Carbohydrate Ingestion on Exercise Metabolism and Physical Performance
Comprehensive Endocrine Reviews synthesis
Scope
A review of more than 160 studies asking what carbohydrate ingestion actually does during exercise — and arguing that the answer is not the one the fuelling literature assumes.
Key Findings (themes)
The thesis: exercise-induced hypoglycaemia, not glycogen depletion, is what ends the effort. The authors report that low blood glucose correlates strongly with exercise termination, while muscle glycogen depletion alone does not produce whole-body fatigue.
Carbohydrate works by preventing that hypoglycaemia. It reduces liver glycogenolysis and preserves blood glucose — and, paradoxically, ACCELERATES muscle glycogen breakdown, which is the opposite of the glycogen-sparing story usually told for it.
The benefit is largest beyond 2-3 hours, and in individuals with insufficient hepatic gluconeogenesis. Carbohydrate improved performance even from a glycogen-depleted state, which the authors read as evidence for the glucose mechanism rather than the fuel-store one.
Fat-adapted athletes performed equivalently on much lower carbohydrate oxidation and lower glycogen — the authors' argument that carbohydrate is not an obligatory fuel.
The paper explicitly questions universally high carbohydrate intakes, both habitual and during exercise. It is the contrarian pole of that argument, not a source for a grams-per-hour number.
Practical Application
Where it changes the reading of a session: the athlete who collapses suddenly with willing legs. Symptoms of low blood glucose are explained better here than by glycogen accounting, and the answer is fuelling earlier rather than more.
Present it as a disagreement, not as consensus. The current high-carbohydrate position is held by REF-334, REF-066 and SYNTH-02; this review argues against its universality. An athlete asking whether the high intakes now used in racing (Not from this paper: ≥100 g/h, REF-334) are really necessary deserves both sides and the fact that the question is open.
Both poles agree that carbohydrate helps past 2-3 h. The dispute is about WHY, and the why decides how much is enough.
Limitations
Author-prior note: the authors (Noakes, Prins, Buga, D'Agostino, Volek, Koutnik) are the low-carbohydrate and ketogenic research group, and the conclusions run with that prior. That does not make the argument wrong — the hypoglycaemia evidence is real — but this is an argued position, not a neutral summary of the field.
Narrative review, not systematic: no protocol, no risk-of-bias assessment, no pooled effect sizes. Study selection across 100 years of literature is the authors'.
It reframes the mechanism; it does not test a fuelling protocol. Nothing here tells an athlete how much to eat.
Endocrine-journal lens — strong on metabolism, lighter on field-practical detail
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: narrative review without systematic method (the level grades design, not the journal); kept above C+ as an authoritative review in Endocrine Reviews
Content correction 2026-09-27
Level A -> B+: a narrative review of >160 studies with no systematic method does not meet the rubric's A (SR/MA or consensus); regraded, author-prior caveat kept.
The "90-120 g/h" figure in the Practical section was not from this paper; relabelled as the racing range carried by REF-334.