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.