How much water should you drink when you exercise in the heat?
At 35 degrees the bottle empties fast and the advice pulls in every direction. Here is what health authorities say about drinking enough, not too much, and spotting the signs that should stop you.
The essentials
- There is no single figure: drink more than usual, and do not wait until you are thirsty.
- In very hot weather, WHO advises a cup of water an hour and at least two to three litres daily.
- Dizziness, nausea or cramps: stop, get somewhere cool and drink.
Is there one number to remember?
Not one, but two useful guides. Outside hot spells, the UK’s healthy eating guide suggests six to eight glasses of fluid a day, and more when it is hot or when you are active for long periods. During very hot weather, the World Health Organization recommends drinking water regularly: one cup an hour and at least two to three litres a day.
Your real needs shift with the temperature, the length of the session and how much you sweat. The simplest check is the colour of your urine, which the NHS suggests as a guide: pale yellow means you are drinking enough. If it darkens, drink more.
When should I drink: before, during or after?
All three, and without waiting for thirst. That is how the US Centers for Disease Control and Prevention puts it for people exercising on hot days: drink more water than usual, and do not wait until you are thirsty to drink more.
- Before: turn up already hydrated by drinking through the day.
- During: a few sips at every break.
- After: keep going at your own pace until your urine runs clear again.
Drinking too much is possible as well. The International Olympic Committee reminds athletes to replace what sweat takes away, and not to drink beyond that. The point is to drink steadily, not to force it.
Is water enough, or do I need something else?
Water is enough for most sessions. Other low-sugar drinks count too: coconut water with no added sugar, a lightly sweetened bissap (hibiscus drink), home-made lemonade or milk. None of them is essential.
Salt matters when you sweat heavily for a long time. The International Olympic Committee recommends including it when sweat losses are high, especially if exercise lasts more than about two hours. For a shorter session, water while you train and then a meal salted as usual replace the water and salt you lost.
The NHS does mention isotonic sports drinks and rehydration powders mixed with water, for one specific case: helping someone with heat exhaustion replace the salt lost through sweating.
Which signs should make me stop?
Feeling faint or weak: stop everything and get to a cool place. That is the instruction from the US Centers for Disease Control and Prevention, which adds that muscle cramps may be an early sign of heat-related illness.
The NHS describes heat exhaustion as tiredness, dizziness, headache, nausea, heavy sweating, cramps and intense thirst. Move into the shade, take off unnecessary clothing, drink water and cool the skin. You should feel better within 30 minutes. If not, or if there is confusion, hot skin without sweat, fast breathing or loss of consciousness, call the emergency services.
The best protection is the timetable: train early in the morning or late in the day, start gently, and wear loose, light-coloured clothing.
Heat: the right habits
Do
- Train early in the morning or late in the day, away from the hottest hours.
- Fill your bottle before you leave and sip at every break.
- Start gently and wear loose, light-coloured clothes.
- Train in pairs and keep an eye on each other.
Avoid
- Waiting for thirst before you drink.
- Hard exercise between 11am and 3pm.
- Carrying on through dizziness, nausea or cramps.
When to seek advice
Call the emergency services at once if someone is still unwell 30 minutes after moving somewhere cool, becomes confused or loses consciousness. Talk to your doctor before training in very hot weather if you are pregnant or live with diabetes, or a heart or kidney condition.
General guidance for healthy adults. It does not replace advice from a health professional who knows your situation.