I am always trying to balance intake with exertion-induced upset stomach while hill training.
For me, it helps to really watch what I'm eating, and when, before an exertion...if I know I have a big hill to do 30 minutes from now, I'll eat a couple bites of energy bar NOW, then stick to a little gel maybe 10 minutes before the hill...
In other words, I watch the timing of longer digesting fuels (including proteins and fats) vs quicker, like sugars. That said, I also can't just dump sugar into my system when exerting...for me, timing is everything.
Also, I have had a LOT, LOT, LOT fewer digestive problems since I switched to bars & gels for all my calories, and just using electrolyte tabs in plain water, and another bottle of just water, for my liquids. The sports drinks just weren't my consistent friends.
"The best rides are the ones where you bite off much more than you can chew, and live through it." ~ Doug Bradbury