Anorexia is a lack of appetite that results in significant weight loss. For people with cancer, anorexia can be a result of many different factors.
One cause of anorexia is the chemicals released by the cancer itself. Cancerous tumours can release substances which alter gut functions, digestion of nutrients, and hormones affecting the appetite. Radiation and chemotherapy can also worsen the initial effects of the tumour by altering the taste and smell of foods, as well as causing nausea and vomiting.
Another potential source of anorexia in people affected by cancer is depression and pain, which can decrease the desire to eat by causing a loss of appetite or an aversion to food. 
Anorexia can lead to cachexia, which is a progressive wasting disease marked by the loss of fat and muscle. This is associated with increased toxicity of treatment, and therefore treatment interruptions and dose reductions may help. Nutritional interventions can improve outcomes by helping patients maintain weight staying on the intended treatment regimen with fewer changes, improving quality of life, and producing better surgical outcomes.