Everyone says 60 to 90 grams per hour. For a 95-kilo rider at 2.3 W/kg, the right number is 63. For a 70-kilo rider at 4.0 W/kg, it’s 75. Same race, same bracket, different bodies. Which number is yours?
A bike has no motor. You are the motor. You convert chemical energy — food stored in your body — into mechanical energy at the pedals. The rate of that conversion is power, measured in watts.
How fast the bike moves depends on what those watts must overcome: gravity, air resistance, rolling resistance. Control for those, and more watts = faster. Period.
So the first question isn’t what should I eat? It’s: how much energy does my body need to produce, and where does that energy come from?
Your body runs on two fuels: fat and carbohydrate. Fat is an enormous tank — even a lean rider carries 30,000–80,000 kcal of it. But fat burns slow and demands extra oxygen. Carbohydrate is a small, fast tank — only 1,600–2,400 kcal of glycogen — but it produces energy quickly under pressure.
At every intensity, your body burns both. But carbs always dominate. Even on the easiest ride, carbohydrate provides 60%+ of your energy. As intensity rises, the carb share grows. At threshold, it’s nearly 100%.
Fat never runs the show. It supplements. The question is how much supplementing it can do — and that depends on your fitness.
The answer is oxygen. Fat oxidation is aerobic — it requires more O2 per calorie than carbs. You can only burn fat when you have oxygen to spare.
A pro’s VO2max is roughly 2× a recreational rider’s (75 vs 40 ml/kg/min). At the same absolute pace, the pro has massive O2 headroom. Their mitochondria can run the slow fat-burning pathway alongside carb burning.
The recreational rider at that same pace is near their oxygen ceiling. Every O2 molecule is spoken for. The body triages: all oxygen goes to glycolysis (fast ATP from carbs). Fat oxidation is shut down — not by choice, but by physics.
Four structural differences drive this: heart stroke volume (1.7× more blood per beat), capillary density (2–3× more oxygen delivery routes), mitochondrial density (2–3× more engines per cell), and Type I fiber proportion (more fat-preferring muscle fibers).
Put both riders on the same gravel road at 2.5 W/kg (188W for a 75 kg rider). Same pace. Same watts. Same energy cost.
The pro is in Zone 1 — a recovery ride. Fat covers 35% of the energy bill. Heart rate barely above resting. Could hold this pace all day.
The recreational rider is at threshold. Heart rate 170+. Gasping. Fat oxidation has collapsed to near zero because there is no O2 headroom. ~100% of energy comes from carbs. Every watt is draining glycogen. And they can’t sustain it for more than an hour.
The pro is still in Zone 1 at 3.0 W/kg — that’s active recovery (54% of FTP). The rec rider’s FTP is 2.5 W/kg. The pro’s recovery pace exceeds the rec rider’s maximum sustainable effort.
At 188W, both riders burn 677 kcal/hr (3.6 × watts). But the split is everything.
The pro at 35% fat / 65% carb: 110 g/hr of carbs. The rec rider at ~3% fat / ~97% carb: 164 g/hr. Same watts. 50% more carb burn for the rec rider.
Now introduce the gut ceiling. Your intestines can absorb 60–90 g/hr of carbs untrained, up to 90–120 g/hr with trained dual-transport (glucose + fructose). That ceiling doesn’t care how hard you’re riding. Both riders exceed it at this pace.
But the rec rider can’t even hold 2.5 W/kg. Their actual race pace is Z2: 1.4–1.9 W/kg, burning 60–90 g/hr — manageable, if they eat. The pro cruising at endurance pace (3.0–3.5 W/kg) burns 140–175 g/hr. No gut can keep up.
Two recreational riders, both 2.5 W/kg. One weighs 75 kg, the other 90 kg. Same fitness. Same speed uphill. Same perceived effort.
But the 75 kg rider at 80% FTP pushes 150W and burns 88 g/hr of carbs. The 90 kg rider pushes 180W and burns 106 g/hr. That’s 20% more carbs — with the same gut.
The gut doesn’t scale with body weight. SGLT1 transporter density — the molecular bottleneck for glucose absorption — is set by dietary carbohydrate exposure, not body mass (Jeukendrup, 2014).
W/kg normalizes performance. It does not normalize fueling. The heavier rider climbs at the same speed but bonks first.
Women have genuinely different substrate utilization — not just lower body weight. Estrogen upregulates fat metabolism: women burn ~7 percentage points more fat at the same relative intensity, their crossover point is higher (~58% vs ~50% VO2max), and they use 25–50% less muscle glycogen than matched men.
So why are men faster? The engine is bigger. ~20% higher VO2max per kg lean body mass. Larger hearts. More hemoglobin. More muscle mass producing force. Men are the V8 — more power, worse fuel economy. Women are the hybrid — less power, better efficiency per watt.
In short-course racing, the bigger engine wins. But as races get longer, fuel economy starts mattering more than horsepower. The male fueling deficit compounds every hour. The female efficiency advantage accumulates. This is why the performance gap narrows in ultra-distance — and why some women win 200+ mile races outright.
Speed comes from the engine. The fueling crisis comes from the engine too.
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Murphy’s race is 6.5 hours. That puts him in the 4–8 hour bracket: 60–80 g/hr. The bracket is his floor and ceiling.
