Ultra Marathon Nutrition Calculator

The gut can only absorb carbohydrate at a limited rate during sustained exercise, typically around 60-90 grams per hour for most athletes, so trying to replace calories burned one-for-one would exceed that absorption capacity and often causes nausea or GI distress instead of delivering more usable energy. The 40%-of-expenditure target (capped at 350 calories per hour) reflects that practical absorption ceiling, not a nutritional recommendation to under-fuel.

Higher temperatures drive up sweat rate substantially, and sweat carries both water and sodium out of the body, so hot-weather racing requires meaningfully more of both to avoid dehydration and low blood sodium (hyponatremia). The calculator scales both fluid and sodium targets upward as temperature climbs above a moderate baseline, reflecting this well-established heat-stress relationship.

No — these are reasonable population-level starting targets, but individual tolerance for carbohydrate intake, fluid volume, and sodium varies significantly from person to person. The standard guidance in ultrarunning is to practice your fueling plan extensively during long training runs well before race day, since testing a new nutrition strategy for the first time during an actual race is a common cause of race-day GI problems.

Caffeine is commonly used in endurance sports because it can reduce perceived effort and improve alertness during long events, with a widely cited effective range around 3-6 milligrams per kilogram of body weight spread strategically across the race rather than taken all at once. The calculator's total target scales with body weight and divides it into doses spaced roughly every three hours, since taking it all early would burn through its benefit long before the race is over.

Higher effort increases calorie burn per hour and accelerates fluid and electrolyte loss through heavier breathing and sweating, so the calculator scales calorie expenditure, fluid needs, and electrolyte needs all upward together as effort level rises from easy to hard. Choosing the effort level that honestly matches your expected race-day pace and terrain gives a more accurate fueling plan than defaulting to moderate.