Hair Health & Diet

Does anyone have any ideas about whether improving your diet, taking supplements and exercising can improve the condition of the hair that you already have? Obviously it will benefit the new hair that grows, but does any of the above have an effect on your current hair that`s perhaps a bit dull, dry and lacklustre? I often hear people say that hair is dead and can`t be altered once it`s damaged. Any opions on this would be much appreciated.