Actually, the first thing that came to mind was "intensity?" So Cranium and I do agree to a large extent. Intensity means greater sweat, higher energy output, and therefore the need for faster replenishment of both fluids and food.
I also have to agree that in tandem with intensity, the osmotic effect can have an effect on gastrointestinal function.
For the life of me, I simply cannot understand why people still drink Gatorade on long-distance rides. We have Powerade here and the maker, Coca-Cola, at least goes to some effort to include maltodextrin as the majority carbo in it, not plain sucrose like in Gatorade.
The electrolyte intake that Carbonfibreboy discusses (along with the recommendations for V8) helps to restore the osmotic effect. It's also worth saying, I think, that excess water intake without adequate electrolyte intake can lead to issues of energy uptake.
At least, that is all in my experience. Others' experiences may differ.