When you're sick with the stomach flu everything goes through a lot faster than normal.
Food is absorbed in the intestines mostly.
When you're sick and the food just goes zippy-do, right on through, you don't absorb it, and indeed, you lose nutrients. Of course, that's the basis for the products that block fat absorption. The normal process is that the stomach churns things up, mixes in some acid, mixes in some enzymes, and sends the mix off to the intestines to be absorbed in little, tiny pieces. Think of food as beads on a string, beads of protein, beads of amino acids, beads of sugars and carbohydrates, etc. After absorbing them, the body metabolizes them into energy molecules - ATP, for example.