Water is generally sufficient for exercises that span less than an hour.

Isotonic drinks are suitable for:

1. Sustained exercise lasting more than 60 minutes.
2. Exercise in extreme environments, such as high heat and humidity
3. Excessive sweat loss
4. Excessive clothing is required so more sweat is produced

Not taking sports drinks during events like these may result in cramping, fatigue and poor performance.

How to select an isotonic drink?

1. Containing a 6-8% carbohydrate solution (low carbs and sugar) which is absorbed into the body more rapidly than water and provides energy immediately
2. Contains at least 50mg of sodium per 100ml, along with smaller amounts of the other electrolytes, such as potassium and chloride, which are lost in sweat.
3. Palatable

Not all sports drinks are isotonic. Those that contain more than 10% of sugars (hypertonic) such as carbonated beverages and fruit juices are not suitable for exercise as these drinks are digested slowly.

Reference:

The effects of the intake of an isotonic sports drink before orienteering competitions on skeletal muscle damage, National Institute of Health.