Carbohydrate is the most important fuel during exercise because, unlike proteins and fat, it can be broken down quickly to give your muscles a boost of energy.
How many carbs you can ingest is limited by the rate of absorption from the intestines into the bloodstream. Peak rates from a single carbohydrate (i.e. glucose) appear to be around 1.2 grams per minute.
However, when different types of carbohydrates are combined—most commonly glucose and fructose—the rate can increase to about 1.75 grams per minute, almost a 50 percent increase. This is because fructose and glucose use different transporters that absorb faster into the bloodstream, delivering energy to your muscles quicker.
Consuming a mixed carbohydrate sports drink or gel rather than a glucose-only drink allows you to use carbohydrate at a faster rate, and the sugar blend reduces feelings of stomach discomfort. Most sports products will use a mix of carbohydrate sources.
How many carbs you should take in per hour is influenced by exercise intensity and duration, carbohydrate availability, your fitness level and environmental conditions. Here are my carb intake recommendations for specific workouts or races.