Lactate Is a Signal, Not a Waste Product

EN-05 in the VAT-Trap / SCVC Evidence Notes series. Companion to WP-06. The commonest objection to prescribing hard interval training is the discomfort of working above the point at which lactate accumulates. This note sets out what that discomfort represents — and what cannot be concluded from it. Lactate is not a metabolic waste product and is not evidence that muscle has run out of oxygen. It is released by contracting muscle as a signalling molecule, acts as an upstream regulator of PGC-1α, is a candidate mediator of exercise-induced appetite suppression through GPR81 on gastric ghrelin-releasing cells, and is taken up and oxidised by the myocardium in rising quantities as effort increases — more than threefold within five minutes of moderate cycling in coronary sinus catheter studies. Following the Evidence Note format, the note carries an explicit section on what the evidence does not show: that lactate causes fat loss, that hard exercise substitutes for pharmacotherapy, or that generating lactate feeds or protects the heart. It also distinguishes the transient physiological rise of hard exercise from raised blood lactate as a marker of inadequate tissue perfusion — a conflation worth guarding against in lay writing. A downloadable HTML version is included alongside the PDF: its references are hyperlinked with superscript citations and return-links, making it easier to cite, search and link to specific sources.