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Advice about race fluid balance and nutrition in runners.

Running is a fantastic recreational sport which continues to grow in popularity thanks to Mo Farah and Paula Radcliffe. It’s easy to do, requires minimal kit, it’s great for aerobic fitness and is open to all-comers.

What to drink and eat is never far from a runner’s thoughts and can become a bit of  an obsession especially considering what to do during a race. It’s not uncommon to see runners armed with a water bottle even on the shortest of outings and there’s plenty of myth circulating round the Running Community about continuous drinking, flushing out toxins – even down to the ideal colour of the urine. So what is sound advice? How long is it safe to run before needing to hydrate or refuel? Are there dangers to over-hydrating?

Performance starts to be impaired when there is a 2% loss of body weight – about 1.5 litre of water in a 70kg individual. Runners lose around 1 litre/hour in ideal conditions where the weather is mild and cool.

Race Hydration

Do I need to drink before a race or at every water stop? Answer no – the current medical advice is only to drink to quench thirst.

Over-hydrating may lead to a dangerous lowering of plasma sodium (exercise associated hyponatraemia EAH), and can lead cerebral oedema and death. Certain runners are vulnerable to this and cannot correct the imbalance by excreting water in their urine. It is thought that they are unable to detect over-hydration and continue to secrete too much of the water-retaining hormone ADH. Sweating cools the body but involves losing water and salt. Sweating a litre of fluid results in the loss of about ½ teaspoon of salt. The hotter/more humid the conditions the more you will sweat lose salt.

Race Nutrition

Time running Intra-race fluids & nutrition Post-race fluids & nutrition
Less than 1 hour Unnecessary Hypotonic / water
More than 1hour Sports drink Isotonic sports drink

To maximise the uptake of carbohydrate by the gut requires a specific mix of sugars: fructose, maltose and glucose. Nutrition is recommended when running for more than an hour to prolong the available glucose needed by the muscles. After 2.5 hours or longer the body begins to run out of glycogen/glucose and runners describe ‘hitting the wall’ as their metabolism converts from a glucose/fat burn to fat only burning and their pace grinds to halt. They may feel emotional, disorientated or completely “spent”.

Isotonic Sports Drinks – quickly replaces fluids and salt lost and supplies a boost of carbohydrate. This drink is the choice for most athletes in middle and long distance running. There are plenty of sports drinks on the market although most have only about 50% of the required salt content. By drinking a sports drink containing salt the thirst stimulus is maintained so salt loss is minimised. At the same time carbohydrate and water are taken on board.

My recommendations for a smooth marathon/half marathon are:

  • Assume you are fully hydrated prior to the race. No need to drink right up to the starting gun.
  • Practice drinking and eating on the run before the race.
  • Eat  2 or 3 jelly beans every mile.
  • Start rehydrating (and further refuelling) after an hour – drinking up to a maximum of 250mls every 15-20 minutes until the end of the race – but being guided by thirst.