system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what is it about the structure of vegetables that makes them so nutritious yet contain relatively few calories?
gpt
We cannot digest cellulose. That is the physical structural component of plants. Because we cannot digest it we cannot get nutritional value from it. We call it dietary fiber and it passes through our bodies mostly intact. We are able to digest other components of plants and so get many vitamins and such from them, but the sugars that make up their cellulose are locked away from us. 

This is perhaps the greatest proof that we are omnivores and not herbivores.