I wonder if Sanderson ever answered it on any panels. Do the metals simply magically disappear after they are burnt or are they in any way excreted from the body? Do they provide any nutritious value for the allomancers? If allomancers can control what metal they burn, can they also control what foods they burn?
No, burning is a special Allomantic act distinct from digesting.
From one of Kelsier's first lessons for Vin on Allomancy:
“First,” Kelsier said, “remember to burn away any unused metals you have inside of you at the end of the night. Some of the metals we use can be poisonous if digested; it’s best not to sleep with them in your stomach.”
This means that the metals disappear after burning, but the way they are 'consumed' is different from regular digestion. They're not digested by the stomach or excreted like normal foods, so there's no way for them to provide any nutrition. Allomancers don't have completely different bodies that can digest metal; they just have a different thing they can do ('burning') to use that metal for another purpose
“Also,” Kelsier said, “never try to burn a metal that isn’t one of the ten. I warned you that impure metals and alloys can make you sick. Well, if you try to burn a metal that isn’t Allomantically sound at all, it could be deadly.”
This answers your other question comparing metals to food. Only specific metals can be burned: not food, and not even other metals that aren't Allomantically pure. That's why Allomancers generally pay for their little bags of metal rather than just taking scrapings from pewter cups and so on.