All you think about is food.
Is all you think about is food?
The above would be fairly common in informal speech, and easily understood. Most people would probably not notice it as peculiar. But many would likely say that it varies from standard grammar patterns. We can perhaps more easily see how it's nonstandard by simplifying the structure to:
Is everything is ok?
At the same time, we can see why it would be said if we understand that questions are formed generally by inverting subject-verb order, and we recognize the following declarative sentence as standard:
It's all you can think about, food is.
We can render the question onto more solid standard ground in several ways:
Is that all you can think about, food?
Is that all you can think about, food is?
Is food all you can think about?
Is all you think about food?
This is standard grammar, structurally like
Is everything you eat food?
But it is not easy to interpret correctly because it the subject is long and contains a verb. We are more likely to understand it with a short dummy subject:
Is that/it all you think about, food?
The remaining questions are all grammatical. The most common and standard question forms are the firt triplet:
Is X Y?
Is tomorrow Saturday?
Is she the mayor?
Is this mine?