The science behind your fueling plan

The maximum rate at which muscles can oxidize carbohydrate is substrate-limited, not delivery-limited. A single carbohydrate source (glucose or maltodextrin) saturates the SGLT1 intestinal transporter at roughly 60 g/hour. Adding a second source that uses a different transporter — fructose via GLUT5 — raises total exogenous carbohydrate oxidation to 90–120 g/hour when supplied in a 2:1 glucose-to-fructose ratio.

We apply Jeukendrup's carb oxidation model (2014) to calculate the per-hour carbohydrate target for your distance, target pace, and estimated intensity. At intensities below ~65% VO2max, fat oxidation contributes meaningfully and carb needs are lower. At intensities above ~75% VO2max (typical for shorter events or faster athletes), carbohydrate dependency rises sharply.

For events under 75 minutes, our plans recommend ~30–45 g/hour. For 75–150 minutes: 45–60 g/hour. For 150–300 minutes: 60–90 g/hour. For ultra events exceeding 300 minutes: 60–90 g/hour on the bike, reduced to 40–60 g/hour on the run where gut tolerance is typically lower after prolonged exercise.

Sweat rate varies enormously between athletes (0.5–2.5 L/hour) and is influenced by bodyweight, exercise intensity, heat, humidity, and acclimatization status. Dehydration beyond ~2% of bodyweight begins to impair endurance performance; hyponatremia (dangerous overdrinking) is a greater risk in slower athletes and cool conditions.

Where you provide a measured sweat rate (from a pre/post-exercise body mass test), we use it directly. Where you don't, we apply the ACSM fluid needs estimation model adjusted for your bodyweight, estimated intensity, and race-day temperature. We target drinking to thirst for most athletes — replacing 70–80% of sweat losses to allow a manageable fluid deficit without risking overhydration.

Sodium recommendations are based on sweat sodium concentration research (typically 460–1,840 mg/L with high inter-individual variability). We use a conservative mid-range figure and flag salty sweaters (visible salt crust, white streaks on kit) to increase their intake.

Carbohydrate loading — elevating muscle glycogen above resting concentrations — is well established for events exceeding 90 minutes. The protocol we prescribe follows Burke's contemporary model: 10–12 g/kg/day of carbohydrate for 36–48 hours pre-race, with reduced training volume. This reliably increases muscle glycogen by 20–40% compared to habitual diet.

We also include race-morning carbohydrate timing guidance: 1–3 g/kg of easily digestible carbohydrate 1.5–3 hours pre-start. This tops up liver glycogen depleted overnight without requiring stomach-emptying time that many athletes underestimate.

The gastrointestinal system is trainable. Repeated exposure to carbohydrate intake during training increases intestinal SGLT1 transporter expression, improving absorption capacity over 4–6 weeks. Athletes who attempt 90 g/hour on race day without gut training frequently experience bloating, nausea, and diarrhea.

Our gut-training schedule progressively increases carbohydrate intake during long training sessions over 4–6 weeks before race day, starting at ~45 g/hour and building toward race-day targets. We also flag FODMAP-sensitive athletes — for whom lactose, fructose excess, and certain gel ingredients are common GI triggers — with product substitution guidance.

Caffeine is one of the few ergogenic aids with unambiguous evidence across endurance disciplines. At 3–6 mg/kg bodyweight, caffeine reliably reduces perceived exertion and improves time-to-exhaustion and time-trial performance. Timing is critical: plasma caffeine peaks 45–60 minutes post-ingestion and remains elevated for 4–5 hours.

Our plans time caffeine intake to coincide with the anticipated hardest portion of your race — typically the back third of the run — while avoiding pre-race intake that conflicts with habitual patterns (abrupt withdrawal can trigger headaches). Athletes who flag caffeine sensitivity receive caffeine-free alternatives for every gel and chew recommendation.

Beyond individual studies, our methodology adheres to the following position stands and consensus documents: