Weight loss can be intentional, such as from dieting and exercise, or unintentional and be a manifestation of illness. Weight loss can result from a decrease in body fluid, muscle mass, or fat. A decrease in body fluid can come from medications, fluid loss, lack of fluid intake, or illnesses such as diabetes. A decrease in body fat can be intentionally caused by exercise and dieting, such as for overweight or obesity. Weight loss is normal after pregnancy. Other causes of weight loss include, but are not limited to, cancer, viral infection (such as CMV or HIV), gastroenteritis, parasite infection, depression, bowel diseases, and overactive thyroid (hyperthyroidism).
The loss of weight and lack of nutrition associated with a chronic illness is referred to as cachexia. Unexplained, unintentional weight loss is often a result of illness and should be evaluated by a health-care professional.