The simplest and most effective fluid mix I've found for those hot summer running days is just an electrolyte, complex carbohydrate (Maltodextrin) and icecubes. During a race, I'll eat one GU every hour with a bit of caffiene just to keep fatigue at bay and put a little bit more spring into each step but that's it.
Maltodextrin (MD) provides roughly 112 calories per oz and it is recommended to consume 1.5 oz per hour of activity.
I'll actually finish the entire ~21oz in one hour (~6 miles) on a normal training run during the summer. All depends on how hot, pace and how many hills.
Summer Fluid Mix:
- CamelBak Elixir - NUUN and Nathan are also popular. I happen to like the Elixir Berry flavor.
- Maltodextrin (MD) - I use Carbo-Pro but bulk food grade is probably just as good. Normally just one or two scoups per 21 oz per instructions.
- Icecubes - Nothing like cool water to help cool you from the inside out as your body quickly warms up on a hot day.
- 21-oz Hand-held - I like the CamelBak hand-held with little pouch because it fits well and you can synch the strap down for a good fit.