What are problems associated with excessive thinness (underweight)?
Being too thin (underweight, often defined as having a BMI of less than 18.5) can occur with anorexia nervosa, with other eating disorders, or loss of appetite. Many chronic medical conditions, cancers, and infections can also result in weight loss to the point of being underweight. Being underweight is linked to menstrual irregularity (which can lead to infertility) and osteoporosis in women, and greater risk of early death in both women and men.
Many people -- especially women -- are concerned about body weight, even when their weight is actually normal. Excessive concern about weight may cause or lead to unhealthy behaviors such as excessive exercise, self-induced vomiting, and the abuse of laxatives or other medications. These practices may only worsen the concern about weight.
Unexplained weight loss is sometimes an early clue to a health problem. If
someone experiences sudden weight loss for unknown reasons when not attempting
to reduce or lose weight, he or she should visit a doctor to determine if a medical condition is responsible for the weight loss.
Medical supervision is important when trying to regain a significant amount of weight. Just as weight loss involves taking in fewer calories than one burns through daily activities, weight gain involves the consumption of more calories than are needed to maintain body functions and activities. Even those trying to take in extra calories should pay attention to the nutritional content of their foods and limit high-fat foods, refined sugars, and other poor nutritional choices. Some experts also recommend weight training or other exercises to promote muscle development while attempting to gain weight. If
someone needs to gain weight, a doctor can help decide on an eating and exercise plan to best help
him or her accomplish this.
For those who are severely underweight and attempting to regain weight, a condition known as refeeding syndrome may occur as a complication of attempts to regain weight too rapidly. This syndrome is characterized by a number of metabolic abnormalities and imbalances in electrolyte levels that may result in serious or even fatal complications. Refeeding syndrome occurs most often in people who are extremely underweight, such as those suffering from severe anorexia nervosa (those with less than 75% of a normal body weight). Sometimes severe underweight requires hospitalization during the initial weight gain phase in order to monitor the individual's overall nutritional and metabolic status.