Hydration

Why does water matter so much for your body?

Your body is about 60% water, and it needs that water for digesting and absorbing food, removing waste and controlling your internal temperature¹. Water also carries nutrients and waste around the body and acts as a lubricant and shock absorber².

You don't have to manage this by hand. Your body controls both how much water comes in and how much is lost, and tiny changes in the concentration of your blood are the main trigger². In ordinary daily life, your kidneys and thirst do most of the work. During long exercise, sweat loss becomes the main challenge⁵.

There is no fixed amount that suits everyone, because needs change with climate, physical activity and diet². Treat any daily figure as a rough guide, not a target to hit.

Older adults are more prone to dehydration, partly because thirst can be weaker and the kidneys are less able to concentrate urine⁶. Outside extreme cases, the signs of dehydration in adults are subtle and unreliable⁶, so you can't always tell by how you feel.

How much should you drink while exercising?

For light to moderate activity in normal weather, there is no fixed amount to aim for. Most people who exercise for less than 60–90 minutes a day in normal weather are unlikely to become dehydrated⁷, and drinking to thirst may be enough for short, lower-intensity or cooler sessions³.

Plain water is the best choice for most activity⁸. For an easy walk in mild weather, water when you feel thirsty is usually all you need. No set amount per hour is clear yet for people just starting out, so let thirst and comfort guide you on short, easy sessions.

Longer or hotter sessions are different. Sweat loss during exercise generally exceeds the fluid you take in, and the gap is bigger in hot, humid conditions⁹. On those days, take it easier and watch how you feel.

More is not always safer. Drinking more than you lose in sweat, especially plain water over a long session, can cause low blood sodium, which can be life-threatening¹⁰. Never drink so much that your body weight goes up during activity. Early signs include vomiting, swollen hands and feet and confusion, and they can be mistaken for heat illness.

How does dehydration affect performance?

Losing about 2% of your body mass through fluid is the point where endurance, temperature control and the work your heart and circulation do start to suffer³.

Dehydration raises heart rate and body temperature¹¹. It also reduces sweating and cooling, so the body stores more heat³. If it is allowed to progress, it can affect physical and mental performance, cause fainting from low blood pressure, and lead to heat illness that can be fatal⁹.

Keep this in proportion. The 2% figure comes mostly from athletes, often in the heat, so the effect during everyday walking or similar activity is probably smaller. For short, gentle activity in mild weather, dehydration is unlikely to be a problem⁷, so don't let worry about it stop you being active.

Some people are more at risk, especially in hot weather. Causes of dehydration in the community include older age, long-term conditions, taking many medicines, limited access to drinking water and very hot weather⁶.

When do you need electrolytes?

For most people doing everyday activity, you don't. Most people don't exercise enough to need to replace lost electrolytes (salts such as sodium), and sports drinks are not necessary⁴. A healthy person doing moderate physical activity may need no drink other than water⁹.

Electrolytes matter more in long, intense or hot activity. Athletes doing long or intense exercise may need extra carbohydrates, electrolytes or both⁹. Sports drinks may be useful if your activity is moderate to vigorous for more than 60 minutes, but they can be high in sugar, so use them only if necessary⁸.

After exercise, a carbohydrate-electrolyte drink may restore hydration when whole foods are not available. If clean drinking water is available, its lower cost makes it an acceptable alternative⁹. What suits you can vary, because people differ in how fast the stomach empties, how well the gut absorbs fluid, kidney function and sweat rate⁹.

For walks, gentle classes and similar everyday activity, plain water is a good default. If you do choose a sports drink for a long, hot or hard session, check how much sugar it contains.

Is thirst a good guide to how much to drink when you're active?

For short, lower-intensity activity in cooler conditions, mostly yes: drinking to thirst is likely to be enough³. Thirst works well at rest but is less sensitive during exercise, and when sweat rates are very high, people drinking as they wish tend to under-drink³. Thirst also tends to ease before you have fully rehydrated³.

Age may matter. Older adults can have a weaker thirst response⁶, though it isn't clear that age changes thirst for everyone¹². Because that isn't settled, if you are older, don't rely on thirst alone.

A simple check is the colour of your urine. Most people should aim for a clear pale yellow¹³; darker yellow suggests you may need more. In hot weather or longer sessions, keep checking that it stays pale yellow rather than relying on thirst.

When to check with a health professional

If you're starting exercise after a long break, it's always a good idea to see your doctor first.

For an injury or a health condition, see a health professional, such as your doctor or a physio.

Older adults, people with long-term conditions such as diabetes or heart problems, and people on regular medicines (including diuretics) may need different fluid advice⁶. Ask your doctor or another health professional for a plan that suits you. If you have been told to limit salt, ask before using electrolyte products. If bladder leaks or needing the toilet often make you want to cut back on drinks, talk to a doctor or other health professional first.

If you feel dizzy, sick, headachy, crampy or have a racing heart while active, especially in the heat, stop and cool down. If it has settled, see a doctor soon. If it doesn't settle, or you become confused or faint, treat it as an emergency.

Call your local emergency number for chest pain or pressure (or pain spreading to the arm, neck, jaw or back, especially with sweating or feeling sick), severe breathlessness, fainting or collapse, signs of a stroke, a serious injury, or someone who isn't safe.