Sometimes this is the result of the disease, sometimes secondary to a nutrient deficiency such as zinc, and sometimes due to side-effects of drugs or other forms of treatment. Tumors may physically interfere with eating and digesting food. For example, tumors of the oral cavity may cause difficulties with chewing and swallowing. Tumors of the stomach or intestines may obstruct the normal passage of food or the absorption of nutrients. Cancer therapies may also have a direct impact on the gastrointestinal tract.
Some drugs cause nausea and vomiting.
Others may actually injure the cells lining the intestines. Radiation is often used to treat tumors of the oral cavity which can cause inflammation and ulceration of surrounding mucous membranes. Surgical removal of tumors involving parts of the digestive tract can obviously affect a patient's ability to eat or digest food. For example, it might be necessary to remove part of a patient's jaw, tongue, or intestines. Surgery and radiation therapy require anesthesia which involves periods of fasting before and after each episode.