Constant poor nutrition will make you vulnerable to all types of illnesses and diseases. Your body will be starved for the essential vitamins it needs to remain healthy and it will catch up to you.
Poor nutrition can adversely affect your body differently, depending on exactly what is deficient in your diet. Insufficient calories will cause fatigue as well as loss of muscle mass.
Poor nutrition is really bad because your body doesn't get all of the vitamins, minerals, proteins, etc, that it needs. If you don't get these nutriets, you could become sick, lose or gain weight, or feel lethargic.