When designing a nutritional program to supplement the body's energy stores, the rate of digestion and absorption of foods must be taken into account. The time needed for the stomach to start the digestive process, empty its contents into the small intestine, and have the food components absorbed into the bloodstream will directly affect how quickly any food will be available to the muscle to provide the Calories for exercise.
You have some control over four major factors influencing the digestive process.
- Solid versus liquid - liquids are emptied from the stomach more quickly than solids.
- Fat content of the food - fat slows the digestive process and delays the availability of any Calories to the muscles.
- Sugar concentration - especially in liquids, a sugar content of more than 10% will slow stomach emptying. (The use of complex carbohydrates, will offset this to some degree and offers an alternative strategy to maximize Caloric intake.)
- Physical activity level of the cyclist - the mechanical activity of digestion is slowed by any vigorous activity, usually starting at 70% VO2 max. Except in short, all out events, this is rarely an issue, and it is much less so for cycling than for running where the additional component of mechanical stimulation of abdominal contents from the sport itself slows digestive tract functioning.
From the above four points, it is easy to see that the optimal food for a rapid, high energy boost during a ride would be a semi-liquid or liquid carbohydrate with minimal if any fat. On the other hand, an endurance athlete, competing at a lower VO2 max., might prefer a complex carbohydrate with some fat added to improve taste (and generally in a solid form), in order to slow emptying from the stomach and even out absorption over a longer period of time.