I fill my bottles up with water and stick a flask of Hammergel in the little pocket. That way I have the option. I don't carry anything on runs less than an hour, and I find I need calories on runs longer than that. Not thousands of calories, but I feel much better during and after my run if I ingest 50 or so calories every 15 minutes or so. It makes a huge difference on my long runs. I did a few 2 hours runs with no water/food, and I had to resort to the slow shuffle on the way home. Everyone is different, what tends to work for you?