Weight loss, in the context of medicine or health, is a reduction of the total body weight, which can mean loss of fluid, muscle, bone mass, or fat. The loss of weight associated with a chronic illness is referred to as cachexia. Unexpected, unintentional weight loss is a common symptom of illness and should be evaluated by a healthcare professional. Poor management of Type 1 diabetes mellitus (IDDM) can be a cause of significant weight loss. Lack of insulin triggers the release of fatty acids into blood from adipose tissue, as well as the release of protein from muscle tissue. This causes a reduction in both lean tissue and fatty tissue mass. Also of interest is to consider the weight loss as a product of the energy imbalance, because much biochemical energy is lost in the urine due to the excretion of glucose (glycosuria) and ketone bodies.

Medications, lack of fluid intake, or illnesses such as diabetes can trigger fluid loss.Infections such as HIV may alter metabolism, leading to weight loss. Hormonal disruptions, such as an overactive thyroid (hyperthyroidism) , may also exhibit as 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. Overweight and obese individuals face a greater risk of health conditions such as type 2 diabetes, heart disease, high blood pressure, and osteoarthritis. 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.

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.