Lactate Is Not Your Enemy. It Never Was.

For 200 years, science blamed the wrong molecule. Here is what lactate actually does — and why getting this right changes everything from athletic performance to cancer treatment.

Based on research by Dr. Iñigo San Millán & Dr. George Brooks

In 1780, a Swedish pharmacist named Carl Wilhelm Scheele isolated an unknown compound from curdled milk and named it mjölksyra — acid of milk. The Latin root gave us lactic acid, a name that would stick for 245 years and carry a reputation the molecule never deserved.

In 1843, a German physician found lactic acid in the blood of patients dying from sepsis and hemorrhagic shock. By the time Otto Meyerhof won the Nobel Prize in 1922 for showing that glycogen breakdown produces lactic acid during muscle contraction, the case was closed in the scientific mind: lactate was a waste product of oxygen-deprived tissue, the culprit behind the burn, the cause of fatigue.

There was one problem. Meyerhof built his model on isolated frog muscle in a dish, deprived of oxygen. The living, breathing, intact organism was a different story entirely — one that would take another six decades to tell.

The man who got it right

George Brooks grew up running 400 meters competitively in New York. When his coach told him poor times were caused by too much lactic acid, Brooks read the Nobel Prize-winning science. Something about it did not sit right. Why would the body mass-produce something during the very activity it was designed to support, if it were purely a waste?

That question became a career. From a basement laboratory at UC Berkeley, Brooks spent the 1980s using isotopic tracers to track lactate in living, exercising organisms with intact circulation and functioning mitochondria. What he found overturned a century of consensus.

Lactate is produced continuously under fully aerobic conditions — not as a consequence of oxygen deficiency, but as an obligatory product of normal cellular metabolism.

First, a correction of chemistry: at physiological pH, the molecule exists almost entirely in dissociated ionic form. It is lactate, not lactic acid. That distinction is not merely semantic — lactic acid essentially does not exist in the body at normal pH.

Second, and more consequential: the enzyme lactate dehydrogenase (LDH) consumes a proton when it converts pyruvate to lactate. Lactate does not cause acidosis — it modestly reduces it. The burn athletes feel comes from ATP hydrolysis during muscle contraction, which releases protons. Lactate was at the scene of the crime, not the criminal.

A fuel, not a failure

Brooks formalized his findings in the Lactate Shuttle — now one of the most important frameworks in metabolic physiology. Fast-twitch muscle fibers produce lactate rapidly and export it into the bloodstream. Slow-twitch fibers, the heart, brain, and kidneys take it up and burn it as fuel. Lactate is the body's primary carbohydrate energy currency, distributed continuously through the circulation to any organ with mitochondria.

Source - Glycolytic (fast-twitch) fibers

Destination - Heart, brain, slow-twitch muscle, kidneys

Role - Fuel + pH buffer + signaling molecule

This reframes what blood lactate actually measures. An athlete with high mitochondrial density clears lactate as fast as it is produced — barely a trace appears in the blood at high work rates. An athlete with low mitochondrial capacity sees lactate accumulate rapidly at the same power output. Blood lactate is not a measure of anaerobic desperation. It is a direct readout of the gap between glycolytic production and mitochondrial clearance.

Two cyclists at 300 watts. One shows 1.8 mmol/L blood lactate. The other shows 7. The difference is not effort — it is mitochondrial function. And from that single number you can infer fitness, predict performance, and design a training intervention.

Lactate talks to your entire body

Brooks has characterized lactate as a lactormone — a molecule with hormone-like properties that functions as an endocrine, paracrine, and autocrine mediator. During high-intensity exercise, lactate binds to receptors in adipose tissue and suppresses fat breakdown, redirecting the body toward carbohydrate as the primary fuel. It inhibits the enzymes that import fatty acids into mitochondria. It does not merely reflect the metabolic shift — it orchestrates it.

In the brain, circulating lactate is taken up by neurons and astrocytes, where it stimulates production of BDNF, supports synaptic plasticity, and promotes neurogenesis in the hippocampus. The cognitive lift and mood elevation that follow aerobic exercise are not incidental — lactate is a direct molecular signal to the brain that physical work is occurring, and the brain responds with adaptation. Lactate also modifies histone proteins through a process called lactylation, reaching inside cell nuclei to alter which genes are expressed.