I'm glad you asked, because I've got a solution to that problem that I've contemplated, blogged and posted on numerous times.
Let's provide universal health insurance.
Once done, it's not an invasion of privacy for a certified provider to weigh you.
If they find you are obese, they put you on a program to lose weight. We're not talking boot camp here, but just good sense.
You will be provided with all the help you could possibly need from mental health counseling to shopping advise. The program will even go so far as to go shopping with you and go into your home and help you cook healthy. You will be required to log all caloric input to your body. That will be easy also as it will be done by computer that will automatically download to a central system and to your provider.
This will all be gradual, but with goals in mind.
If after a certain length of time you have made no progress toward getting your health under control, you will be unplugged from the system and only provided with emergency health care which will consist of little more than "comfort measures only."
With this program, we not only lead the person to water, but we provide extreme measures that encourage, help and motivate them to drink.
Same type of program applies for tobacco, alcohol and drug addicts.
People can eat good tasty food. People can eat luxurious food. People just need to learn how.
If I've left anything out that might be required for such a program or overlooked some major or minor flaw, I would love to hear about it.
I don't see how this could be considered unconstitutional. Healthcare is not a constitutional right.