Since I've come to this discussion late (just joined textkit today) I will give my totally different answer to the question of what is the inverse of donut.
Let us first define donut in terms of it's food characteristics.
It is sweet, with lots of carbs. It satisfies the appetite quickly, but has little staying power - within an hour you find yourself needing something more.
So what would it's food inverse be?
not(sweet) = savory
not(carb) = protein/meat
So the inverse of donut would be a solid chunk of roast beef, or maybe a nice T-bone steak.