Hydration & Performance

Dietitian and nutrition consultant Zoë Watt examines the importance of hydration during exercise, focusing on the roles played by water and sports drinks.

Sports dietitians frequently get asked how much people need to drink when exercising. The answer is not straightforward, because everyone’s sweat rate varies and hydration when you are active is about replacing what you lose when you sweat.

If you are dehydrated your muscles won’t work optimally, thereby affecting performance. Dehydration can increase heart rate and body temperature and cause headache, nausea, increased risk of gastrointestinal upsets and impaired stomach emptying, as well as deterioration in motor skills and performance. If your clients complain about getting tired more quickly than usual or finding it harder to run up hills, then inadequate hydration may be the culprit. The following guide can help you decide whether sports drinks should feature in yours and your clients’ hydration plans.

Sports drink or water?

Concerns have been raised that sports drinks might contribute to weight gain. When a sports drink is consumed in moderation, and during periods of physical activity, it can be part of a healthy diet. Energy balance is key: clients need to find a balance between kilojoules consumed via food and beverages and kilojoules burned during daily life and exercise.

Sports drinks, such as Gatorade, replace what you lose more quickly than water does, and assist in the hydration process, by making the body absorb fluid faster, and retain it longer – making it more effective than water for sports hydration. Because they contain sodium, which makes them taste slightly salty, the thirst mechanism is stimulated, encouraging you to drink more. Sodium also stimulates sugar and water uptake in the small intestine and assists with hydration. An additional role of sodium in sports drinks is to replace sodium chloride, the major salt lost during sweating.

Flavour – or palatability – also plays a role in increasing fluid intake, by stimulating your brain to tell you to drink more. Carbohydrate (the preferred fuel source for working muscles) in sports drinks provides muscles with energy and assists absorption of sodium by acting as a ‘co-transporter’ within the intestine.

Are sports drinks really necessary?

Yes. The performance benefits derived from sports drinks come from their ability to hydrate faster than water. Fluid, sodium and the addition of carbohydrates have all been shown to have beneficial effects on performance. Water can actually turn off the thirst mechanism too quickly, making it harder to stay hydrated over the long haul. A beverage with electrolytes and carbohydrate will hydrate faster than water. Replacing the electrolytes lost in sweat and providing carbohydrate will promote rapid absorption and supply energy.

How much, and when?

To maintain adequate hydration during activity, you should drink enough during exercise to minimise dehydration (weight lost during exercise), but to avoid the over-drinking (weight gain during exercise) that can increase the risk of hyponatremia (low blood sodium level).

The question of what is ‘enough’ depends on how much sweat is being lost. You can develop a good sense of your fluid replacement needs by stepping on a scale before and after workouts. If you lose more than two per cent of your body weight (e.g. 1.4kg for a 70kg individual), increase your fluid intake the next time you train. If you’ve gained any weight at all, cut back in future sessions. Sweat rates can vary depending on environmental conditions, and everyone sweats at their own rate, so don’t copy what others are doing, and try to calculate your sweat rate in various conditions. After some trial and error, you’ll become good at gauging your hydration needs in varying conditions.

Make sure you begin your physical activity well hydrated, drink enough during exercise to prevent dehydration and continue replacing ongoing fluid losses after exercising.

Zoë Watt
Zoë is an Accredited Practising Dietitian and nutrition consultant with a passion for sports nutrition and weight management. She completed the IOC (International Olympic Committee) diploma of sports nutrition in 2010 and served on the board of Sports Dietitians Australia for four years. For more information visit
www.theathleteskitchen.com.au or call 0419 933303.