It is generally regarded as a medical problem when at least 10% of a persons body weight has been lost in six months or 5% in the last month. Another criterion used for assessing weight that is too low is the body mass index (BMI). However, even lesser amounts of weight loss can be a cause for serious concern in a frail elderly person.Unintentional weight loss can occur because of an inadequately nutritious diet relative to a persons energy needs (generally called malnutrition). Disease processes, changes in metabolism, hormonal changes, medications or other treatments, disease- or treatment-related dietary changes, or reduced appetite associated with a disease or treatment can also cause unintentional weight loss. 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.Continuing weight loss may deteriorate into wasting, a vaguely defined condition called cachexia. Cachexia differs from starvation in part because it involves a systemic inflammatory response. It is associated with poorer outcomes. 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. This leads to a condition called anorexia cachexia syndrome (ACS) and additional nutrition or supplementation is unlikely to help.
Myths
Some popular beliefs attached to weight loss have been shown to either have less effect on weight loss than commonly believed or are actively unhealthy. According to Harvard Health, the idea of metabolic rate being the "key to weight" is "part truth and part myth" as while metabolism does affect weight loss, external forces such as diet and exercise have an equal effect. They also commented that the idea of changing ones rate of metabolism is under debate. 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. Health effects
Obesity increases health risks, including diabetes, cancer, cardiovascular disease, high blood pressure, and non-alcoholic fatty liver disease, to name a few. Reduction of obesity lowers those risks. A 1-kg loss of body weight has been associated with an approximate 1-mm Hg drop in blood pressure. Intentional weight loss is associated with cognitive performance improvements in overweight and obese individuals. See also
Anorexia
Cigarette smoking for weight loss
Dieting
Physical exercise
Weight gain

References
External links

Weight loss  at Curlie
Health benefits of losing weight By IQWiG at PubMed Health
Weight-control Information Network Archived 12 February 2015 at the Wayback Machine  U.S. National Institutes of Health
Nutrition in cancer care By NCI at PubMed Health
Unintentional weight loss