The varying risks of IBD in different countries are not entirely due to purely inherited or genetic reasons. The increasing incidence of Crohn’s disease observed in Japan suggests that environmental factors have an important effect on the risk of developing IBD. In addition, studies of South Asian immigrants to North America have shown that the individuals who immigrate keep the lower risk of IBD that is seen in their country of origin, whereas their children, who are generally born and raised in North America, have a higher risk of developing IBD in their lifetime. These variations in the incidence of IBD provide clues as to the possible contributing factors or causes and have led to a number of interesting theories and questions that are undergoing further testing.
North-South Gradient
The incidence of IBD has generally been highest in North America and Northern European countries and lower in the countries at more southerly latitudes. This has been described as a “north-south gradient.” However, this gradient is not unique to the north-south comparison but probably reflects an underlying gradient between developed and developing countries.
Where in the Body Does IBD Occur?
Inflammatory bowel disease occurs in specific sections of the gastrointestinal tract, or gut. Before considering what has gone wrong in Crohn’s disease and ulcerative colitis, we need to understand how a healthy gut works. The normal functioning of the intestinal immune system may go awry in inflammatory bowel disease.
Critical for Life
The gastrointestinal tract serves several critical functions that help to keep us alive. It allows nutrients, water, minerals, and vitamins to enter our body while keeping out harmful substances.
Functions of the Gastrointestinal Tract
The gastrointestinal tract is a tubular structure that extends from the mouth all the way down to the anus. The gut has two vital functions — nutrient absorption and immune protection.
Principal Parts of the Gastrointestinal Tract
Nutrient Absorption
The primary job of the gut is to take in and absorb nutrients. These nutrients provide the building blocks and fuel needed to maintain all other bodily functions. The gut absorbs water, minerals, and vitamins from the food and drinks that are ingested.
Immune Protection
At the same time that it allows or promotes absorption of nutrients, the gastrointestinal tract must keep numerous potentially harmful items out of the body. These include microscopic organisms, such as bacteria, viruses, and parasites, as well as certain dangerous proteins that may cause disease if absorbed into the body from the gut. The gastrointestinal tract is, therefore, an important part of the body’s immune system.
Principal Parts of the Gastrointestinal Tract
The gastrointestinal tract has six major components: mouth, esophagus, stomach, small intestine, large intestine, and anus. These may all be affected by inflammatory bowel disease.
Mouth
The mouth and the structures within it (lips, teeth, tongue, and palate) are involved in the ingestion of food. The teeth allow the grinding of food into small particles that are more easily broken down and digested by the enzymes present farther down in the intestine. The lips, tongue, and palate assist with the chewing and swallowing of food.
Esophagus
The esophagus (or gullet) is a tube that transports food, once it is swallowed, from the mouth to the stomach. A valve at the bottom of the esophagus prevents food and stomach acid from coming back up into the esophagus and into the mouth, where it can cause heartburn, which can result in damage to the inner lining of the esophagus. When you vomit, this valve opens up to allow acid and food to come out, and when you burp, it opens to allow gas to come out.
IBD Ulcers
An area that has lost its mucosal lining is called an ulcer. When people talk about ulcers, they are usually referring to duodenal or gastric (stomach) ulcers, which are typically different from the ulcers that may occur in inflammatory bowel disease. In Crohn’s disease and ulcerative colitis, the ulcers usually occur in the small intestine and large intestine and much less commonly in the stomach and duodenum.
Stomach
This saclike structure lies in the upper part of the abdomen. It receives and holds food that has recently been eaten and slowly pushes it down into the small intestine, where most of the absorption of nutrients occurs. There is a valve at the lower end of the stomach that helps to regulate how quickly the food leaves the stomach to enter the small intestine. The stomach provides an important signal to the brain to indicate when you have eaten enough.
The stomach also secretes acid from its lining. This helps to protect against infections caused by harmful bacteria that might inadvertently be ingested during a meal. The stomach acid also helps with the initial digestion of proteins in food. An enzyme called pepsin, also produced by the stomach, provides additional help with breaking down proteins.
Small Intestine
The small intestine (or small bowel) is a tubular structure approximately 12 to 15 feet (4 to 5 m) long. It is divided into three segments: from top to bottom, these are the duodenum, the jejunum, and the ileum. In the small intestine, most of the nutrients in food are absorbed into the body.
Mucosa
The absorption of nutrients is dependent upon the presence of a highly specialized inner lining (or mucosa). The mucosa lining is made up of cells whose main reason for being is to absorb nutrients from the inside (or lumen) of the intestine and pass them through into the body, where they are available as building blocks or fuel for other body functions. The surface of the mucosa is folded into many tiny fingerlike projections, called villi, which effectively increase the surface area and, therefore, the number of cells available for absorption of nutrients.
The surface of these cells contains enzymes that help break down food into smaller components so as to be absorbed more easily. When the intestine is inflamed, as is the case in inflammatory bowel disease, the villi may be reduced in number or size — or may be wiped out altogether so that the inner lining of the intestine appears flat. This loss of normal villi results in a reduced ability to absorb nutrients. When the inflammation is severe, the mucosa lining may be completely gone, leaving the underlying tissue exposed to the inside of the intestine.
Large Intestine
The large intestine, also known as the colon, is approximately 3 to 4 feet (1 to 1.2 m) in length. Although shorter than the small intestine, it is called the large intestine because its width or diameter is greater than that of the small intestine. It is divided into several sections: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.
Primary Function
The large intestine’s primary function is to absorb fluid (water) and minerals, such as sodium and potassium, from the intestinal contents back into the tissues and into the bloodstream. By absorbing fluid, the colon causes the stool to be solid rather than liquid and helps to prevent fluid loss and dehydration.
Small Intestine