitine handling changes dramatically at supra-physiological doses and guides replacement strategies in deficiency states or hemodialysis.
Limits
The abstract does not provide systematic review methodology, search criteria, study quality grading, or aggregated sample sizes. Pharmacokinetic parameters represent summarized narrative ranges rather than meta-analytic effect estimates.
Cited by
- supports L-carnitine is poorly absorbed when taken orally compared to injectable forms.