Estimate fuel for trail running.
Trail running fuelling is governed by carbohydrate absorption rate rather than by total calorie need, because fat oxidation covers a large share of energy at endurance effort while glycogen is the limiting store. Intake targets are therefore expressed per hour: around 40 g at easy effort, 60 at steady, and up to 80 or more at race pace where carbohydrate oxidation dominates. Runs beyond about three hours justify pushing toward the upper end, and the gut can be trained to tolerate it. Climbing is reported separately because it adds real energy cost — lifting body mass through 900 metres is several hundred calories — but it does not raise the absorption ceiling.
Carbohydrate target
g per hour = base rate x duration uplift
Climbing energy
kcal = weight kg x 9.81 x gain m / 1000 / (4.184 x 0.23)
General guidance for recreational endurance activity, not nutritional or medical advice. Individual tolerance varies substantially and fuelling strategies should be tested in training, never introduced on race day. Consult a qualified sports dietitian or your doctor before adopting a fuelling plan, particularly if you have diabetes, a gastrointestinal condition or any metabolic disorder.
Because the constraint is absorption, not appetite. The gut can only move a limited amount of carbohydrate into the bloodstream per hour, and exceeding it causes nausea and cramping rather than more energy. Fat covers the remaining energy demand from stores you already carry.
Only with training and the right mix. Glucose transport saturates near 60 g an hour, but adding fructose uses a separate transporter and lifts the ceiling to roughly 90. Practise it in training — attempting a high intake for the first time on race day commonly causes stomach problems.