hmm...
***thinks about it
They're never filled with jelly or anything, like doughnuts sometimes are( spudnuts=too soft ).

It's mainly whatever icing/coating is put around it( other than plain icing, and chocolate, I think it varies according to location ).

P.S.
I feel sorry for this guy.

Made with a potato flour mixture, they are super lite on the inside with a little bit of a crust. So what happens ? I move to Oklahoma. Only two donut shops in town other than WalMart and one store bakery. What horrible torture.