Trail Nutrition Calculator

Trail race nutrition planning means calculating the per-hour targets for calories, carbohydrates, fluids, and sodium your body needs to sustain effort across a multi-hour event. Unlike a road marathon where you take a gel every 45 minutes and call it done, trail racing demands a detailed fueling strategy because effort varies enormously between climbing, descending, and flat sections - and aid station gaps can exceed two hours.

This calculator outputs personalised per-hour targets based on your body weight, race duration, intensity level, temperature, and altitude. It also generates a 2-hour aid station schedule you can use to plan exactly what to carry between checkpoints.

These are general guidelines - consult a sports dietitian for personalised race nutrition advice.

1. Enter your body weight Calorie and sodium targets scale directly with body weight - heavier runners burn more and lose more sodium in sweat.
2. Set your race duration Enter your goal finish time. The calculator uses this to compute total targets and break them into per-hour figures.
3. Choose intensity level For most ultras, select moderate. Hard applies to shorter faster races (under 6-7 hours). Easy applies to very long efforts where you hike large sections.
4. Set environmental conditions Adjust temperature and altitude. Both significantly affect fluid needs and calorie burn. UTMB runners should set altitude to high; Templiers runners should account for Mediterranean heat.
5. Read your per-hour targets The output shows calories, carbohydrates, fluids, and sodium per hour, plus total race targets and a 2-hour aid station schedule.

Calorie targets are derived from MET (Metabolic Equivalent of Task) values for running, scaled to body weight: kcal/hr = weight x MET x 0.0175 x 60. Trail running at race effort uses a MET of approximately 14 (Burke, 2001). Altitude increases metabolic cost by 5-12% above 1500m due to increased ventilation and reduced mechanical efficiency.

Carbohydrate absorption is limited by gut transport capacity. Jeukendrup & Moseley (2010) showed that glucose alone saturates the SGLT1 intestinal transporter at ~60g/hr. Combining glucose + fructose (roughly 2:1, as in most modern trail gels) adds the GLUT5 pathway, raising the ceiling to 80-90g/hr. More recent work (Podlogar et al., 2022) suggests gut-trained athletes using a 1:0.8 glucose:fructose ratio can absorb up to 90-120g/hr after 3-6 weeks of dedicated gut training - treat 90g/hr as a starting ceiling, not an absolute limit. Exceeding absorptive capacity without gut training causes osmotic distress - water is drawn into the intestine, resulting in cramping and nausea. The targets in this calculator respect the 80-90g/hr range for untrained guts.

Fluid and sodium targets follow Maughan & Shirreffs (2010) sweat rate research. Sweat loss increases with temperature (roughly +200-600ml/hr across the cool-to-very-hot range) and with altitude due to respiratory water loss. Sodium replacement prevents hyponatremia - Hew-Butler et al. (2005) documented hyponatremia in 13% of Comrades ultramarathon finishers, with slower runners at highest risk from prolonged sweat-rate-exceeding plain water intake. The calculator targets 400-1000mg/hr of sodium depending on conditions, derived from sweat volume. Importantly, sweat sodium concentration varies 5-10x between individuals (Baker et al., 2016) - far more than sweat volume alone. If you see white salt deposits on clothing or skin after long runs, your personal sodium requirement may be significantly above these estimates. A sweat composition test or supervised salt titration is more reliable than any formula.

Caffeine is one of the most evidence-backed ergogenic aids for endurance events (Spriet, 2014; Graham, 2001). Optimal dose is 2-3mg/kg, capped at 400mg - above this, side effects (anxiety, GI distress, cardiac arrhythmia) can outweigh benefits. The calculator suggests timing caffeine at approximately 50-60% into the event when fatigue compounds, and restricts intake in the final 3-4 hours to protect sleep quality during multi-day events.

Fat vs carbohydrate oxidation shifts with intensity. At easy pace, fat supplies 40-60% of energy. At threshold, carbohydrate dominates at 80-90%. Trail racing involves both extremes within a single race - which is why carbohydrate intake cannot be eliminated even on easy segments. Fat oxidation is too slow to fully cover a hard climb.

Jeukendrup AE, Moseley L. (2010). Multiple transportable carbohydrates enhance gastric emptying and fluid delivery. Scand J Med Sci Sports. Maughan RJ, Shirreffs SM. (2010). Dehydration and rehydration in sport. Scand J Med Sci Sports. Hew-Butler T et al. (2005). Consensus Statement: 1st International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med. Graham TE. (2001). Caffeine and exercise. Sports Med. 31(11):785-807. Spriet LL. (2014). Exercise and sport performance with low doses of caffeine. Sports Med. 44(S2):175-184. Burke LM. (2001). Energy needs of athletes. Can J Appl Physiol. 26(S):S202-S219.

UTMB (171km, 10,000m D+, ~20-46 hours). A 75kg runner finishing in 30 hours needs approximately 12,000-13,000 kcal total - around 400-450 kcal/hr. The critical section is the night traverse from Les Chapieux through Italy. Altitude at Col de la Seigne (2516m) suppresses appetite exactly when you most need fuel for the long descent to Courmayeur. Strategy: eat proactively on the climb before appetite fails. At Courmayeur (79km, major crew point), stock up on savoury food - broth, soup, rice - for the long overnight section. Courmayeur to Champex-Lac (125km) is 10+ hours for mid-pack runners with no crew access except La Fouly.

Lavaredo Ultra Trail (120km, 5,850m D+, ~13-30 hours). A warmer race in the Dolomites compared to UTMB. Temperatures in late June can reach 25-28°C in valley sections, pushing fluid needs to 700-900ml/hr. Aid stations are more frequent (roughly every 10-15km), reducing the need to carry large quantities. Focus on the Sesto to Dobbiaco section (15km, hot valley) - this is where dehydration typically sets in. Plan for higher sodium intake across this segment.

Templiers (80.7km, 3,443m D+, ~7-15 hours). A faster race in the Tarn and Dourbie gorges around Millau, southern France. The shorter duration means less total calorie volume but a higher per-hour carbohydrate demand at race pace. Fluid needs are high - Aveyron in late October can still see 20°C afternoons. Because most finishers complete within 15 hours, gut fatigue is less of an issue and gels remain viable throughout, but carrying savoury backup from the Saint-Andre-de-Vezines aid station (36.3km) is still recommended.