It depends on whether your mental model is "stuff goes into the stomach and then magically ends up in the body intact" or "stuff goes into the stomach, gets broken down, and then synthesized into what the body needs".
The first model implies, for example, that dietary fat is uniquely fattening and that, conversely, a low-fat diet is an easy way to reduce or eliminate your body fat. I believe we can mark this model down as "trivially disprovable". :) (It also implies that eating sugar should have some impact; sugar deposits forming around the body...again, nothing of the sort happens.)
And yet, belief in the first model persists. I don't think either model is inherently more intuitive, given our current understanding. People will believe whatever the popular wisdom is as it shows up in USDA guidelines, highschool texrbooks, etc. The question is, why does the obviously wrong model persist in popular wisdom?
