Many modern sports drinks contain a mix of simple carbohydrates and salt, designed to replace fluid and electrolytes lost during prolonged exercise while enabling exercisers to maintain and replenish their energy stores both during and after activity.
These typically come in two forms:
- Isotonic drinks contain similar concentration of salt and sugar as found in the human body (for combined rehydration and carbohydrate replacement).
- Hypotonic drinks contain a lower concentration of salt and sugar than the human body (more efficient hydration with limited carbohydrate replacement).
Different studies have indicated that ingesting carbohydrates during exercise is acceptable when:
- The exercise session continues longer than an hour.
- The match or race is longer than 90 minutes in duration.
- A pre-exercise meal is not possible (such as early morning intensive training).
The consumption of isotonic drinks (containing 4-8g sugar) during exercise has been shown to delay the onset of fatigue and to help improve performance for endurance athletes. Some athletes find it difficult to consume even a light meal before exercise without causing gastrointestinal discomfort, or simply may not have the time to eat before their planned training session.
People often have to train early in the morning, making a pre-exercise meal unrealistic.
The replacement of lost fluids by ingesting an istonic drink is also an obvious advantage. However, the intake of carbohydrate during longer exercise does not have to come from a fluid; it can be taken through food items where appropriate.
This can be observed in sports like tennis and distance cycling where athletes carry food with them and eat small amounts at a time through the competition.