An extremely obese man shows up at his doctor's office and claims that he's tried every possible way to lose weight, to no avail. So the doctor proposes a radical diet: rectal feeding. Reassuring the fattie that he won't starve to death, the doctor explains that he can actually take in enough nutrients through the rectal walls to sustain life, but that he's sure to lose weight in the process.
Three weeks later the patient comes in for a follow-up appointment, and he's down from 360 to a trim 175 pounds. The doctor shows him into his office and asks him how he's feeling, noticing that he's bouncing up and down in his seat quite energetically.
"I'm feeling great, doc; never felt better!" is the reply.
"In that case, why are you bouncing up and down like that?" asked the doctor.
"Just chewing some gum!"