Weight loss, in the context of medicine, health, or physical fitness, refers to a reduction of the total body mass, due to a mean loss of fluid, body fat or adipose tissue or lean mass, namely bone mineral deposits, muscle, tendon, and other connective tissue. Weight loss can either occur unintentionally due to malnourishment or an underlying disease or arise 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 conditionIntentional weight loss is commonly referred to as slimming.

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,[1][2][3] and there is substantial evidence this can prevent progression from prediabetes to diabetes type 2 with a 7-10% weight loss and manage cardiometabolic health for diabetic people with a 5-15% weight loss.[4]

Weight loss in individuals who are overweight or obese can reduce health risks,[5] increase fitness,[6] and may delay the onset of diabetes.[5] It could reduce pain and increase movement in people with osteoarthritis of the knee.[6] Weight loss can lead to a reduction in hypertension (high blood pressure), however whether this reduces hypertension-related harm is unclear.[5][failed verification]

Weight loss is achieved by adopting a lifestyle in which fewer calories are consumed than are expended.[7] According to the UK National Health Service this is best achieved by monitoring calories eaten and supplementing this with physical exercise.[7]

Depression, stress or boredom may also contribute to weight increase,[8] and in these cases, individuals are advised to seek medical help. A 2010 study found that dieters who got a full night's sleep lost more than twice as much fat as sleep-deprived dieters.[9][10]

Though hypothesized that supplementation of vitamin D may help, studies do not support this.[11] The majority of dieters regain weight over the long term.[12]

According to the Dietary Guidelines for Americans those who achieve and manage a healthy weight do so most successfully by being careful to consume just enough calories to meet their needs, and being physically active.[13]