Sports drinks like Lucozade and Gatorade are the jack of all trades. They replace the three things that you lose when you’re exercising: fluids, carbs and electrolytes (sodium, predominantly). They replace a bit of each. If they’re biased towards anything it’s the delivery of carbohydrates rather than fluids and salts, so they tend to work well when energy demand is really high. For team sports that require intermittent sprints – playing 60 or 90 minutes of football, for instance – they’re a decent way of getting a little bit of everything back in your system. Those drinks are called isotonic drinks, because they’re a similar thickness and similar osmolality to your blood.
Hypotonic drinks are thinner than your blood, lower-calorie and higher in electrolytes and they’re the best thing for hydration because they’re absorbed a lot more quickly. Carbohydrate has quite a lot of mass to it, so if you reduce the carbohydrate content but keep or increase the electrolyte content, then you’ve got a hypotonic drink. They often come either in a tablet form or in a little packet. People will know medical brands like Dioralyte, which rehydrate you if you’ve got sickness and diarrhoea, but they share quite a few characteristics with a good hypotonic sports drink.
Hypertonic drinks go the other way with a much higher carbohydrate content. They’re for when you want to deliver energy as the priority and fluid is less important. A hypertonic drink is thicker than your blood, which means that it’s not as easily absorbed in the gut, but it does deliver a very intense hit of energy. The classic one that athletes use is not really a sports drink – it’s flat Coke.
A hypotonic drink will be 0-4% carbohydrate. Normally, an isotonic drink is about 6% carbohydrate, then a hypertonic drink like Coke will be 10-12% carbohydrate.