Kinetics, pharmacokinetics, and regulation of L-carnitine and acetyl-L-carnitine metabolism.
Level 5 - mechanism / opinion, no new human data
Narrative review of pharmacokinetic, physiological, and in vitro data without systematic review methodology
PubMed 15591001 · doi:10.1196/annals.1320.003
What was done
This narrative review summarizes physiological, in vitro, and in vivo human pharmacokinetic data regarding the absorption, tissue distribution, metabolism, and renal elimination of L-carnitine and acetyl-L-carnitine.
What was found
Dietary L-carnitine bioavailability is 54–87%, whereas bioavailability from dietary supplements (0.5–6 g) drops to 14–18%, with unabsorbed carnitine largely degraded by colonic microbiota. Whole-body turnover time in humans at normal dietary intake is 38–119 h. Oral acetyl-L-carnitine (2 g/day) increased circulating acetyl-L-carnitine concentrations by 43%, indicating partial intact absorption. Single-dose intravenous acetyl-L-carnitine (0.5 g) is rapidly hydrolyzed, with acetyl-L-carnitine and L-carnitine returning to baseline concentrations within 12 h. Normal renal L-carnitine reabsorption is 90–99% efficient (clearance 1–3 mL/min) but saturable, leading to rapid renal clearance at elevated circulating concentrations.
Why it matters
These kinetic characteristics demonstrate that high-dose oral L-carnitine supplementation faces both limited fractional intestinal absorption and rapid saturable renal excretion.
Limits
The abstract describes a narrative review without reporting underlying sample sizes, subject characteristics, search methodology, or confidence intervals. The review focuses on pharmacokinetic parameters rather than clinical or functional outcomes.
Cited by
- supports L-carnitine is poorly absorbed when taken orally compared to injectable forms.