Lactate as a fulcrum of metabolism
Brooks' review: lactate is formed aerobically, shuttles between cells as fuel, gluconeogenic precursor and signal, and controls fat/carbohydrate partitioning
Key Findings — Core Thesis (abstract)
Lactate was mistakenly thought to be the consequence of oxygen lack in contracting muscle; the L-lactate anion is formed under fully aerobic conditions and used continuously in diverse cells, tissues, organs and at whole-body level
Lactate moves from cells that make it ("driver") to cells that use it ("recipient"), and in doing so does at least three jobs: (1) it is a major fuel for mitochondrial respiration; (2) it is the main precursor for gluconeogenesis; (3) it acts as a signalling molecule
Mechanisms of influence: lactylation of histone lysine residues that reprograms chromatin, allosteric binding, regulation of cell redox, and mass action
Scale of the signal: in tissue, lactate spans 0.5-20 mM under physiological conditions, and the lactate/pyruvate ratio inside cells spans 10 to >500; the swings seen in exercise and other stress responses are far larger in size and range than any other metabolic signal, so they act quickly and strongly, short- and long-term, on cell redox and the other control systems
Substrate partitioning: through HCAR-1 lactate blocks fat breakdown (lipolysis) in adipose tissue, and through malonyl-CoA and CPT1 it blocks fatty-acid entry into muscle mitochondria — so lactate controls the fat/carbohydrate mix
Training effect: when regular exercise exposes the body to lactate again and again, the expression of the enzymes that regulate glycolysis and mitochondrial respiration changes substantially
Lactate is not a fatigue-causing waste product (the framing of the whole review)
Practical Application
Narrative review (Brooks 2020): lactate is produced under fully aerobic conditions and shuttles between cells as a major mitochondrial fuel, the main gluconeogenic precursor and a signalling molecule; tissue lactate spans 0.5-20 mM and the lactate/pyruvate ratio 10 to >500 during exercise — a signal that dwarfs other metabolic cues — and lactate suppresses fat use (adipose lipolysis via HCAR-1, mitochondrial fatty-acid uptake via malonyl-CoA/CPT1) while repeated exposure through training upregulates glycolytic and mitochondrial enzymes. Meaning: a rising lactate reading is a fuel-partitioning signal — the body switching toward carbohydrate — not a toxin to "clear".
Lactate at sub-threshold/threshold intensities is a signalling stimulus as well as a substrate — a mechanism argument for threshold-zone work; the training evidence lives in the MIT-block entries (REF-137 trained at about 2.8 mmol/L, REF-421) and whether the lactate itself drives that adaptation is inference, not tested
Reframes "lactate clearance": what improves with training is shuttling and oxidation capacity (MCT transporters, mitochondrial uptake — full text), not waste removal
Limitations
Narrative review, no systematic method; conceptual
"Lactormone" signalling and histone lactylation still being mechanistically detailed
Some claims extrapolated from cell/animal models
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: authoritative narrative review from the originator of the lactate-shuttle concept, no systematic method
Content correction 2026-09-27
"Justifies MIT-block training at ~3 mmol/L — the lactate level itself drives mitochondrial adaptation" -> not in the abstract; relabelled as inference, pointing to REF-137 (2.8 mmol/L) for the training evidence.
Added the abstract's own content: 0.5-20 mM range, L/P ratio 10 to >500, HCAR-1 / malonyl-CoA / CPT1 partitioning mechanism, histone lactylation, training effect on enzyme expression.
Level A -> B+ (narrative review); added Tier/Sport lines and an abstract-based lead.