As a friend of mine said (mocking those who say that they cannot lose weight): there were no fat people in Auschwitz.
Part of the key to this is that people are largely free to choose their own diets (within economic constraints). I can make anyone lose weight an keep it off--if they give me total control over their diet forever.
Plan B is drugs and/or surgery... There was a recent five-year study of older men with low testosterone. They lost, on average, 35 lbs. http://digitaljournal.com/article/324685