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.