Longer races need fewer carbs per hour — not more. As intensity drops, fat oxidation rises and carbohydrate demand falls. A 14-hour Unbound needs less per hour than a 4-hour SBT GRVL.
60–80 is still a 20 g/hr gap. Over a 6.5-hour race, that’s the difference between 390g and 520g of carbs. Your fitness narrows it down.
Murphy’s FTP is 220W at 95 kg — that’s 2.3 W/kg. He lands near the bottom of his 60–80 bracket. His personalized number: 63 g/hr.
A 70 kg rider with 280W FTP — 4.0 W/kg — lands near the top. Same bracket, 12 g/hr apart. His number: 75 g/hr.
The relationship isn’t linear. It steepens at higher W/kg because carbohydrate oxidation accelerates near threshold. The 1.4 exponent captures this: gains are compressed at low W/kg and amplified at high W/kg.
Before the science: here’s what to do.
Go to your race page. Enter your weight, FTP, and expected finish time. That’s your starting number — not a ceiling, not a floor.
The calculator defaults to the bracket midpoint. Conservative — and that’s fine. Undershooting by 10 g/hr is an inconvenience. Overshooting is a porta-potty emergency.
Your stomach is the weakest link in your fueling chain. Gut training increases exogenous carbohydrate oxidation by ~16% over 28 days and reduces GI symptoms by up to 60%. A perfect number on paper is worthless if you’re retching at mile 90. Start 8–10 weeks out.
For your first Leadville or your first Unbound, aim for the lower third of your bracket. GI distress cascades — once it starts, your gut becomes less tolerant, not more.
By hour 10, the thought of another gel is worse than the climb ahead. Rice cakes, PB+J, boiled potatoes — real food keeps your stomach in the game.
SBT GRVL (4–6 hours) gives you 60–80 g/hr. Unbound 200 (10–14 hours) gives you 50–70. Five brackets. Your race picks the floor and ceiling.
A 2.3 W/kg rider lands near the bottom of the bracket. A 4.0 W/kg rider lands near the top. The formula uses your FTP and weight to find your spot.
Your muscles can burn more carbs than your stomach can absorb. Max intake tops out around 90–120 g/hr no matter who you are.
Personalized fueling for 328 races.
Two riders. Both 3.5 W/kg. Both riding Mid South in Oklahoma. One has a diesel engine — slow-twitch dominant, burns fat like a furnace. The other is glycolytic — fast-twitch heavy, tears through carbs like they’re free. The diesel rider’s glycogen lasts 12 hours. The glycolytic rider’s lasts 5.
| VLaMax (mmol/L/s) | Rider Type | CHO at ~65% VO2max | CHO at Threshold |
|---|---|---|---|
| 0.25–0.3 | Diesel — the rider who never bonks | ~60–80 g/hr | ~100–120 g/hr |
| 0.4–0.5 | All-rounder — most gravel racers | ~90–120 g/hr | ~140–170 g/hr |
| 0.6–0.7 | Glycolytic — the rider who surges and suffers | ~120–150 g/hr | ~180–220 g/hr |
The counterintuitive finding: the phenotype effect on total carbohydrate combustion is massive, but the effect on the exogenous fueling recommendation is constrained by the gut. Maximum exogenous absorption caps at ~90–120 g/hr regardless of how fast the muscles burn carbs. The practical effect at gravel-relevant intensity is ~20 g/hr — exactly one bracket width.
Gravel races are not ridden at constant power. You surge up a 15% gravel wall, coast down the other side, and sprint to close a gap. Your average watts might be 180 — but you spent half the race at 250 and the other half at 110. Mathematicians call this Jensen’s Inequality. For fat oxidation, the error is dramatic:
The formula takes three inputs: body weight (kg), FTP (watts), and estimated race duration (hours). It works in two stages. First, your race duration picks a bracket. Then your W/kg positions you within it.
| Duration | Bracket (g/hr) | Basis |
|---|---|---|
| 2–4 hrs | 80–100 | High-intensity race pace |
| 4–8 hrs | 60–80 | Classic endurance |
| 8–12 hrs | 50–70 | Sub-threshold, fat oxidation rising |
| 12–16 hrs | 40–60 | Ultra pace, glycogen depletion |
| 16+ hrs | 30–50 | Survival pace, GI distress >90% |
Inside a metabolic lab, riders pedal on an ergometer while a mask captures every breath. As intensity rises, the body’s fuel mix shifts. At low watts, fat dominates. Cross a threshold — and carbohydrate takes over. Where that crossover happens depends almost entirely on how fit you are.
Notice that even the well-trained rider’s crossover (~5.4 W/kg) is below typical gravel race intensity for most athletes. This is the core insight: at race pace, riders across all fitness levels are carb-dominant. The question is how much carbohydrate they burn — and therefore how much they need to replace.
| Fitness Level | Crossover (W/kg) | Relative |
|---|---|---|
| Well-trained endurance | ~5.4 | |
| Competitive age-group | ~4.4 | |
| Trained masters (40+) | ~3.8 | |
| Recreational | ~3.2 | |
| Minimally trained | ~2.1 |
Ten things this framework does not do, cannot do, or does imperfectly.
Three inputs. Your weight, your FTP, your race. Find your race page and the calculator does the rest.
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