ALL of these things are factors...but don't overlook the psychological effect of knowing you are doing good things for yourself. It is similar to the placebo effect...but the "pill" in this case is no overeating to deal with the winter doldrums or blue feelings, and knowing you are going in a positive direction with what you are doing to/for yourself. Negative self image and despair about being overweight is not going to make a recipe for feeling good and having enough energy...even on a sunny day, and using the BEST anti-depressant for you!!
Doing good things for yourself is the only "medicine" I know of that does not interact with your other Rx or OTC meds.