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.

The least intrusive weight loss methods, and those most often recommended, are adjustments to eating patterns and increased physical activity, generally in the form of exercise. The World Health Organization recommends that people combine a reduction of processed foods high in saturated fats, sugar and salt, and reduced caloric intake with an increase in physical activity.Both long-term exercise programs and anti-obesity medications reduce abdominal fat volume. Self-monitoring of diet, exercise, and weight are beneficial strategies for weight loss, particularly early in weight loss programs. Research indicates that those who log their foods about three times per day and about 20 times per month are more likely to achieve clinically significant weight loss.

Permanent weight loss depends on maintaining a negative energy balance and not the type of macronutrients (such as carbohydrate) consumed. High protein diets have shown greater efficacy in the short term (under 12 months) for people eating ad libitum due to increased thermogenesis and satiety, however this effect tends to dissipate over time.

Medications

Other methods of weight loss include use of anti-obesity drugs that decrease appetite, block fat absorption, or reduce stomach volume. Obesity has been resistant to drug-based therapies, with a 2021 review stating that existing medications are "often delivering insufficient efficacy and dubious safety".

Weight loss industry

There is a substantial market for products which claim to make weight loss easier, quicker, cheaper, more reliable, or less painful. These include books, DVDs, CDs, cremes, lotions, pills, rings and earrings, body wraps, body belts and other materials, fitness centers, clinics, personal coaches, weight loss groups, and food products and supplements. Dietary supplements, though widely used, are not considered a healthy option for weight loss, and have no clinical evidence of efficacy. Herbal products have not been shown to be effective.