According to Robert E. Black of the Johns Hopkins School of Public Health (JHSPH), "Underweight status ... and micronutrient deficiencies also cause decreases in immune and non-immune host defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes." People who are malnourished raise special concerns, as not only gross caloric intake may be inadequate, but also intake and absorption of other vital nutrients, especially essential amino acids and micronutrients such as vitamins and minerals.In women, being severely underweight, as a result of an eating disorder or due to excessive strenuous exercise, can result in amenorrhea (absence of menstruation), infertility or complications during pregnancy if gestational weight gain is too low.Malnourishment can also cause anemia and hair loss. Being underweight is an established risk factor for osteoporosis, even for young people. This is seen in individuals suffering from relative energy deficiency in sport, formerly known as female athlete triad: when disordered eating or excessive exercise cause amenorrhea, hormone changes during ovulation leads to loss of bone mineral density. After this low bone mineral density causes the first spontaneous fractures, the damage is often irreversible. Although being underweight has been reported to increase mortality at rates comparable to that seen in morbidly obese people, the effect is much less drastic when restricted to non-smokers with no history of disease, suggesting that smoking and disease-related weight loss are the leading causes of the observed effect. Treatment
Diet
Underweight individuals may be advised to gain weight by increasing calorie intake.
It may also occur as a result of oral surgery, laparoscopy, and cricothyrotomy. In a pneumonectomy, in which an entire lung is removed, the remaining bronchial stump may leak air, a rare but very serious condition that leads to progressive subcutaneous emphysema. Air can leak out of the pleural space through an incision made for a thoracotomy to cause subcutaneous emphysema. On infrequent occasions, the condition can result from dental surgery, usually due to use of high-speed tools that are air driven. These cases result in usually painless swelling of the face and neck, with an immediate onset, the crepitus (crunching sound) typical of subcutaneous emphysema, and often with subcutaneous air visible on X-ray.One of the main causes of subcutaneous emphysema, along with pneumothorax, is an improperly functioning chest tube. Thus subcutaneous emphysema is often a sign that something is wrong with a chest tube; it may be clogged, clamped, or out of place. The tube may need to be replaced, or, when large amounts of air are leaking, a new tube may be added.Since mechanical ventilation can worsen a pneumothorax, it can force air into the tissues; when subcutaneous emphysema occurs in a ventilated patient, it is an indication that the ventilation may have caused a pneumothorax. It is not unusual for subcutaneous emphysema to result from positive pressure ventilation. Another possible cause is a ruptured trachea. The trachea may be injured by tracheostomy or tracheal intubation; in cases of tracheal injury, large amounts of air can enter the subcutaneous space.