This doctor was NICE about it.
Me to Fatty: PUT THE BOX OF TWINKIES DOWN!!!!(through megaphone, two feet from patient) GET OFF YOUR ASS AND EXERCISE!!!!
Then again, I'm not that nice.
A doctor was once rude to me (not about losing weight, and it was even over an issue that was none of his business), and I read him the riot act so loud everyone in the waiting room could hear. I then proceeded to the front desk, told the woman I refused to pay, and explained why. I made sure I spoke loudly enough that the waiting patients could hear.
Of course, I found another doctor. Doctor's don't have a need to be rude to their patients. They can speak firmly and plainly (as this doctor seems to have done) without being stupid about it.