REF-1338 · Brooks, 2018 · Cell Metabolism · Paper · PubMed

Is lactate a waste product of hard exercise?

No: in its founder's review, lactate forms all the time, even with ample oxygen, and other cells burn it as fuel or make glucose from it; it is also a signal (B+, abstract).

Main caveat: Abstract only. The free copy at the publisher and the eScholarship repository copy listed by OpenAlex sit behind bot checks, and the paper is not in PubMed…
moderate evidenceMeasured on non-athletes to world-class athletes

Brooks reviews the science and the clinical translation of lactate shuttle theory.

In the abstract lactate, once thought a waste product of anaerobic metabolism, is now known to form continuously under aerobic conditions, and shuttling between producer and consumer cells serves at least three purposes: a major energy source, the major gluconeogenic precursor (the main raw material for making new glucose) and a signalling molecule.

"Lactate shuttle" concepts describe its roles in delivering oxidative and gluconeogenic substrate and in cell signalling.

In medicine a rise in blood lactate has long been known to track the severity of illness or injury; with the shuttle in mind, the review says, some clinicians now read lactatemia as a "strain" and not a "stress" marker, and clinical studies are using lactate to treat pro-inflammatory conditions and to deliver fuel to working muscle in sports medicine.

Our reading: the abstract restates, in the theory author's words, what the base already holds from his later reviews (REF-471, REF-172) and from Gladden (REF-493): lactate is made with oxygen present, is a fuel and a signal, and a blood value is a balance of production and use.

It does not address acidosis, fatigue or soreness, so it cannot be cited on those; the dispute over the intracellular (mitochondrial) part of the shuttle, reported in REF-493, cannot be judged from this abstract either.

Protocol

Design: narrative review of historical and recent studies of lactate metabolism and shuttling; no search method or inclusion rules described in the abstract (n = not applicable).
Scope (from the abstract): lactate formation under aerobic conditions; shuttles between producer and consumer cells; lactate as fuel, gluconeogenic precursor and signal; blood lactate in illness and injury; clinical and sports-medicine uses of lactate.
Not given in the abstract: any number, method or population; the author's declaration of interests.
Funding (as registered, not as printed): Crossref lists the Pac-12 Conference (a US collegiate athletic conference) and UC Berkeley as funders; Europe PMC lists NIA NIH R01 AG059715, the Pac-12 Conference (#3-02-Brooks-17) and UCB. The printed acknowledgments and declaration of interests were not read.

Key Results

Lactate forms continuously under aerobic conditions — it is not only a product of oxygen lack (abstract).
Three roles for shuttled lactate: a major energy source, the major gluconeogenic precursor, and a signalling molecule (abstract; "at least three").
Lactate shuttle concepts cover delivery of oxidative and gluconeogenic substrates and cell signalling (abstract).
Clinical reading: a rise in blood lactate correlates with the severity of illness or injury; some clinicians now treat lactatemia as a "strain" rather than a "stress" biomarker (abstract).
Clinical use: studies are using lactate to treat pro-inflammatory conditions and to deliver fuel to working muscles in sports medicine (abstract; no study or result named).
No numbers: the abstract gives no rates, concentrations or study counts.

Mechanisms

Authors' interpretation (abstract): lactate links cells — producer cells export it, consumer cells oxidise it or turn it into glucose, and it signals along the way. The abstract states the roles; the tracer and transporter evidence behind them is in the full text and, in the base, in REF-471, REF-172 and REF-493.
Contested (not in this abstract): whether lactate is oxidised inside mitochondria through a mitochondrial lactate dehydrogenase (the intracellular shuttle) is reported as disputed by Gladden 2004 (REF-493, full text: two laboratories found insignificant mitochondrial LDH activity); this abstract does not take up the point.Whether lactate production causes the acidosis of intense exercise is a separate dispute (REF-1339 against the strong-ion reading in REF-493), and whether acidosis causes fatigue another (REF-455).
Inference: the "strain, not stress" framing points the same way as the base's reading of a blood value in training: a high lactate marks a high rate of glycolytic flux that other tissues are clearing, not a poison to be flushed (SYNTH-83).

Practical Application

Is lactate a waste product of hard exercise? No: in its founder's review, lactate forms all the time, even with ample oxygen, and other cells burn it as fuel or make glucose from it; it is also a signal (B+, abstract).
What it is good for as a source: that lactate is an aerobic-condition fuel, gluconeogenic precursor and signal, in the theory author's own words. It does not show anything about the burn, fatigue, acidosis or next-day soreness, and gives no training rule or dose.
Reading a lactate value: it is the balance of production and clearance, read with the protocol, the meter and the day (SYNTH-53); not from this paper.
"Flush out lactic acid" has no support in this abstract or elsewhere in the base (SYNTH-83); faster clearance with a cool-down did not bring less soreness (REF-913).
The clinical lines (lactate as a therapy) are not advice for athletes and are not tested in this entry.

Limitations

Abstract only. The free copy at the publisher and the eScholarship repository copy listed by OpenAlex sit behind bot checks, and the paper is not in PubMed Central; the evidence, the figures, the acknowledgments and the declaration of interests could not be read.
The theory's author reviewing his own theory. It is an argued synthesis, not a systematic one, and parts of the theory (the intracellular shuttle) are disputed by other laboratories (REF-493).
No numbers. Nothing quantitative may be cited from this entry.
Funding registered with a sports body: the Pac-12 Conference, a collegiate athletic conference, is listed as a funder in Crossref and Europe PMC; the printed statement was not read.
How this entry was checked and changed
Why this level

authoritative narrative review from the leading group on lactate shuttles (Brooks, UC Berkeley, who proposed the theory), the rubric's line; read from the abstract only. It is the theory's author reviewing his own theory, and one part of it, the intracellular (mitochondrial) shuttle, is reported as disputed elsewhere in the base (REF-493)