Fasted Half Marathon — Endurance Under Constraint
Executive Abstract — Performance Under Constraint
On September 21, 2025, I executed the Colmar Half Marathon in 1:31:54 fully fasted, without carbohydrate intake before or during the race. This was not a demonstration of deprivation; it was a controlled audit of metabolic alignment. The objective was to test whether sub-threshold pacing, supported by structured aerobic development, disciplined threshold positioning, hydration and electrolyte stability, and recovery elasticity, could sustain competitive half-marathon performance without mid-race fuel redundancy.
The result was stable perceived effort, no glycogen collapse, and structured post-race recovery. This field execution demonstrates that when intensity aligns with substrate availability, and volatility is minimized through system architecture, performance can remain durable under executive life constraint. The lesson is not anti-carbohydrate. It is pro-alignment.
A Physiological Field Report from Colmar (1:31:02, Fully Fasted)
On September 21, 2025, I ran the Colmar Half Marathon in 1:31:02.
Fully fasted.
No breakfast. No gels. No mid-race carbohydrate intake.
That sentence tends to trigger immediate conclusions. Some assume metabolic superiority. Others assume recklessness. Both interpretations miss the only question that matters:
Under what physiological conditions can a half marathon be executed fully fasted without performance collapse?
Colmar was not a stunt. It was an audit of a system.
The foundation was built inside the Applied System — protein consistency, structured timing, recovery elasticity, and disciplined aerobic development. It rested on the baseline established in the Metabolic Reset Protocol and echoed the structured transitions explored in the 48-Hour Fast — Fuel Transition Analysis.
The race simply asked: does the system hold under load?
To answer that honestly, we need to move beyond ideology and into mechanics.
The Substrate Curve: The Invisible Architecture of Endurance
Endurance performance is not powered by a single fuel. It is powered by a blend, and that blend shifts continuously with intensity.
Imagine a graph with intensity on the horizontal axis and fuel contribution on the vertical axis. At very low intensities, fat oxidation dominates. As pace increases, carbohydrate contribution rises gradually. Near threshold intensities, carbohydrate reliance increases sharply. Above that, it becomes dominant.
The curve is nonlinear. That is the key.
A modest increase in pace near threshold does not produce a modest increase in carbohydrate use. It produces a disproportionately large one.
That is why fasted racing is not a question of bravery. It is a question of positioning. If you operate in the region where carbohydrate demand becomes extreme, glycogen depletion becomes the limiting factor. If you operate slightly below that region, fat oxidation continues to contribute meaningfully, and glycogen use remains within manageable bounds.
The margin between those two positions can be as small as 5–10 seconds per kilometer.
Colmar was won in that margin.
Lactate Threshold Mechanics: Respecting the Curve
Lactate threshold is often described as a line. In reality, it is a bend in the curve.
At lower intensities, lactate production and clearance remain balanced. As intensity rises, lactate begins to accumulate more rapidly. Near the second lactate threshold (LT2), the slope steepens sharply.
The danger is not crossing some mystical line. The danger is operating close enough to the bend that small fluctuations push you into a metabolically expensive zone.
In a fueled race, carbohydrate intake can buffer that cost. In a fasted race, there is no buffer. That means pace selection must be deliberate.
During Colmar, the early kilometers were intentionally conservative. Breathing stayed controlled. Effort felt restrained. That restraint was not caution — it was strategic preservation of substrate alignment.
Fasted racing narrows the safe pacing corridor. Discipline replaces redundancy.
Glycogen Modeling: A Feasibility Calculation
Now to the arithmetic.
Running costs roughly one kilocalorie per kilogram per kilometer. For a 75 kg runner, a half marathon costs approximately:
75 × 21.1 ≈ 1,583 kcal.
If we assume that 60% of that energy comes from carbohydrate at a controlled sub-threshold intensity:
1,583 × 0.60 ≈ 950 kcal from carbohydrate.
At four kilocalories per gram, that equals approximately 238 grams of glycogen.
A trained runner often stores 400–500 grams of muscle glycogen when properly fueled in the days leading up to a race. Liver glycogen is reduced after an overnight fast, but muscle glycogen remains largely intact unless deliberately depleted.
This simplified model demonstrates something important: a well-paced half marathon does not necessarily exhaust glycogen stores, even in a fasted state.
Contrast that with a marathon at similar intensity and the math shifts dramatically. The margin disappears quickly. That is why fasted half marathons are metabolically plausible under discipline, while fasted marathons at aggressive pace are far less forgiving.
The math does not glorify fasting. It defines boundaries.
