The Best Ways to Stay Hydrated in the Heat

Your body keeps its core temperature steady by sweating. As sweat evaporates off your skin, it carries heat away. That system works well, but it costs fluid, and in hot conditions the cost climbs sharply. During hard exercise in the heat, sweat rates of 0.5 to 2 litres per hour are normal, and some people well beyond that. A casual habit of a few sips here and there simply cannot match that rate.

This matters because dehydration is not just about feeling thirsty. Once you lose more than about 2 percent of your body weight as fluid, endurance, strength, and mental focus all start to slip. Push further, past roughly 3 percent, and you become more vulnerable to heat illness. For a 75 kilogram person, 2 percent is only about 1.5 litres, which is easy to lose in a single hot hour.

The single most useful thing you can do is stop guessing and measure once. Sweat rates vary enormously between people and conditions, so a friend's plan may be far too much or too little for you. One weigh-in session tells you your real number.

1. Weigh yourself before. Empty your bladder, strip to minimal dry clothing, and note your weight just before a typical hot session.
2. Track what you drink. Note how much fluid you take in during the session, in litres or millilitres.
3. Weigh yourself after. Dry off, remove sweat-soaked clothing, and weigh again straight after finishing.
4. Do the sum. Weight lost in kilograms plus fluid drunk in litres equals your sweat loss. Divide by the hours you exercised to get your hourly rate. Every 1 kilogram lost is roughly 1 litre of sweat.

Once you know, say, that you lose about 1.2 litres an hour on a hot run, you can plan to replace most of that during and after, instead of finishing badly behind. You do not need to replace every drop during the activity, but aim to limit losses to under that 2 percent mark and top up afterwards.

For short and moderate activity, plain water is genuinely fine and is what most people need most of the time. The case for adding electrolytes, mainly sodium, grows when you sweat heavily for longer than an hour, because sweat contains salt and replacing only water can leave your sodium diluted. You do not need expensive products for this. Salty food, a pinch of salt in a drink, or a standard electrolyte tablet all do the job.

Food and drink both count toward your intake, and this is true whatever your diet. Fruit, salads, soups, dairy or fortified plant milks, beans, and cooked grains all carry water and, in many cases, useful minerals. A bowl of lentil soup or a smoothie made with soy milk hydrates you just as a meat-based meal would. There is no hydration advantage to any particular food group here.

You do not need a spreadsheet. A workable approach is to arrive already topped up, drink to thirst with a rough target during activity, and rebuild afterwards. For everyday life in the heat, thirst plus urine colour is a reliable guide. For long or hard efforts, lean on the sweat-rate number you measured.

1. Before: drink a normal glass or two of fluid in the hours beforehand so you start well hydrated, not scrambling to catch up.
2. During: for efforts over an hour, sip regularly toward replacing a good share of your sweat losses, adding sodium if you are sweating heavily.
3. After: replace remaining losses over the next few hours. A useful target is around 1.25 to 1.5 litres for every kilogram of weight lost, because you keep losing some through urine.
4. Check: pale straw-coloured urine means you are roughly on track. Dark and scant means top up.

In the heat, the early signs of dehydration are easy to miss because they overlap with just feeling hot and tired. Learn to spot them so you act before it becomes serious.

• Thirst, a dry mouth, and dark, infrequent urine.
• A headache, dizziness, or feeling unusually flat and heavy-legged.
• Poor concentration and a sense that the effort is much harder than it should be.
• In hot conditions, watch also for cramping, nausea, or clammy skin, which can signal heat illness.