After being duped by her husband more times than she cared to mention, Duffus's wife decided to turn the table at his birthday party celebrations. All the guests collectively brought enough snack to feed a small Middle Eastern Country. But Duffus could only have the snacks if he could answer their riddle.

A unit of measure to help you keep track.
And once taken we're hard to give back.
We come in many things, some good & some bad.
You burn us many a way through many a fad.
But the type Duffus likes are empty you see.
Little nutritious value though labeled fat free.
So tell us dear Duffus, yes Duffus do tell.
If you know what we are you'll eat really well.

Duffus immediately knew the answer but after thinking about it decided not to eat any more snacks.
But why, what is the answer to this riddle?