What if it is hot on race day?
Above 25 °C, fluid needs rise to 800–1,000 mL/h and sodium becomes limiting before carbohydrate. It is the one parameter of the plan to revise upwards in the days before the race, once the forecast firms up.
Sweating increases to shed heat, and it carries sodium with it — between 300 and 1,500 mg per litre depending on the individual, a spread wide enough to explain why two runners of the same level do not react alike. Replacing volume without replacing salt dilutes blood sodium: that is the mechanism of hyponatraemia, whose risk becomes real beyond a litre an hour without sodium, especially on efforts over twelve hours.
A second, less known effect: heat slows gastric emptying. Fatty and fibrous products, already slow to digest, then go down very badly. This is the moment to simplify — liquid and simple sugars — rather than introduce rich solid food, even food that sat well in cool-weather training.
On a real race: Mont-Blanc — UTMB®
On Mont-Blanc — UTMB®, a runner with an Index of 600 is projected at 37h10 and drinks 26 L in mild conditions. Above 25 °C, at 900 mL/h instead of 700, the same runner would approach 30 litres — and the sodium requirement would rise in proportion. That is not a marginal adjustment: it is a different plan.
| UTMB® Index | Estimated time | Carbs | Fluid | Sodium |
|---|---|---|---|---|
| 500 | 43h01 | 2,581 g | 30.1 L | 21,508 mg |
| 600 | 37h10 | 2,230 g | 26 L | 18,583 mg |
| 800 | 24h36 | 1,476 g | 17.2 L | 12,300 mg |
Temps repris des passages réels des coureurs de chaque tranche de cote sur les éditions précédentes, arrêts aux ravitaillements compris. Besoins calculés à 60 g/h de glucides, 700 mL/h de liquide et 500 mg/h de sodium, par temps tempéré.
In practice: watch the forecast from five days out, revise the fluid and sodium targets rather than the carbohydrate, and check that the aid stations let you carry that volume. On long legs it sometimes takes two extra flasks — which changes the pack, so the weight, so the pace. The whole plan is recalculated.
RAVIT applies this calculation to 2,778 races across 49 countries, with 6,547 aid stations whose contents are taken from the official rulebooks.