What kinds of things have energy that we can't digest that other things can? For example I imagine that if I ate some jellied gasoline then I wouldn't get any energy from it, but an airplane might be able to. What about the stuff that cows digest that we can't digest? Is there a lot of this stuff in food? Does lettuce actually have a lot of energy but we can't digest it to get at it? I mean bunnies would starve if it didn't, right?
Cellulose is the main thing that we cannot digest that other animals can. The cell wall of plants is made of cellulose and about 50% of a plant can be cellulose. So lettuce contains indeed much energy that is wasted when we eat it, while cows and bunnies can digest and use it.