Fluids and running exercise

Woman drinking water

When we exercise, our muscles only use about 25% of the energy for work, with the rest released as heat – which is why exercise makes you hot. The main way the body is kept cool is by sweating. Heat from the working muscles is transferred to the blood. The blood flow to the skin is increased, and heat is lost by evaporation – sweating. Sweat comes from the water in blood, so we need to replace this vital fluid to prevent dehydration. If we run or exercise while dehydrated, our temperature can rise quickly and cause heatstroke, which is potentially fatal.

Fluid needs for runners
In general, we need to drink about two litres of fluid a day to be properly hydrated. However, it’s quite likely that exercise will increase our fluid needs. The more you sweat, the more you need to drink to replace the lost fluid. Some people naturally sweat heavily, but even small losses can cause fatigue. Plus, the fitter you are, the more effectively you keep your body cool – so the more you sweat! Training harder, running longer or exercising in hot and humid surroundings will also make you sweat more.

During exercise we lose on average 500-1000ml of fluid an hour. The easiest way to get a better estimate of how much fluid you lose is to weigh yourself before and after exercising – preferably naked so as not to include the sweat absorbed within your clothing.

Each kg of weight loss is equivalent to a litre of fluid loss. However, you will lose more fluid as urine, so you should drink at least 1.5 litres of fluid for every kg of weight lost. The colour of your urine is also a good indicator of fluid loss – if it’s pale and plentiful, you’re well hydrated, but if it’s dark and sparse, you need more fluid. A loss of just 2% in your body weight may affect your ability to exercise; a 4% loss can cause exhaustion. If you’re competing, for every 1% drop in body weight there’s about a 5% drop in performance, which could mean the difference between coming first and last in a running race.

Woman drinking water

If you keep exercising without replacing fluid, you will become increasingly dehydrated. You will no longer be able to keep your body cool, your temperature will rise, you will feel lightheaded and nauseous and, ultimately, get fatigue or heatstroke. The only way to prevent this is to start off well hydrated and stay that way.

Drinking schedule for running
It is vital that you drink plenty of fluid to avoid compromising your health when running. To succeed, you need to plan your drinking strategies and get into the habit of drinking, so that your body can gradually adapt to increased fluid intakes. Don’t leave it to chance; take your beverage choice with you.

It is unlikely that you will drink too much water - not drinking enough is usually the problem. The only time drinking plain water may cause a problem is if you’re sweating very heavily for a prolonged period of time. In this situation, a sports drink containing sodium would be better than plain water to prevent low blood sodium levels (hyponatraemia).

Hydration and running...

Before running exercise
Always start every running exercise session well hydrated. Drink 300-500ml of fluid in the 15 minutes prior to your workout. During exercise aim to drink 150-250ml every 15 minutes to offset fluid losses – drinking smaller volumes more frequently minimises stomach discomfort. Remember, the more you sweat, the more you need to drink.

After running exercise
How much fluid you need depends on how much you lost, but you’ll probably need at least 500ml. Try to drink 1.5 litres of fluid for every kg of weight lost during exercise, or keep drinking until you pass light-coloured urine.

Which fluid for runners?
Which fluid you opt for depends on how hard you exercise, and for how long. However, choose a flavour you like to encourage you to drink more. If you’re exercising at a low-to-moderate intensity for less than an hour, then water is great. If you find it difficult to drink large quantities of plain water, try adding some juice or squash, which will also provide you with some carbohydrates to help restock glycogen stores.

Woman drinking water

Sports drinks for runners
If you work out continuously for more than an hour, then a sports drink would probably be a good idea. Sports drinks help maintain better fluid levels, plus the added carbohydrates provide vital glucose to help avoid fatigue. Most sports drinks are 5-8% carbohydrate, which makes them ‘isotonic’ – a similar concentration to blood – and, therefore, quickly absorbed.

In addition, sports drinks contain sodium to stimulate sugar and water absorption, and replace the sodium lost in sweat. This added sodium is particularly useful if you’re a ‘salty sweater’ – where your sweat is opaque, tastes salty, and leaves white marks on your clothes. The sodium has the added benefit of encouraging you to drink more. In fact, the drive to drink is present for several hours following exercise (it stops when you eat).

However, when your mouth is moistened with fluid, your body automatically signals your brain to stop drinking. This inhibition can happen before the body’s fluid levels have been completely restored. This means that even if you don’t feel thirsty, you’re not necessarily well hydrated, so it’s important to keep drinking fluid throughout the day.

Alcohol and running
Although alcohol in moderation is fine, it’s not a good idea to drink it just before exercise as it has a detrimental effect on co-ordination skills and exercise performance and also increases injury risk. You also need to rehydrate properly before drinking alcohol after running - alcohol can cause dehydration and slow down recovery from injury.