An old New York Times article I just ran across. "95% Regain Lost Weight. Or Do They?" Note the source of the alleged fact:
The success stories might not have been so surprising had it been clear that the 95 percent failure rate was so poorly founded. The figure comes not from any kind of random sampling, but from a study of 100 patients treated for obesity at a nutrition clinic at New York Hospital in the 1950's. In 1959, its authors, Dr. Albert Stunkard and Mavis McLaren-Hume, published a paper in which they concluded, ''Most obese persons will not stay in treatment, most will not lose weight, and of those who do lose weight, most will regain it.''