Factors influencing substrate oxidation during submaximal cycling: a modelling analysis
434 studies: sex, dietary intake and exercise duration have the largest effects on RER; models explain at most 59% of the variation
Background (abstract)
Which fuel the body burns during exercise depends on how long and how hard the session is, on sex, and on what is eaten before and during it — but how much each factor counts, and how they interact, had not been clear. Aim: model the respiratory exchange ratio (RER) during continuous cycling to see which factors best explain it and how much each matters.
Methods (abstract)
Data extracted from 434 studies reporting RER during continuous cycling exercise
General linear mixed-effect models relating RER to purported influences (exercise duration and intensity, muscle glycogen, dietary intake, age, sex, …); standardized coefficients used to rank relative influence
Protocol
Modelling analysis of published data: RER during continuous cycling exercise extracted from 434 studies; the total number of participants is not stated in the abstract (from the abstract).
General linear mixed-effect models related RER to purported influences (exercise duration and intensity, muscle glycogen, dietary intake before and during exercise, age, sex, VO2max, percentage of type I fibres); standardized coefficients ranked their relative influence.
Outcome: share of RER variation explained by the full model and by a model limited to easily modified factors (exercise duration and intensity, dietary intake before and during exercise).
Key Findings (abstract)
RER decreases (more fat oxidation) with: longer exercise, more dietary fat, older age, higher VO₂max, and a larger share of type I muscle fibres
RER increases (more carbohydrate oxidation) with: more dietary carbohydrate, higher exercise intensity, male sex, and carbohydrate taken before and during exercise
Largest effects on RER: sex, dietary intake, and exercise duration — not intensity
Among diet factors, everyday fat and carbohydrate intake matter more than carbohydrate taken during exercise
Explanatory power: the full model explained up to 59% of the variation in RER; a model using only easily modified factors (duration, intensity, diet before and during exercise) explained 36%
Conclusion: RER variability cannot be fully accounted for by participant, diet, exercise and physiological characteristics; more research is needed on older subjects and on females
Practical Application
Modelling of RER data from 434 cycling studies: fat oxidation is higher with longer duration, higher dietary fat intake, older age, higher VO₂max and more type I fibres, and lower with higher dietary carbohydrate, higher intensity, male sex and carbohydrate taken before or during exercise; the biggest levers were sex, habitual diet and duration — and even the full model explained only 59% of the variation (36% with the modifiable factors alone). Meaning: two athletes at the same %FTP can burn very different fuel mixes, and what they ate in the days before the test moves the needle more than the gel they took during it.
Fat-oxidation rate is plastic and context-dependent — not a fixed trait
Interpreting an athlete's metabolic test: standardize the previous days' diet and the fed state or the result is confounded; do not over-read a single FatMax test
Sex and age are built-in offsets: women and older athletes tend toward lower RER at a given workload — compare an athlete to their own history, not to a generic curve
Limitations
Modelling of published study means — synthesizes existing data, not new measurement; study-level, not individual-level, variation
Explains at most 59% of RER variation; the authors call for more data on older subjects and females
Cannot fully separate diet from training-status effects
Cycling only
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: systematic quantitative synthesis of heterogeneous measurement studies, not controlled trials
Content correction 2026-09-27
"Exercise intensity (largest factor)" -> abstract: sex, dietary intake and exercise duration had the biggest effect on RER; corrected.
"Genetics — explains residual inter-individual variation" -> not in the abstract (the abstract names percentage of type I fibres and unexplained variance); removed.
Added the abstract's numbers (434 studies; 59% / 36% explained; direction of every factor; daily diet > intra-exercise carbohydrate). Level A -> A- (quantitative synthesis of heterogeneous measurement studies); added Tier/Sport lines and an abstract-based lead.
2026-09-28: added Protocol from the abstract (Europe PMC, DOI 10.1007/s40279-022-01727-7, PMID 35829994). The abstract gives studies (434), not participants, so no n was added.