By Ian Wilson

Anyone who thinks all this sports drinks stuff is just modern marketing should come for a ride with some of my older friends. For show me the drink and I’ll give you the age of the rider. Water and salt tablets? Over 50. Lime cordial? Over 40. Gatorade? Over 30. Red Bull? Probably too young to be consuming that much caffeine. There’s a little truth in all good marketing campaigns and drinking to improve sporting performance has been a recognised concept for nearly half a century. But our understanding of exercise physiology has changed almost as much in that time as the way sports drinks are marketed, and this summer you’re going to see a marketing push for new ranges of “isotonic” sports drinks. Are they better than what has come before or just a new angle on an old story? There are three physiological reasons to drink while exercising: to rehydrate, to replace lost electrolytes, and to provide fuel. Unless you’re riding around Darwin at midday, none of these will really affect your performance for the first hour, but after that the amount of sweat you produce in an attempt to stay cool will rob enough fluid from your cells to have an effect on your performance. If it was only water you were losing it would be easy to replace, but sweat contains large amounts of sodium and smaller amounts of calcium, magnesium and potassium which are all vital for normal muscle and nerve function. The concentration of these electrolytes (particularly sodium) in your bodily fluids is referred to as tonicity. Any fluid that has the same ratio of water to electrolytes as body fluid is called isotonic. If it has more electrolytes (more salty) it’s known as hypertonic. Less electrolytes than body fluid is called hypotonic. This wouldn’t be relevant to cycling except the tonicity of the fluid you drink determines how quickly it is absorbed and how it’s distributed around your body. Water has a tendency to move towards salt in a process called osmosis. To reduce your water loss your body takes advantage of osmosis to pump salt out of the urine that is collected in your kidneys and put it back into your blood. A large amount of the water in your urine follows the salt back into the blood, keeping
your tonicity in balance and you hydrated. When the fluid loss becomes too great there is simply not enough water in your urine to follow the salt. The resulting increase in blood tonicity triggers your thirst reflex telling you to replace the lost fluid. If it’s just water you want to replace then drinking water is great. Water is hypotonic so once it hits your stomach it wants to move towards your salty blood, meaning it’s rapidly absorbed and
distributed. But by reducing your blood’s tonicity it also reduces your thirst reflex and makes you less likely to continue drinking. Water also doesn’t replace any of those lost electrolytes or provide any fuel. Which means you might want to add some electrolytes and fuel to your beverage.
Electrolytes is a big word that sounds scientific but its role in muscle and nerve function is quite difficult to explain. The role of sugar on the other hand is quite simple: it fuels your body. Sugar also tastes a lot better than salt, and together that’s why most sports drinks contain a lot of sugar, or as it’s described in the ads: “carbohydrates for energy”. But adding large amounts of sugar to a liquid quickly affects its tonicity, and with the
amounts added to most sports drinks to make them palatable, it makes them hypertonic. The effect of this is to slow their absorption into the blood. Getting the balance between rehydration and the replacement of fuel and electrolytes is why a number of sports drink manufacturers have introduced isotonic drinks to their ranges. Absorbed more quickly than sugarladen drinks, they don’t suppress your thirst too quickly and they also replace lost electrolytes. Give them a try and you might find the less sweet taste appealing. You might also find yourself incredibly fit, stunningly attractive and absolutely unbeatable on the bike. At least that’s what the marketing suggests.

H2O Info
1. Our bodies are approximately 55-65% fluid by weight.
2. Moderate exercise produces 0.5 and 1.5 litres of sweat per hour.
3. Thirst is not a good indicator of fluid requirements.
4. Dehydration can occur in as little as 20 minutes.
5. Drinking when dehydrated may cause gastrointestinal distress.
6. Alcohol and caffeine based drinks such as tea, coffee and Coke are diuretics and may contribute to dehydration.