Is it possible to reverse diffuse hair loss due to nutritional deficiency?

If you have nutritional deficiency and it is the only cause of your hair loss, it is possible to reverse the hair loss.  However it would be highly unlikely hair loss in the crown area is due to eating junk food and nutritional deficiency.  

A full answer can only come by seeing your scalp "up close"Given the information you have provided, there's likely to be a few things going on. A FEW GENERAL PRINCIPLES a) Hair loss from stress and poor diet cause hair loss all over the scalp - front, back, top. Fairly equal loss, left and right back and front. We call this telogen effluvium b) Hair loss from genetics occurs in a "pattern" - especially crown and temples in men. It's be advised to get an evaluation from a hair loss specialist to determine how much genetic hair loss you have and how much telogen effluvium you have or once had. It's likely both are playing a role here, and possibly other causes too