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 increase in exercise is called cachexia and may be a symptom of a serious medical condition.

[13] According to the UK National Health Service and 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.

[18] Meal replacements, orlistat, a very-low-calorie diet, and primary care intensive medical interventions can also support meaningful weight loss.

[19][20] 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.

[21] The World Health Organization recommends that people combine a reduction of processed foods high in saturated fats, sugar and salt,[22] and reduced caloric intake with an increase in physical activity.

[29] 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.

[33] Other methods of weight loss include use of anti-obesity drugs that decrease appetite, block fat absorption, or reduce stomach volume.

[citation needed] Obesity has been resistant to drug-based therapies, with a 2021 review stating that existing medications are "often delivering insufficient efficacy and dubious safety".

[36] Both can be effective at limiting the intake of food energy by reducing the size of the stomach, but as with any surgical procedure both come with their own risks[37] that should be considered in consultation with a physician.

[48] The National Institute for Health and Care Excellence devised a set of essential criteria to be met by commercial weight management organizations to be approved.

[57] Unintentional weight loss can occur because of an inadequately nutritious diet relative to a person's energy needs (generally called malnutrition).

[52][53][58][59][60] Poor nutrient utilization can lead to weight loss, and can be caused by fistulae in the gastrointestinal tract, diarrhea, drug-nutrient interaction, enzyme depletion and muscle atrophy.

[52][57][58] In the advanced stages of progressive disease, metabolism can change so that they lose weight even when they are getting what is normally regarded as adequate nutrition and the body cannot compensate.

[54] 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.

[56] Disease-related malnutrition can be considered in four categories:[56] Inability to eat can result from: diminished consciousness or confusion, or physical problems affecting the arm or hands, swallowing or chewing.

[54] Loss of hope, status or social contact and spiritual distress can cause depression, which may be associated with reduced nutrition, as can fatigue.

[67] Diet plans in fitness magazines are also often believed to be effective but may actually be harmful by limiting the daily intake of important calories and nutrients which can be detrimental depending on the person and are even capable of driving individuals away from weight loss.