Hydration and exercise

The body loses water through sweat and breath during exercise, and more in heat. Replacing it keeps the heart, muscles and temperature control working properly. For most people and most exercise, drinking water according to thirst is enough.

Everyday needs

The government's guide, as given by the NHS, is six to eight cups or glasses of fluid a day. Water, lower-fat milk and sugar-free drinks, including tea and coffee, all count, and some fluid comes from food. More is needed in hot weather, during long periods of physical activity, in pregnancy and when breastfeeding, and during illness. The simplest check is the colour of the urine, which should be a clear pale yellow.

Before, during and after exercise

• Before: starting well hydrated matters more than drinking large amounts at the last minute. A glass of water in the hour or two beforehand is usually enough.
• During: for sessions of up to about an hour, water taken when thirsty is sufficient. In longer or hotter sessions, small amounts are drunk at regular intervals.
• After: fluid is replaced over the following hours, with a meal or snack to restore salt.

Athletes sometimes weigh themselves before and after training: each kilogram lost corresponds to about a litre of sweat.

Water or sports drinks?

• Water is the right choice for ordinary exercise.
• Sports drinks contain sugar and salts. They are useful in hard exercise lasting more than about an hour, such as a long run or cycle ride, and add unnecessary sugar to shorter sessions.
• Milk provides fluid, protein and salts, and is a good recovery drink.
• Energy drinks are a different product, high in caffeine and sugar, and are not designed to replace fluid.
• Fruit juice and smoothies are limited by the NHS to one small glass, 150 ml, a day.
• Alcohol increases fluid loss and slows recovery.

Signs of dehydration

The NHS lists thirst, dark yellow and strong-smelling urine, passing urine less often than usual, headache, dizziness or light-headedness, tiredness, and a dry mouth, lips and tongue. Those most at risk include older people, children, people with diabetes, and anyone exercising hard or for a long time in the heat.

Urgent advice from NHS 111 is needed for unusual tiredness, dizziness on standing that does not pass, a fast heartbeat or breathing, or very little urine. Confusion, extreme drowsiness, cold or blotchy skin and rapid, shallow breathing need a 999 call.

Drinking too much

In long endurance events, drinking far more than is lost can dilute the salt in the blood, a condition called hyponatraemia. It causes headache, nausea, bloating and confusion, and can be fatal. It has occurred in marathon runners who drank at every opportunity. Endurance athletes are advised to drink to thirst, not to a fixed schedule.

Heat

In hot weather exercise is moved to the cooler parts of the day, the pace is reduced, and light clothing and a hat are worn. Headache, dizziness, nausea, cramps and heavy sweating are signs of heat exhaustion: the person stops, moves to a cool place, lies down and drinks. A person who has not recovered within 30 minutes, or who becomes confused or stops sweating, needs emergency help.

Other precautions are covered under avoiding exercise injuries.