The gastrointestinal tract and disease

Considering the importance of the gastrointestinal tract for the entry of essential compounds into the body, it is surprising that its function is so rarely impaired by the massive variations in quantity and composition of the food consumed by humans. Nonetheless, when it is impaired the consequences can be severe. Examples are  

Coeliac disease probably arose around 10000 bc when humans switched from the hunter-gatherer way of life to cultivation of cereals (barley, wheat and oats). An illness 

The healthy small intestine contains only a small bacterial population, unlike the colon. However, an acute infection of the mucosa by a virus, bacterium or other parasite can reduce its motility, allowing a huge proliferation of the resident bacteria. Absorption of both macro- and micronutrients is impaired, resulting in the disorder known as sprue. Folic acid is particularly poorly absorbed, causing reduced rates of repair of mucosal cells. Hence, the damage persists and worsens to create a vicious circle. Treatment involves administration of an antibiotic to kill the bacteria and folic acid to allow damaged tissue to recover. The clinical presentation includes bulky stools, steatorrhoea (fatty faeces) and weight loss. 

The intestine usually constitutes an effective immunological barrier to an invasion by pathogens but this protection can be overwhelmed by frequent repeat attacks. This may explain the prevalence of sprue in the tropics (hence the name, tropical sprue) especially for visitors who are unaccustomed to contaminated food and water and hence have no immune protection against the bacteria specific to that part of the tropics. 

Huge amounts of digestive enzymes can be produced by the gastrointestinal tract in order to digest vast quantities of food that may be ingested. Nonetheless, low activities of three enzymes can occur sucrase, lactase or pancreatic amylase, which can lead to problems. 

---