I did an informal, rather unscientific poll/survey a couple of days ago on dailymile.com to see how people use energy gels/drinks/bars/whatever during training runs. About half responding said they either did not use them, or used them inconsistently for long workouts. The other half selected an option that suggested they use sports energy supplements consistently on long training runs.
I used to wonder how regular supplementation during long workouts would affect pure endurance. Pure endurance depends on the efficiency of the metabolic systems that convert glycogen and fats to metabolizable sugars that can be used to make "fuel" (ATP) in our cells. When we do endurance work, we place these systems under great demand and the theory goes that our bodies adapt over time to produce more power plants and restructure muscles tissues to become more efficient at delivering the sugars to the cells and processing the sugars to ATP at the cells, so we can keep working.
If we supplement these sugars routinely, I have wondered if we aren't reducing the effective stress on energy systems caused by a given intensity of exercise and possibly reducing the training effect on our energy systems. On the other hand, using the supplements may simply allow us to increase our performance enough so that the net same level of demand for production of sugars/energy from our internal stores is required anyway, in which case, there is a net benefit of training with gels, because it adapts our bodies to also use them for some performance benefit in a race.
Personally, I use sports energy supplements inconsistently on long workouts, because I have a suspicion that I might become dependent on them for good performance. I use them often enough to get used to using them, and otherwise just run with water. But maybe it doesn't matter, or maybe there is a real net benefit of using them consistently in training, and we all could see some fitness/peformance benefits from using them regularly on long workouts?
Anyway, I'm a science nerd.