Fatigue is defined as a complete lack of energy or feeling of debilitating tiredness. It is a symptom often associated with many digestive system conditions. The process of digestion delivers nutrients that provide energy to all body tissues. Any condition that affects that process may produce numerous symptoms including fatigue.

Fatigue may also be a normal physical response to factors such as:

• Lack of sleep. People who get even an hour less sleep than their body requires are likely to feel drowsy. The older people get, the less soundly they sleep. Sleep requirements vary by age. Most adults need about eight hours of sleep, whereas children’s sleep requirements range from 16 to 20 hours for newborns to around 9.5 hours for adolescents.
  
  
• Physical exertion or inactivity. It is normal to feel fatigued after strenuous activity. However, the longer-term effect of physical exertion is actually an increase in energy level. By contrast, people who are inactive may become easily fatigued by even moderate exertion.
  
  
• Stress or anxiety. People who feel anxious or on edge may not be able to relax or to get proper rest, resulting in fatigue.
  
  
• Boredom or depression. These emotions can sap a person's energy, creating feelings of lethargy and fatigue.
  
  
• Lack of proper diet. People who eat too little or do not consume enough fluids are likely to have less energy and to feel more fatigued. Patients should consult their physician about the proper intake of food and fluids for their age and size.
  
  
• Vitamin deficiency. Inadequate amounts of certain vitamins (e.g., vitamin B1, folic acid, vitamin B12) can cause feelings of fatigue. Patients who do not consume a sufficient amount of these vitamins in their diet may benefit from taking a daily multivitamin.

Fatigue associated with these factors may be acute or chronic in duration. The fatigue often disappears after the patient becomes rested. Even though fatigue can greatly affect patients' lives, many individuals do not report it to healthcare professionals. Patients experiencing fatigue that lasts for at least two weeks and has no obvious cause are encouraged to notify their physician. This may indicate the presence of a condition called chronic fatigue syndrome (characterized by severe, persistent and often debilitating fatigue for six months or longer) or another underlying medical condition.

There are no specific medical tests for measuring fatigue. However, the physician will typically perform a physical examination, including various tests to identify any underlying medical causes of fatigue.