It's probably salt, not calories. Even if you are not overweight your body stores calories in fat and glycogen that it can utilize in a pinch. You don't run out of calories over a matter of hours. And your body will compensate. I disagree that's it's a bad idea to compensate later (after the ride). Because your body wants to focus on one thing at a time. It can send blood to your muscles so they can pedal. Or it can send blood to your gut for digestion. It doesn't like to do both at the same time. (That doesn't mean you shouldn't eat on a ride--you should eat if you're hungry. It works even better if you take a break to eat AND digest.)

But you do run out of water and salt in a matter of hours.

Becoming conscious of my salt intake--making sure I get enough--has made the biggest difference for me.