It depends. If obesity is accompanied by illness, then it is morbid obesity, and it is a disease. Otherwise, it is not a disease. It will be very odd for a healthy obese adault to tell his manager (or an obese child to tell his teacher) that he need to take sick leave for one or two month because he is obese.
For morbid obesity, daily regular restrictive eating, i.e., only eating the food that enough keep the homeostasis of the gastrointestinal tract, is the only effective method to keep one in health. As the body of a morbid obese person is efficienty recycling and reusing the dysfunctional cell components by the upregulated autophagy, and no extra energy and nutrition are needed for the main body.