Loss of appetite can have potentially devastating consequences to the health of cancer patients. It may result from several factors. For instance, the following symptoms associated with cancer may make patients less hungry and lead to appetite loss:

• Pain
• Nausea and vomiting
• Difficulty swallowing (dysphagia)
• Changes in taste and/or smell
• Premature satiety (fullness)
• Diarrhea or constipation
• Cramps
• Sore or dry mouth
• Anxiety, stress or depression 

Cancer treatments also may cause some degree of anorexia in patients. Such treatments include:

• Surgery
• Chemotherapy
• Radiation therapy
• Hormone therapy
• Biological therapy
  

In addition to killing cancer cells, these treatments typically destroy some healthy cells. This can trigger side effects that lead to anorexia, such as premature satiety (fullness), changes in taste and smell that make food less appealing, dry mouth, nausea, vomiting and others.

Two primary factors related to cancer make it more difficult for patients to process nutrients:

• Metabolic changes. Cancer can physiologically alter the body’s metabolism, the chemical processes necessary for the maintenance of life. Patients may experience an increased basal metabolic rate (measure of the rate at which a person's body "burns" energy) and an increased overall total energy expenditure. This means that patients must eat more to provide the calories necessary to replace the lost calories.
  
  
• Malabsorption. For a number of reasons, cancer patients are less likely to properly absorb nutrients. In some cases, the cancer itself may cause these problems. For instance, pancreatic cancer may cause a decrease in the production of the digestive juices that regulate absorption. In other cases, surgery to correct a problem can affect the absorption of nutrients. For example, this might occur in patients who have undergone stomach resection as a treatment for stomach cancer.
  

These factors may require patients to consume more calories to maintain their current weight and body mass, known as body mass index (BMI).  BMI is a calculation of a person’s body fat relative to their height and weight. Patients need to maintain an appropriate BMI for optimal health. However, anorexia makes this difficult because it deprives patients of the appetite necessary to consume enough foods to maintain their weight. 

As a result of this combination of appetite loss and metabolic changes, cancer patients frequently experience significant weight loss, usually defined as at least 10 percent loss of body weight over a period of six months. For example, by the time of diagnosis, 80 percent of patients with upper gastrointestinal cancer and 60 percent of patients with lung cancer have experienced significant weight loss, according to the National Cancer Institute.