Demo Mountain 50K, aid station by aid station.

• Carbohydrate. The joint position stand of the American College of Sports Medicine, the Academy of Nutrition and Dietetics and Dietitians of Canada gives 30–60 g an hour for efforts of one to two and a half hours, and up to 90 g an hour beyond that. Under about 45 minutes nothing is needed.1 One sugar on its own is absorbed at about 60 g an hour at most, so anything above that has to mix glucose and fructose.2
• The ratio. Most gels still advertise 2:1 glucose to fructose. A meta-analysis of 14 studies found the best delivery and performance closer to 1:0.8, and a 2022 review agreed and found the near-equal ratio easier on the gut.3
• 120 g an hour. In a 2020 study, elite runners at a 42 km mountain marathon with 3,980 m of climbing took 60, 90 or 120 g an hour. Of 31 starters, 20 finished, and only finishers were analysed, so six or seven runners per group.4 A trial in 11 cyclists found 120 g was burned faster than 90 but did not spare the body's own glycogen and did not measure performance.5 No study has shown that 120 g an hour is faster than 90. It is a ceiling for trained athletes, not a target.
• Gut training. Four to six weeks, one or two fuelled hard sessions a week, adding about 10 g an hour every one to two weeks, with the products you will race with. This is coaching practice, not a trial result.
• Sweat. Across 506 athletes, sweat rate averaged 1.2 litres an hour but ranged from 0.26 to 5.73. Sweat sodium averaged about 1,000 mg per litre, with values from under 300 to over 2,400.6 That is why the calculator gives a band rather than a number.
• Sweat sodium cannot be predicted. A 2022 analysis using individual, environmental and dietary factors explained only 17–23 % of the variation between people.7 The only way to know yours is to measure it.
• The fluid and sodium bands. 300–500 ml an hour in the cold, 600–800 in the heat, and 300–600 mg of sodium an hour past 90 minutes are the ranges in common coaching use. They are starting points. No consensus statement sets an hourly sodium target, and the 2026 cycling nutrition consensus declines to give one.8
• Drinking too much is the dangerous error. Exercise-associated hyponatremia, where blood sodium is diluted by drinking more than you sweat, has killed endurance athletes. The international consensus recommends drinking to thirst and states that fixed intake ranges are not appropriate.9 Salt tablets do not prevent it. Take salt to replace what you sweat, not as a reason to drink more.
• Why it ignores your watch. A 2022 systematic review of 42 wrist devices from 23 manufacturers found an average error above 30 % for every brand, and from 15 % to 211 % across studies of one model. None was accurate enough for energy expenditure.10 Duration, climb and temperature are rougher inputs, but they are not wrong by a factor of two.

Sources

1. Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise, 48(3), 543–568. doi:10.1249/MSS.0000000000000852 Joint position stand of three professional bodies.
2. Podlogar, T., & Wallis, G. A. (2022). New horizons in carbohydrate research and application for endurance athletes. Sports Medicine, 52(Suppl 1), 5–23. doi:10.1007/s40279-022-01757-1 Review.
3. Rowlands, D. S., Houltham, S., Musa-Veloso, K., Brown, F., Paulionis, L., & Bailey, D. (2015). Fructose–glucose composite carbohydrates and endurance performance: critical review and future perspectives. Sports Medicine, 45(11), 1561–1576. doi:10.1007/s40279-015-0381-0 Meta-analysis of 14 studies.
4. Viribay, A., Arribalzaga, S., Mielgo-Ayuso, J., Castañeda-Babarro, A., Seco-Calvo, J., & Urdampilleta, A. (2020). Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients, 12(5), 1367. doi:10.3390/nu12051367 20 finishers, 6–7 per group.
5. Podlogar, T., Bokal, Š., Cirnski, S., & Wallis, G. A. (2022). Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g h⁻¹ versus 90 g h⁻¹ at different ratios. European Journal of Applied Physiology, 122(11), 2393–2401. doi:10.1007/s00421-022-05019-w 11 trained cyclists.
6. Baker, L. B., Barnes, K. A., Anderson, M. L., Passe, D. H., & Stofan, J. R. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences, 34(4), 358–368. doi:10.1080/02640414.2015.1055291 506 athletes. Whole-body sweating rate was estimated from regional patches, not measured directly.
7. Baker, L. B., De Chavez, P. J. D., Nuccio, R. P., et al. (2022). Exploring the role of individual, environmental and dietary factors in sweat sodium concentration. Journal of Applied Physiology. doi:10.1152/japplphysiol.00391.2022
8. Jeukendrup, A. E., Redegeld, M., Martins, G., et al. (2026). UCI Sports Nutrition Project: Race nutrition for road cycling. International Journal of Sport Nutrition and Exercise Metabolism, 36(3), 215–232. doi:10.1123/ijsnem.2025-0239 Expert consensus.
9. Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303–320. doi:10.1097/JSM.0000000000000221 International consensus statement.
10. Germini, F., Noronha, N., Borg Debono, V., et al. (2022). Accuracy and acceptability of wrist-wearable activity-tracking devices: systematic review of the literature. Journal of Medical Internet Research, 24(1), e30791. doi:10.2196/30791 Systematic review, 42 devices from 23 manufacturers.

Links go to the publishers. Where a paper is behind a paywall, the abstract carries the figure quoted. The note after each source says what kind of evidence it is, so you can weigh it yourself.