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Marathon hydration guidelines

Around 50-60% of the human body is made up of water. There are a number of ways in which we lose body water, one of which is through sweating during exercise. It is just as important to drink little and often throughout the day as it is to drink during prolonged exercise. Dehydration as little as 2% is known to impair performance however, being regularly dehydrated throughout the day can increase circulating levels of stress hormones, cause headaches and increase the risk of illness.

You should aim to drink 2-3 litres of water a day. This can include squash, water, fruit juice, teas and coffee and soups. Be careful not to hydrate all day on caffeinated beverages as these are diuretics and you will struggle to retain the fluid. A good indication of your hydration status is the colour of your urine. If your urine is dark this indicates dehydration and you need to drink more. If your urine is a pale straw colour you are sufficiently hydrated.

Before Exercise:

The most important goal, and often the most overlooked, is to start exercise well hydrated. If you have been drinking plenty of fluids throughout the day, this should be sufficient to start you off on the right foot. Official guidelines suggest drinking 5-7 ml of fluid per kilogram of body mass (ml/kgBM) at least 4 h before exercise. If you still don’t produce urine, or the urine is dark or highly concentrated then more fluid should be consumed about 2 h before exercise (e.g. 3-5 ml/kgBM). To help retain the fluid, choose drinks with sodium or eat small amounts of salted snacks or sodium contain­ing foods.

Worst case scenario, if that’s all just too hard to remember, in the 1-2 hours before exercise, top up your fluid levels by drinking a pint of water or squash. This allows plenty of time for you to go to the toilet before you set off!

During Exercise:

There are a number of different factors that influence how much you need to drink during exercise. These include starting hydration levels, exercise intensity, climate, humidity and sweat rate.

Carry a water bottle with you and drink little and often (150 ml every 10-15 min). You should aim to avoid feeling thirsty as this indicates you are already in a dehydrated state and your performance may be impaired.  Use a sports drink containing electrolytes when your session lasts longer than 90 min. You should be aiming to drink a sufficient amount to avoid a 2% loss in body weight. This can be calculated, and monitored, by measuring body weight changes immediately before and after exercise.

Post Exercise:

Rehydrate with a sports drink after training if you find your urine colour is a dark yellow or you have lost a significant amount of weight (2% of your body weight). Drink 1.5 L of fluid for each kg of body weight loss. Drink the fluid over time to help retain the fluid. If you don’t have the time to weigh yourself, start off by drinking 500 ml of a sports drink or water with a salty snack and monitor urine colour as an indicator of hydration status.