RER: The Laboratory Compass
VO₂max testing produces impressive numbers, but the most actionable metric for fasted racing is often Respiratory Exchange Ratio (RER).
RER provides insight into substrate use at specific intensities. Lower values correspond to greater relative fat contribution. Higher values reflect increased carbohydrate reliance. (pmc.ncbi.nlm.nih.gov)
The important variable is not maximal RER. It is RER at race-intended pace.
If RER climbs meaningfully at a pace only slightly faster than target, that is a warning sign: carbohydrate reliance is increasing. In a fueled race, that may be acceptable. In a fasted race, it reduces margin.
RER becomes a pacing compass.
It translates laboratory data into race-day discipline.
VO₂max and Threshold: Ceiling vs Sustainability
VO₂max defines capacity. Threshold defines sustainability.
Two athletes may share identical VO₂max values but differ significantly in the percentage of VO₂max they can sustain at LT2. The athlete with a higher sustainable percentage can run faster without crossing into the steep region of the substrate curve.
In fasted racing, that distinction matters. A higher threshold relative to VO₂max widens the safe sub-threshold window.
This is why aerobic base development and structured threshold work matter. They do not eliminate carbohydrate reliance. They reposition the curve.
Race Morning: Structure Over Spectacle
Race day began without carbohydrate intake, but not without structure.
Hydration was prioritized. Electrolytes were taken to support fluid balance and neuromuscular stability. During the fasting window, I used Unicity Unimate as my consistent morning anchor — not as a metabolic shortcut, but as a behavioral stabilizer.
Consistency reduces volatility. Volatility magnifies error.
There were no gels in my pocket. No mid-race redundancy. That meant the only lever available was pacing discipline.
The Three Phases of the Race
The race unfolded in three clear phases.
Phase One (0–5K): Discipline.
Adrenaline is high, and the temptation to drift is real. The goal was not to feel powerful; it was to lock into the correct metabolic lane.
Phase Two (5K–15K): Audit.
This is where glycogen limitation would reveal itself if intensity were misaligned. Instead, effort remained steady. No surge, no crash. Just controlled output.
Phase Three (Final 6K): Permission.
Only after confirming stability was pace allowed to rise slightly. The finishing time — 1:31:54 — was not a product of aggression. It was a product of alignment.
Conceptual Diagram: Pacing vs Glycogen Depletion
Visualize two curves.
Curve A represents disciplined sub-threshold pacing. Glycogen depletion rises steadily but remains below critical levels through the finish.
Curve B represents threshold-adjacent pacing. The slope steepens mid-race, glycogen use accelerates, and depletion approaches critical territory before the end.
The difference between these curves can be measured in seconds per kilometer.
That is the fragility of fasted endurance.
Fueled vs Fasted: A Clear Distinction
Fueling guidelines for endurance performance are well established and supported. (pmc.ncbi.nlm.nih.gov)
A fueled half marathon offers:
- Replenished liver glycogen
- Exogenous carbohydrate intake
- A wider pacing corridor
A fasted half marathon removes redundancy.
That removal does not create superiority. It creates constraint.
Constraint demands discipline.
Recovery Elasticity: The Final Test
The finish line is not proof of wisdom. Recovery is.
Post-race refeeding was structured. Protein anchored the first meal. Fiber stabilization preceded intake to reduce volatility. Hydration was maintained. Sleep was protected.
The system absorbed stress without swinging.
That is recovery elasticity.
Performance Under Constraint — Executive Framing
Objective: Execute a sub-threshold half marathon fully fasted without collapse.
Controlled variables:
- Aerobic base
- LT2 positioning
- Hydration and electrolyte stability
- Structured fasting routine (including Unimate)
- Pacing discipline
Outcome:
1:31:54
No bonk
Stable perceived effort
Structured recovery
Conclusion:
When intensity aligns with substrate availability and recovery elasticity is preserved, performance can survive constraint without reliance on mid-race carbohydrate intake.
Final Reflection
Colmar was not about proving carbohydrates unnecessary.
It was about proving that discipline can substitute for redundancy.
It was not about pushing harder.
It was about understanding the curve — and staying on the correct side of it.
Intensity creates headlines.
Structure creates durability.
That is what a fasted half marathon done correctly actually demonstrates.
Références
- Carbohydrate intake during endurance exercise: current recommendations. (pmc.ncbi.nlm.nih.gov
)
- RER and substrate utilization in exercise physiology. (pmc.ncbi.nlm.nih.gov
)
- Carbohydrates for training and competition (review). (pubmed.ncbi.nlm.nih.gov
)
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