Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration
The foundational substrate-crossover tracer study (n=5)
Participants
n=5 trained subjects (endurance-trained men, cycle ergometer — full text; not checked against the abstract).
Protocol
Stable isotope tracers and indirect calorimetry to evaluate endogenous fat and glucose metabolism
Three exercise intensities: 25%, 65% and 85% VO₂max
Duration effect studied over 2 h of exercise at 65% VO₂max
Recovery after high-intensity exercise also measured (lipolysis and fatty-acid release)
Outcomes: plasma glucose tissue uptake, muscle glycogen oxidation, peripheral lipolysis, fatty-acid release into plasma, muscle triglyceride lipolysis
Key Findings (abstract)
Carbohydrate: plasma glucose uptake and muscle glycogen oxidation increased in relation to exercise intensity
Fat: peripheral (adipose) lipolysis was stimulated maximally already at the lowest intensity (25%), and fatty-acid release into plasma decreased with increasing intensity
Muscle triglyceride lipolysis was stimulated only at the higher intensities
Duration: during 2 h at 65% VO₂max, oxidation of plasma-derived substrates progressively increased over time while muscle glycogen and muscle triglyceride oxidation decreased
Recovery: after high-intensity exercise lipolysis fell immediately, yet fatty-acid release into plasma rose — release of fatty acids from previously hydrolysed triglycerides
Conclusion of the authors: carbohydrate availability is regulated directly in relation to intensity; the regulation of lipid metabolism is more complex
Not from this paper: the "crossover point" intensity and the FatMax rate are not reported in this abstract. FatMax in trained men is measured in Achten & Jeukendrup 2003 REF-451 (0.52 g/min at 62.5% VO₂max, n=55)
Mechanism (as the abstract frames it)
Fat supply to muscle at high intensity is limited at the level of fatty-acid release/delivery, not adipose lipolysis (lipolysis is already maximal at low intensity)
Carbohydrate oxidation scales directly with intensity
Inference, not tested: the intramuscular pathway steps (glycogen phosphorylase, AMPK) named in older versions of this entry are not in the abstract
Practical Application
In 5 trained subjects studied with stable-isotope tracers at 25, 65 and 85% VO₂max, plasma glucose uptake and muscle glycogen oxidation rose with intensity, while adipose lipolysis was already maximal at 25% and fatty-acid release into plasma fell as intensity rose; over 2 h at 65% the fuel mix shifted away from muscle glycogen and triglyceride toward plasma-derived substrates. Meaning: carbohydrate use tracks intensity directly, fat use does not — the tracer basis of the fat/carbohydrate crossover picture (n=5, no performance outcome).
Low-to-moderate intensity is where fat supplies most of the energy; the exact "FatMax" intensity must be measured per athlete (REF-451)
Not from this paper; the base's own practice: long Z2 rides and train-low sessions (REF-157) are placed in this low-to-moderate range to load the fat-oxidation pathway
Sub-threshold race pacing keeps fat oxidation contributing and spares glycogen — inference from the intensity pattern above, not a race outcome tested here
This work + Achten & Jeukendrup 2003 REF-451 = the base's FatMax framework
Limitations
n=5 trained subjects — small sample, men only (full text)
1993 methodology — modern indirect calorimetry more refined
No female cohort
Single fitness level (trained)
Steady-state intensities only — interval/variable not addressed
Stable isotope tracers complex to interpret at high intensities
How this entry was checked and changed
Why this level
regraded 2026-09-27 from A: within-subject mechanistic study, n=5, no performance outcome
Content correction 2026-09-27
"VO₂max ~70 ml/kg/min", "30 min time point", "crossover at ~50-55% VO₂max", "fat oxidation peaks at ~65% (~0.7 g/min)", "FatMax ~60-65%", "Z2 (60-70% VO₂max)" -> not in the abstract; removed or relabelled (FatMax now points to REF-451). Qualitative substrate table replaced by the abstract's own statements (muscle triglyceride lipolysis stimulated only at higher intensities; lipolysis maximal at 25%).
Level A -> B (within-subject tracer study, n=5, no performance outcome); added Tier/Sport lines. "gluten-sparing" typo -> glycogen-sparing.
Cross-reference descriptions checked against the cited entries' current content; wrong years, wrong entries and claims the cited entry does not make were corrected.
2026-10-01: cross-reference repointed from a merged framework to the one that replaced it (SYNTH-01→31, 05→34, 09→70, 12/15→32, 19→26).