Hydration

On most runs, drinking to thirst is enough; for long or hot sessions, plan your drinks so you lose no more than about 2 % of your body weight.

• Long runs: usually 400 to 800 ml per hour, depending on heat, body size and pace.
• Sweat rate: weigh yourself before and after a run; each kg lost is about 1 litre of sweat, plus what you drank.
• Beyond 2 hours, or with heavy, salty sweat, add sodium: about 300 to 600 mg per hour (sports drink, electrolyte tablets).
• Dehydration signs: dark urine, headache, dizziness, a heart rate drifting upwards.
• Don’t drink so much that you gain weight: too much water dilutes blood sodium (hyponatraemia). Confusion, vomiting or fainting need urgent medical help.