For family reasons I won't go into, I watched the news & read New Scientist lighting on articles about Prozac, depression and similar, still do.
One article that sat up and wouldn't be ignored was about dieting. If you loose too much weight too quickly it starves the brain of essentials and it shrinks. The observable effects are to induce symptoms which are uncannily like depression. Is this information any use? From my own experience I can vouch for that but my experience was bound-up in a sense if failure and having to give-up a goal. I found playing badminton a real help then, it is a competetive thing, a co-operative thing, and a physical thing.
Yep I endorse the old excercise wheeze, walking to work makes me feel good, walking home much better, but that job is gone now, boo hoo.