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Drink your way to your PB

A good performance is heavily influenced by fluid intake and taking simple steps to get intake right could be the fine line between success and failure.

Water makes up over half of our bodyweight and we simply cannot survive without it. It is used for many essential processes, controlling temperature, removing waste products and transporting nutrients and oxygen around the body. Regular ingestion of fluids is not only vital for normal functioning of the body, it is also vital for optimal sports performance.

Performance & Dehydration

During prolonged exercise athletes are at high risk of dehydration. Exercise produces heat and as a result lose large amounts of water and electrolytes through sweat to cool down. When an individual is dehydrated by as little as two per cent of bodyweight, performance is impaired by factors such as increased heart rate, increased perceived exertion, stomach upset and loss of coordination. The more dehydrated you become, the affects become more pronounced. In contrast, having too much fluid can be equally as damaging, not only in affecting performance through minor irritations such as stomach discomfort, but in extreme cases can prove fatal. Hyponatraemia causes a dilution of sodium in the blood. Some athletes may believe that “more is better” due to the focus we place on remaining hydrated during exercise, but this can have damaging effects. Slower runners that are out for longer tend to be at higher risk for developing this, compared to the faster runners. So how can we ensure achieving this fine line between too much fluid or too little?


In order to avoid dehydration, an athlete needs to drink sufficient fluid to match their sweat loss. It is impossible to recommend a generic fluid replacement plan as the amount of sweat lost is highly individual, being governed by various factors such as environment, length and intensity of exercise, metabolic rate and even genetics. As such, the amount of water needed to remain in fluid balance is different for every athlete and situation. Kenyan endurance runners are renowned for taking on little fluid when training. A study by Fudge et al (2008) found that although Kenyan athletes took on little fluid even during demanding training sessions, they managed to remain well hydrated.

The ACSM’s main message is that the goal of drinking during exercise is to prevent excessive (>2% body weight loss from water deficit) dehydration and excessive changes in electrolyte balance to avert compromised performance. You can do this by listening to your body and drinking to thirst or by estimating sweat rate during exercise so you can gauge the exact amount of fluid lost and therefore have the ability to be guided on how to match your losses during exercise. It must be noted though, sweat rate can vary from day-to-day and if you are serious about determining your fluid requirements, you will need to estimate your sweat rate under different conditions and exercise intensities (see end of blog on calculations). Methods include tracking pre- and post-body mass or simply checking urine colour.

Water or Sports Drinks?

Studies have shown that flavoured drinks that contain carbohydrate (6-8%) and sodium enhance intake and fluid absorption. This makes sports drinks ideal. Sodium should always be ingested when large salt losses occur, e.g. those with a high sweat rate, exercise over 2 hours or very “salty” sweat. Water is still fine, but it won’t stimulate fluid intake in the same way and therefore a more planned approach to drinking may benefit to ensure consumption matches requirements.

The key message is to be aware, but not obsessed. Think about your own hydration strategy to practice in training and don’t copy the runner in front of you, everyone is different!

Top tips

  • Start the race hydrated, having little and often during the morning (300-600ml)

  • Try and match fluid intake with weight loss (just below and definitely not over)

  • You can work out your sweat rate for more prescriptive hydration or you can drink to thirst (the later is more appropriate if you run at a slower pace)

  • During the marathon, drink early and taking ‘little and often’ is better than large doses

  • Do not over drink, weight gain during a marathon shows an increased risk for hyponatremia

  • Sports drinks or water are the best options

  • If it is hotter than you are used to on race day, be prepared to slow your pace, as drinking more fluid won’t necessarily cool you down

How to Calculate Sweat Rate 1: Record your pre-training weight (kg) 2: Record your post-training weight (kg) 3: Run for 30-60min without drinking fluid 4: Subtract post-run weight from pre-training weight and multiply by1000 (convert to grams) 5: The amount you lost in grams is equal to that lost of fluid 6: If you did a 30min session, multiply that by two to get your ml per hour figure. This is the amount you should aim to drink in subsequent runs (remembering this will change with such things as running intensity and heat

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