> Dr. Kaplan reports that surgery immediately alters the activity of more than 5,000 of the 22,000 genes in the human body.

What does this mean? How is activity of genes altered?

> But surgery only alters the intestinal tract. That tells you, Dr. Kaplan says, that there are whole classes of signals coming from the gut and going to the brain and that they interact to control hunger, satiety, how quickly calories are burned and how much fat is on the body.

This makes sense - you're disrupting signaling. But what does altering genes mean?