Fuel Lab.
Your race has a tank.
Three acts, every race: you start with a full tank. The race drains it faster than you think — the bonk isn't bad luck, it's a point in time. And your plan keeps the tank topped up, down to spoons, bottles and cents. This page explains the engine once, properly; the guides behind it make every chapter practical.
When does your tank run dry?
You don't race with your whole body, you race with your legs — and their store is smaller than the arithmetic suggests. Two switches, two effects: loading raises where you start, eating keeps blood sugar up.
The curve is your store. Once it slips below the dashed mark, your snap fades — not all at once, but steadily: the deeper it goes, the duller the attacks and the sprint. Steady pace holds longest. Eating more on the bike won't fix this one. Starting full will. From 2:30 h you crack: that's when even steady pace goes. Which is exactly why the model stops there — anything past the dashed line is not a prediction.
How your engine works
Two stores. Fat is the huge, slow burner — it powers easy riding for days. Glycogen is the small, fast race tank: a few hundred grams in your legs plus a small reserve that holds your blood sugar, and it's the only one that matters once racing gets hard. Keep the two failure modes apart: empty legs and low blood sugar are different breakdowns — empty legs leave you cooked (no snap, steady pace survives), falling blood sugar is the wall.
The mix tips with intensity. The harder you ride, the bigger glycogen's share of every watt — past your CP it's nearly all race tank, and every spike above it burns a multiple. That's why a surgy crit costs far more than its average watts suggest.
Two gut gates. Glucose alone caps near 60 g per hour. Pair it with fructose (2:1) and the second gate opens — 90+ g/h becomes possible, trained guts reach 120. Absorption is trainable — over weeks, not on race day.
Before: the tank fills over days
Carb-loading isn't a mountain of pasta the night before — it's 8–10 g of carbohydrate per kilo of body weight across one or two days. At 75 kg that's 600–750 g a day: a nutrition project, not a dinner. The payoff is the strongest lever there is: nothing you eat mid-race fixes empty legs — only starting full does.
Carboloading
Grams per kilo, how many days, what goes on the plate — and why gaining a kilo on the scales means it worked.
Read the loading guide →During: eat by the clock, drink by measurement
Mid-race it's all about rate: grams per hour, every 15–20 minutes, by the clock — hunger and thirst always report too late in a race. The mix is no secret science: maltodextrin and fructose 2:1, a pinch of salt, plain water alongside. And how much you need to drink isn't a universal number — it's measurable.
DIY race mix & gel
The pharmacy recipe: the same molecules as the branded sachets, spoon measures, cents instead of dollars per serving.
To the recipe →Sweat rate
Weigh twice, stop guessing — your personal bottle plan from a single measurement, indoors measured separately.
To the protocol →When it goes wrong: the wall
The bonk is the most predictable failure in endurance sport — tank arithmetic on schedule. It announces itself 15–20 minutes ahead, it has a roadside fix (40–60 g of fast carbs, ease off, be patient), and its cause almost always predates the start.
Bonking
The machinery, the warning lights, the roadside fix — and the plan that stops it from happening again.
Read the guide →After: the open window
If the next hard session is inside 24 hours, carbs plus protein (~0.3 g per kg) in the first hour refill fastest — and from your mid-forties the timing stops being optional. Otherwise: eat properly, and one well-salted meal puts the minerals back too.
Trends, honestly sorted
90–120 g/h high-carb: the one trend with real evidence — provided the gut is trained for it. · Lactate/bicarbonate: young evidence, or proven only for short all-out formats, with real stomach risk. · Ketones: expensive, and the meta-analyses see no performance effect.
Sport, not medicine: ranges, not prescriptions. All facts from the sources listed on each guide — no AI-generated numbers. Zwift is a trademark of Zwift, Inc.; this is an independent project with no affiliation to Zwift.