Fueling is a strategy, not a snack. The gut can be trained to absorb carbohydrates, but only within physiogical limits that shift with exercise duration, heat and intensity. The science gives you a ladder; your job is climbing it in training so race day is reflexive.
The carbohydrate ladder
Under ~60 minutes, water is enough — no fuel needed. 1–2.5 hours: 30–60 g/h. Beyond 2.5 hours, the ceiling rises to 60–90 g/h, but only with multiple transportable carbs (glucose + fructose in a 2:1 ratio), because each uses a different intestinal transporter (Jeukendrup 2014). Pushing a single carb type past ~60 g/h just leaves sugar sitting in the gut.
- Train the gut: build up to target intake over weeks, not on race morning.
- Practical: one gel (~25g) every 30 min at the low end; two types of carb above 2.5h.
- Under 75 min — skip it; the cost of GI distress exceeds any marginal glycogen saving.
Sodium and fluid: match the sweat
Sweat rate varies wildly between athletes. The only way to know yours is to weigh before and after a representative session (1 kg lost ≈ 1 L sweat). In heat, sodium needs rise to 400–1000 mg/h and fluid to roughly your measured loss rate — drinking to thirst is a floor, not a plan, on long hot days (Casa 2000, ACSM 2007).
Heat also slows you: the app's temperature model scales pace, trims volume and scales hydration as conditions cross from warm to hot to extreme.
Caffeine as race fuel
Pre-race caffeine (3–6 mg/kg) is the highest-confidence acute boost available. For multi-hour events, trickle it — small doses early and mid-race — rather than one large bolus, and never introduce it on race day if you train without it.
How the app applies this
Every check-in returns a fuel plan — carbs/h, sodium mg/h, fluid ml/h — scaled to today's session duration, intensity and the A-race's expected heat. A heat factor automatically raises your fluid and sodium targets.
Evidence
- › Jeukendrup 2014 — Multiple transportable carbs (Sports Med 44:S25-33)
- › Casa et al. 2000 — Hydration / sweat rate
- › ACSM 2007 — Exercise & fluid replacement
- › Ely et al. 2007 — Heat pacing