Unexplained weight loss means a person is losing weight without intentionally changing diet, exercise or another obvious factor sufficiently to explain the change. It can result from reduced energy intake, increased energy expenditure, impaired nutrient absorption, increased metabolic demand, fluid changes or combinations of these mechanisms. The phrase does not identify a specific disease. Causes range from relatively straightforward factors such as appetite changes or dental problems to potentially serious conditions such as cancer, gastrointestinal disease, endocrine disorders or chronic infection. Persistent or substantial unintentional weight loss should therefore be assessed rather than assumed to be benign.
Reduced food intake is a common mechanism. Appetite can fall because of depression, anxiety, grief, chronic illness, pain, nausea, altered taste or smell, medication effects or social circumstances. Dental disease, difficulty swallowing and problems with chewing can also reduce food intake even when a person wants to eat. Older adults may be particularly vulnerable to inadequate intake because multiple factors can operate simultaneously, including reduced appetite, dental problems, medication burden, changes in mobility and difficulties preparing or obtaining food. Identifying the mechanism is more useful than simply documenting the amount of weight lost.
Gastrointestinal disorders can cause weight loss through reduced intake, malabsorption, vomiting, diarrhoea or increased losses. Conditions affecting the gastrointestinal tract can impair digestion or absorption of nutrients, while chronic diarrhoea can result in substantial fluid and nutrient losses. Examples of relevant disorders include coeliac disease, inflammatory bowel disease, chronic pancreatic disorders and some persistent infections, although the exact differential diagnosis depends on the person's symptoms and circumstances. Abdominal pain, persistent diarrhoea, vomiting, blood in stool or difficulty swallowing are particularly important symptoms to report to a healthcare professional.
Endocrine and metabolic conditions can increase energy expenditure or alter nutrient handling. Hyperthyroidism can increase metabolic rate and may cause weight loss despite maintained or increased appetite, often with symptoms such as heat intolerance, tremor, palpitations or increased bowel frequency. Poorly controlled diabetes can cause weight loss because insufficient insulin activity prevents normal glucose utilisation and promotes breakdown of fat and other tissues, with glucose and water being lost through urine. Other metabolic disturbances can produce weight changes through different mechanisms.
Chronic infections and inflammatory conditions can cause weight loss through appetite reduction, increased metabolic demand and systemic inflammatory signalling. Cancer can also cause unintended weight loss, although weight loss does not mean that a person has cancer. Cancer-related weight loss may involve reduced appetite, altered metabolism, inflammation and tumour-related effects, and it can occur with or without obvious gastrointestinal symptoms. Persistent fever, night sweats, cough, enlarged lymph nodes or other systemic symptoms may provide additional diagnostic clues depending on the underlying condition.
Medications and substances can contribute to unintended weight loss. Some medicines reduce appetite, cause nausea, alter taste, increase gastrointestinal symptoms or otherwise affect nutritional intake. Alcohol and other substances can also influence eating patterns, absorption and metabolism. Mental-health conditions can have major effects on appetite and eating behaviour, including depression and anxiety, while cognitive impairment can interfere with remembering meals or obtaining food. These causes require consideration alongside physical diseases rather than being treated as separate from medical assessment.
The clinical importance of unintentional weight loss depends on its magnitude, duration, baseline body size, age and associated symptoms. A small short-term change may result from ordinary fluctuations, whereas persistent loss over several months is more concerning. Healthcare professionals may review the weight trajectory, dietary intake, medications, gastrointestinal symptoms, endocrine symptoms, mental health, systemic symptoms and functional status and may perform examination and targeted investigations. The evaluation is guided by the history rather than automatically applying the same test to everyone.
Medical assessment is particularly important when weight loss is persistent, progressive or accompanied by symptoms such as difficulty swallowing, persistent vomiting or diarrhoea, blood loss, severe pain, fever, night sweats, marked weakness or other significant changes in health. The appropriate response is not to assume a particular diagnosis from the scale alone. Weight should be considered a clinical signal that can prompt investigation of nutrition, metabolism, gastrointestinal function, medication effects and other causes. A documented pattern of unintentional loss provides substantially more useful information than a single unexpected weighing.
