I've never noticed calories to be an issue. But increased core temperature does increase your heart rate, so I suppose something has to fuel that.

I'm not sure what you mean by feeling "de-fueled." When I need calories on a ride, I get muscle fatigue. Is that what you mean? Maybe you were overheated - did you take your temperature? Maybe dehydrated - did you weigh yourself or note the volume and color of your urine? Maybe mildly hyponatremic - what's your salt intake like?

I'm a firm believer that you CAN acclimate to heat, but be careful and gradual and smart about it.