In very hot weather, try this tip: fill your water bottle half-way with sports drink and freeze it solid. Right before your run, top off the rest of it with cool water. You’ll have cold water to begin, and ice cold sports drink later when you need it most. If you wear a bottle holder, you’ll also enjoy feeling a bit of coldness against your back.
If you use fuel belts with more than one bottle, fill one entirely with sports drink and freeze it, but be sure to leave a little open space at the top to give the freezing water somewhere to expand into.