How much water should runners drink in hot weather in South Africa?

How much do runners sweat?

Sweat rates in athletes are commonly 0.5 to 2 L/h and can pass 3 L/h in hot, humid conditions such as a Durban summer or a Lowveld afternoon. Sweat sodium also varies several-fold (roughly 10 to 90 mmol/L). That is why a generic "drink 250 ml every 15 minutes" rule helps some runners and harms others.

What is the best rule: thirst or a schedule?

The 2015 hyponatraemia consensus supports drinking to thirst. It avoids both significant dehydration and overdrinking. A body-mass loss of about 3% is generally tolerable in cool to temperate conditions, and you should be wary of any weight gain during a run. See what is hyponatraemia.

How do you do a sweat-rate test?

1. Weigh yourself nude (or in minimal, dry clothing) before a 60-minute run at race effort.
2. Run, noting how much you drank.
3. Towel off and weigh again.
4. Sweat loss (litres) = weight lost (kg) + fluid drunk (litres) minus any toilet visits. One kilogram is about one litre.

Worked example: a 70.0 kg runner finishes at 68.8 kg after drinking 0.5 L. Sweat loss is 1.2 + 0.5 = 1.7 L/h. They replaced 0.5 L by thirst and lost 1.2 kg (1.7% of body mass) in an hour. At that pace they would cross 3% (2.1 kg) in under two hours, so a longer hot run needs more fluid, a slower pace, or both.

Repeat the test in hot and cool weather. Your numbers will differ.

What should you drink?

• Under about an hour: water is enough for most people.
• Over 1 to 2 hours, or heavy sweating: a drink with sodium, and carbohydrate if you are fuelling. See do I need electrolytes or just water.
• After the run: replace the deficit over the next few hours with fluid, food and some salt. The 2016 ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada position suggests about 1.25 to 1.5 L per kilogram lost.

Electrolyte sachets cost from about R4.63 (Biogen Electrolyte) to R20 (Rehidrat) each in October 2026; see electrolyte costs in South Africa.

Hot-weather practicalities

• Acclimatise over 10 to 14 days; sweat rate rises and sweat gets less salty.
• Run in the early morning, in shade, with a cap and light clothing.
• Pale yellow urine through the day means you are broadly fine; dark urine means drink more.
• Heat stroke (confusion, collapse, hot dry or very sweaty skin) is an emergency: call 112 or 10177 and cool the person immediately.

Who needs different advice?

People with kidney, heart or blood pressure conditions, or on diuretics, should get medical advice on fluid and sodium targets. Children and teenagers should drink to thirst and eat normally, and pregnant runners should ask their doctor about training in heat.

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