"I want a snack."
"It's late though. You should brush your teeth and go to bed."
"But I had a really small dinner on the way to class, and we've got cookies!"
"Definitely not. Cookies are bad for you, and you know how annoying it is to brush your teeth right after eating them. All those little bits of bread and chocolate get on your toothbrush and it's gross."
"That's true. Okay. I think we have yogurt, that'd be cleaner and a lot healthier."
"Sure, you can have yogurt! lots of calcium."
"And you know what else? Now I can have cookies, because the yogurt will help get the crumbs out of my teeth!"
...and that is why I have food guilt all the time.