Losing more than 100 hairs every time you brush your hair may be a sign you lack iron. “Iron is required for the formation of proteins,” Dr. Pathak says. Hair is a protein thread that grows from follicles found in the skin. The hair follicles in a person with anemia don’t get enough oxygen. The body has prioritized the oxygen it does have for vital functions because it’s in survival mode, Dr. Pathak adds. As a result, the hair falls out.