What is indigestion (dyspepsia, upset stomach)?

Dyspepsia (indigestion) is best described as a functional disease. (Sometimes, it is called functional dyspepsia.) The concept of functional disease is particularly useful when discussing diseases of the gastrointestinal tract. The concept applies to the muscular organs of the gastrointestinal tract, the esophagus, stomach, small intestine, gallbladder, and colon that are controlled by nerves. What is meant by the term, functional, is that either the muscles of the organs or the nerves that control the organs are not working normally, and, as a result, the organs do not function normally, and the dysfunction causes the symptoms. The nerves that control the organs include not only the nerves that lie within the muscles of the organs but also the nerves of the spinal cord and brain.

Some gastrointestinal diseases can be seen and diagnosed with the naked eye, such as ulcers of the stomach and can be seen at surgery, on X-rays, and by endoscopy. Other diseases cannot be seen with the naked eye but can be seen and diagnosed under the microscope. For example, gastritis (inflammation of the stomach) can be diagnosed by microscopic examination of biopsies of the stomach. In contrast, gastrointestinal functional diseases cannot be seen with the naked eye or the microscope. Accordingly, and by default, functional gastrointestinal diseases are those that involve abnormal function of gastrointestinal organs in which the abnormalities cannot be seen in the organs with either the naked eye or the microscope.

In some instances, the abnormal function can be demonstrated by tests (for example, gastric emptying studies or antro-duodenal motility studies). However, the tests often are complex, are not widely available, and do not reliably detect the functional abnormalities.

Occasionally, diseases that are thought to be functional are ultimately found to be associated with abnormalities that can be seen by the naked eye or under the microscope. Then, the disease moves out of the functional category. An example of this would be Helicobacter pylori (H. pylori) infection of the stomach. Some patients with mild upper gastrointestinal symptoms who were thought to have abnormal function of the stomach or intestines have been found to have stomachs infected with H. pylori. This infection can be diagnosed under the microscope by identifying the bacterium in biopsies from the stomach. When patients are treated with antibiotics, the H. pylori and symptoms disappear. Thus, recognition of infections with Helicobacter pylori has removed some patients’ symptoms from the functional disease category.

The distinction between functional disease and non-functional disease may, in fact, be blurry. Thus, even functional diseases probably have associated biochemical or molecular abnormalities that ultimately will be able to be measured. For example, functional diseases of the stomach and intestines may be shown ultimately to be associated with reduced or increased levels of normal chemicals within the gastrointestinal organs, the spinal cord, or the brain. Should a disease that is demonstrated to be due to a reduced or increased chemical still be considered a functional disease? In this theoretical situation, we can’t see the abnormality with the naked eye or the microscope, but we can measure it. If we can measure an associated or causative abnormality, should the disease no longer be considered functional, even though the disease (symptoms) are being caused by abnormal function? The answer is unclear.

Despite the shortcomings of the term, functional, the concept of a functional abnormality is useful for approaching many of the symptoms originating from the muscular organs of the gastrointestinal tract. To repeat, this concept applies to those symptoms for which there are no associated abnormalities that can be seen with the naked eye or the microscope.

While dyspepsia is a major functional disease(s), it is important to mention several other functional diseases. A second major functional disease is the irritable bowel syndrome, or IBS. The symptoms of IBS are thought to originate primarily from the small intestine and/or colon. The symptoms of IBS include abdominal pain that is accompanied by alterations in bowel movements (defecation), primarily constipation or diarrhea. In fact, indigestion and IBS may be overlapping diseases since up to half of patients with IBS also have symptoms of indigestion. A third distinct functional disorder is non-cardiac chest pain. This pain may mimic heart pain (angina), but it is unassociated with heart disease. In fact, non-cardiac chest pain is thought to often result from a functional abnormality of the esophagus.

Functional disorders of the gastrointestinal tract often are categorized by the organ of involvement. Thus, there are functional disorders of the esophagus, stomach, small intestine, colon, and gallbladder. The amount of research that has been done with functional disorders is greatest in the esophagus and stomach (for example, non-cardiac chest pain, indigestion), perhaps because these organs are easiest to reach and study. Research into functional disorders affecting the small intestine and colon (IBS) is more difficult to conduct, and there is less agreement among the research studies. This probably is a reflection of the complexity of the activities of the small intestine and colon and the difficulty in studying these activities. Functional diseases of the gallbladder (referred to as biliary dyskinesia), like those of the small intestine and colon, are more difficult to study, and at present they are less well-defined. Each of the functional diseases is associated with its own set of characteristic symptoms.

Picture of the organs and glands in the abdomen

Picture of the organs and glands in the abdomen