Your muscles store about 1800 calories in the form of glycogen, once you're through that, your workout is over unless you refuel during exercise. I don't know what MMA training burns per hour, but running at a good clip burns about 800-1000 calories an hour. So if you're planning on going non-stop for 2-3 hours, gatorade is a perfectly good way to get additional calories. If you're not going that long without refueling, then water should suffice, although as scarecrow mentioned, the electrolytes can't hurt, although i doubt you run the risk of hyponatremia.
Also, remember that fat burns in a carbohydrate fuelled fire. So a little sugar here and there during workouts (200-300 calories an hour) may actually help you drop weight in the long run.
Demian Maia may be the Fred Astaire of jiu-jitsu, but it’s too bad Silva is the Gene Kelly of kicking everyone’s ass.