Unintentional weight loss may result from loss of body fats, loss of body fluids, muscle atrophy, or even a combination of these.
It is generally regarded as a medical problem when at least 10% of a person's 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 person's 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.
Symptoms of weight loss from ACS include severe weight loss from muscle rather than body fat, loss of appetite and feeling full after eating small amounts, nausea, anemia, weakness and fatigue.
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.