Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, by a mean loss of fluid, body fat (adipose tissue), or lean mass (namely bone mineral deposits, muscle, tendon, and other connective tissue). Weight loss can either occur unintentionally because of malnourishment or an underlying disease, or from a conscious effort to improve an actual or perceived overweight or obese state. "Unexplained" weight loss that is not caused by reduction in calorific intake or exercise is called cachexia and may be a symptom of a serious medical condition. Intentional
Intentional weight loss is the loss of total body mass as a result of efforts to improve fitness and health, or to change appearance through slimming. Weight loss is the main treatment for obesity, and there is substantial evidence this can prevent progression from prediabetes to type 2 diabetes with a 7-10% weight loss and manage cardiometabolic health for diabetic people with a 5-15% weight loss.Weight loss in individuals who are overweight or obese can reduce health risks, increase fitness, and may delay the onset of diabetes. It could reduce pain and increase movement in people with osteoarthritis of the knee. Weight loss can lead to a reduction in hypertension (high blood pressure), however whether this reduces hypertension-related harm is unclear. Weight loss is achieved by adopting a lifestyle in which fewer calories are consumed than are expended.
Symptoms of weight loss from ACS include severe weight loss from muscle rather than body fat, loss of appetite and feeling full after eating small amounts, nausea, anemia, weakness and fatigue.Serious weight loss may reduce quality of life, impair treatment effectiveness or recovery, worsen disease processes and be a risk factor for high mortality rates. Malnutrition can affect every function of the human body, from the cells to the most complex body functions, including:
immune response;
wound healing;
muscle strength (including respiratory muscles);
renal capacity and depletion leading to water and electrolyte disturbances;
thermoregulation; and
menstruation.Malnutrition can lead to vitamin and other deficiencies and to inactivity, which in turn may pre-dispose to other problems, such as pressure sores. Unintentional weight loss can be the characteristic leading to diagnosis of diseases such as cancer and type 1 diabetes. In the UK, up to 5% of the general population is underweight, but more than 10% of those with lung or gastrointestinal diseases and who have recently had surgery. According to data in the UK using the Malnutrition Universal Screening Tool (MUST), which incorporates unintentional weight loss, more than 10% of the population over the age of 65 is at risk of malnutrition. A high proportion (10–60%) of hospital patients are also at risk, along with a similar proportion in care homes. Causes
Disease-related
Disease-related malnutrition can be considered in four categories:
Weight loss issues related to specific diseases include:

As chronic obstructive pulmonary disease (COPD) advances, about 35% of patients experience severe weight loss called pulmonary cachexia, including diminished muscle mass.