Dieters can get depressed if they don't eat enough calories or high-quality foods.  They can run short on tryptophan, an amino acid that becomes serotonin, the important chemical involved in depression. Everyone needs a steady intake of B-vitamins, vitamin C, and an assortment of minerals for a healthy nervous system.  Furthermore, depression is more common in obese people, and food, especially sugar, is a form of self-medication. Depression may come out in full force when food, the coping tool, is removed. To address the problem, eat a balanced diet with adequate calories and fresh food and consult with your doctor and a mental health therapist.