Weight loss, in the context of medicine or health or physical fitness, is a reduction of the total body weight, due to a mean loss of fluid, body fat or adipose tissue and/or lean mass, namely bone mineral deposits, muscle, tendon and other connective tissue.

Unintentional weight loss

A significant loss of total body weight is a serious, chronic illness. Substantial, unintentional weight loss is a symptom of acute or chronic illness, especially if other evidence is present.

Weight loss, for example, accompanied by insatiable thirst and hunger and fatigue may indicate diabetes mellitus, a chronic disease characterized by an abnormal accumulation of carbohydrates in the bloodstream due to insufficient production of insulin, a hormone produced in the pancreas that, when secreted into the bloodstream, permits cellular metabolism and utilization of glucose.

Poor management of insulin-dependent diabetes mellitus (IDDM), also known as diabetes mellitus type I, leads to an excessive amount of glucose and an insufficient amount of insulin in the bloodstream. This triggers the release of triglycerides from adipose tissue and catabolism of amino acids in muscle tissue. This results in a loss of both fat and lean mass, predisposing a significant reduction in total body weight.

A myriad of additional scientific considerations are applicable to weight loss, including but not limited to physiological and exercise sciences, nutrition science, behavioral sciences, and other sciences.

One area involves the science of bioenergetics including biochemical and physiological energy production and utilization systems, that is frequently evidence of diabetes, and ketone bodies, acetone particles occurring in body fluids and tissues involved in acidosis, also known as ketosis, somewhat common in severe diabetes.

In addition to weight loss due to a reduction in fat and lean mass, illnesses such as diabetes, certain medications, lack of fluid intake and other factors can trigger fluid loss. And fluid loss in addition to a reduction in fat and lean mass exacerbates the risk for cachexia.

Infections such as HIV may alter metabolism, leading to weight loss.^[1]

Hormonal disruptions, such as an overactive thyroid (hyperthyroidism) , may also exhibit as weight loss.^[2]

Intentional weight loss

Weight loss may refer to the loss of total body mass in an effort to improve fitness, health, and/or appearance.

Therapeutic weight loss, in individuals who are overweight, can decrease the likelihood of developing diseases such as diabetes ^[3]. Overweight and obese individuals face a greater risk of health conditions such as type 2 diabetes, heart disease, high blood pressure, stroke, osteoarthritis ^[4] and certain types of cancer. For healthy weight loss, a physician should be consulted to develop a weight loss plan that is tailored to the individual.

Weight loss occurs when an individual is in a state of negative energy balance. When the human body is spending more energy in work and heat than it is gaining from food or other nutritional supplements, it will catabolise stored reserves of fat or muscle.

Although weight loss may involve loss of fat, muscle or fluid, weight loss for the purposes of maintaining health should aim to lose fat while conserving muscle and fluid.^{[citation needed]}

It is not uncommon for people who are already at a medically healthy weight to intentionally lose weight. In some cases it is with the goal of improving athletic performance or to meet weight classifications in a sport. In other cases, the goal is to attain a more attractively shaped body. Being underweight is associated with health risks. Health problems can include fighting off infection, osteoporosis, decreased muscle strength, trouble regulating body temperature and even increased risk of death.^[5]

Physical Activity

Aerobic exercise

Anaerobic exercise

Strength training

Dieting

Carbohydrate restriction

Protein restriction

Fat restriction

Combination restriction

Therapeutic weight loss techniques

The least intrusive weight loss methods, and those most often recommended by physicians, are adjustments to eating patterns and increased physical exercise. Usually, health professionals will recommend that their overweight patients combine a reduction of the caloric content of the diet with an increase in physical activity.^[6]

Other methods of losing weight include use of drugs and supplements that decrease appetite, block fat absorption, or reduce stomach volume. Surgery is another method. Bariatric surgery artificially reduces the size of the stomach, limiting the intake of food energy. Some of these treatments may have serious side-effects.

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Please see links below for further informaiton:

Gastric Bypass Surgery
Lap Band (Laparoscopic Gastric Banding)
Laparoscopic Gastric Bypass Surgery
Weight Loss Holiday