Your diet heavily impacts your mood.  When someone has a new onset of a mood or thought disorder (broad categories of specific diagnoses) they require a medical workup to rule out a medical cause for the changes in behavior and thought processes.  Thyroid problems, deficiency in folic acid, deficiency in vitamins B1 and B12, urinary tract infections are just a few of the medical problems that can change thoughts and behaviors.  In fact elderly people who suffer from urinary tract infections often present as confused and psychotic.  Once the infection is cleared up their mental status returns to normal