Many underweight people find themselves feeling tired all the time. This lack of energy and fatigue is a classic symptom of anemia. Other symptoms of anemia include irregular heartbeat, shortness of breath, dizziness and headache. Anemia is a disease of the blood that occurs when there's a deficiency of red blood cells. These blood cells are responsible for transporting oxygen to the organs, and when they don't show up for work, the body doesn't receive the fuel it needs to energize the body. Anemia is caused by nutritional deficiencies of iron, B-12 and folate, which is another reason why underweight people should constantly check with their doctors about whether they're consuming enough of the right foods.