Carnitine shuttle
Fat oxidation is rate-limited here. Without carnitine, long-chain fatty acids cannot reach the enzymes that burn them, however much fat is available.
In depth
Carries long-chain fatty acids across the inner mitochondrial membrane. Also carries the CPT1 enzyme, which is the gate itself.
What drives it
Fat-burning brake reduces it.
Ruderman 1999, Am J Physiol Endocrinol Metab 276:E1 — malonyl-CoA is the allosteric inhibitor of CPT1, the gate governing entry of long-chain fatty acyl-CoAs into mitochondria for oxidation. doi:10.1152/ajpendo.1999.276.1.E1. Malonyl-CoA is a potent direct inhibitor of mitochondrial carnitine palmitoyltransferase 1: it binds CPT1 and reduces the amount of fatty acyl-CoA entering mitochondria, limiting beta-oxidation. This is the gate that decides whether fat is stored or burned, and it is an allosteric block rather than a transcriptional one — it acts in seconds. doi 10.1152/ajpendo.1999.276.1.E1.
Micronutrient status increases it.
Carnitine is obtained from diet and synthesized from lysine and methionine, requiring vitamin C, B6, niacin and iron as cofactors. Carried by the intermediary-metabolism reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
Liver increases it.
Endogenous carnitine synthesis is hepatic and renal. Carried by the intermediary-metabolism reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
Kidney increases it.
The kidney both synthesizes and reabsorbs carnitine; renal impairment alters status. Carried by the intermediary-metabolism reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
What it drives
It increases Fat burning.
CPT1/β-oxidation study (PMC2724792) — carnitine palmitoyltransferase-1 catalyzes the rate-limiting step importing long-chain fatty acyl-CoAs into mitochondria for β-oxidation. PMC2724792. CPT1 is the rate-limiting entry step: fatty acyl-CoA cannot enter the mitochondrion to be oxidized except through the carnitine shuttle, so CPT1 activity sets the ceiling on beta-oxidation. entry is necessary but not sufficient — see the ACC-DKI knock-in on the AMPK → Fatty-Acid β-Oxidation link, where elevated malonyl-CoA did not reduce whole-body fatty-acid oxidation during exercise, showing CPT1-independent routes exist. PMC2724792
It increases Mitochondrial energy.
Substrate delivery to the oxidative machinery. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
It increases Physical performance.
Weaker evidence - mechanistically necessary, but supplementation trials in replete individuals are inconsistent. Carried by the intermediary-metabolism and clinical reviews. doi 10.1016/j.molmet.2023.101755 · PMC9967593
Blends that modulate it
Each blend page names this pathway and carries the study behind its claim.
See it on the map
Open the causal map to follow this pathway upstream and downstream, arrow by arrow, each with its study.