Physiology of digestion and absorption

Cooking food kills bacteria and other pathogens, neutralizes some toxins, e.g. in some beans, and breaks down food. Mastication breaks down food, further increasing the surface area, and thus facilitating enzymatic digestion. The major nutrients (carbohydrate, protein and fat) are broken down enzymatically to low-molecular-weight compounds. Digestive enzymes are secreted by the stomach, pancreas and small intestine.

The transport of the products of digestion into gut epithelial cells and from there to the portal blood is termed absorption. The absorption of some nutrients is passive while others require active transport.

Malabsorption

Failure of digestion is properly called maldigestion. The term ‘malabsorption’ describes impairment of the absorptive mechanisms, but in practice is used to encompass both disorders. Malabsorption is a condition that can occur at any stage of life from a variety of causes (Fig 56.1).

The clinical effects of malabsorption result from the failure to absorb nutrients. The major consequences of generalized malabsorption arise from inadequate energy intake that results in weight loss in adults and growth failure in children.

In suspected malabsorption, a detailed dietary history is essential to establish eating patterns and habits. Provided dietary input is adequate, the presence of malabsorption will often be indicated by diarrhoea and changes in the appearance and consistence of the faeces.

Figure 56.2 illustrates how the normal mucosal structure is designed to maximize absorptive capacity. While mucosal surface enzymes play an important role in digestion, the most important source of digestive enzymes is the exocrine pancreas. As with many other organs in the body, there is more than 50% reserve capacity in both the small intestine and exocrine pancreas. Thus, disorders in these organs are usually quite advanced before malabsorption can be detected by functional tests or is clinically manifest. As a result the role of functional tests of malabsorption has diminished and they have largely been abandoned.

Endoscopy and biopsy are the standard, and by far the most important, tools available for the investigation of gastrointestinal disorders. They allow both the macroscopic and microscopic investigation of the gut. Radiological investigations are important when detecting abnormal anatomy of the bowel and motility.

In the case of fat malabsorption the faeces will contain fat. This can be detected by microscopic examination of the stools. Quantitative fat analysis of the stools is now rarely performed. Where small molecules such as mono- or disaccharides are not absorbed they exert an osmotic effect in the large intestine giving rise to a large volume of watery stools.

Laxative abuse is an important diagnosis that may be missed. In cases of suspected abuse a laxative screen can be performed.

Gastrointestinal disorders

Inflammatory bowel disease

Inflammatory bowel diseases (IBD) include Crohn’s disease, and ulcerative colitis. The former can affect any section of the GI tract from the mouth to the anus and is characterized by inflammation affecting all layers of the gut. The latter is confined to the superficial mucosa of the large bowel. The clinical presentation of both diseases – typically an increase in frequency with blood and/or mucus in the stool – will usually indicate the presence of inflammatory bowel disorders. They are usually diagnosed by a combination of endoscopy, biopsy and radiology. The aetiology of these disorders is unknown and frequently there can be difficulties in making a definitive histological diagnosis. In all inflammatory bowel disorders the faecal calprotectin concentration will be increased when the diseases are active.

IBD must not be confused with IBS – irritable bowel syndrome.

Malignant disease

The small intestine contains a large amount of lymphoid tissue. Mucosal associated lymphoid tumours (MALT tumours) can cause malabsorption. There presentation is variable and they are diagnosed by biopsy.

Colon cancer is a major cause of mortality and morbidity. In its early stages it is asymptomatic hence the introduction of national screening for the condition in those over 50 years of age. Faecal occult blood has a high sensitivity but poor specificity, and is useful as a population screening tool, to select patients for colonoscopy and biopsy, to detect colon cancer.