Undernutrition-type malnutrition can be caused by a decreased food intake, a decreased absorption of food, or an increased demand for nutrients.
A decreased food intake might be due to a lack of available food, a poor appetite, or a medical condition. A decreased absorption might be due to gastrointestinal conditions such as inflammatory bowel disease[1] or gastritis.[2] Other medical conditions can affect the absorption of nutrients as well, especially conditions affecting the liver, pancreas, or gallbladder.[3][4][5][6]
In some situations there is an increased demand for nutrients. Wound healing after an injury or an operation will increase the energy needs of the body, while burn wounds can increase fluid and electrolyte requirements.[7][8]
Overweight and obesity are caused by prolonged periods of eating more calories than the body needs and can use. The extra calories are stored as fat. Factors that affect weight and appetite include genetics, physical activity, nutrition habits, socioeconomic factors, and mental health conditions.[9]