Digestion and absorption

The major components of the diet are starches, sugars, fats and proteins. These have to be hydrolysed to their constituent smaller molecules for absorption and metabolism. Starches and sugars are absorbed as monosaccharides; fats are absorbed as free fatty acids and glycerol (plus a small amount of intact triacylglycerol); proteins are absorbed as their constituent amino acids and small peptides.

The fat-soluble vitamins (A, D, E and K) are absorbed dissolved in dietary lipids; there are active transport systems (section 3.2.2) in the small intestinal mucosa for the absorption of the water-soluble vitamins. The absorption of vitamin B (section 11.10.1) requires a specific binding protein that is secreted in the gastric juice in order to bind to the mucosal transport system.

Minerals generally enter the intestinal mucosal cells by carrier-mediated diffusion and are accumulated intracellularly by binding to specific binding proteins (section 3.2.2.1). They are then transferred into the bloodstream by active transport mechanisms at the serosal side of the epithelial cells, usually onto plasma binding proteins. The absorption of calcium is discussed in section 11.15.3.1, and that of iron in section 4.5.1.

After reading this chapter you should be able to:

  • describe the major functions of each region of the gastrointestinal tract;
  • describe and explain the classification of carbohydrates according to their chemical and nutritional properties and explain what is meant by the glycaemic index;
  • describe and explain the digestion and absorption of carbohydrates;
  • describe and explain the classification of dietary lipids and the different types of fatty acid;
  • describe and explain the digestion and absorption of lipids, the role of bile salts and the formation of chylomicrons;
  • describe and explain the classification of amino acids according to their chemical and nutritional properties;
  • describe the levels of protein structure and explain what is meant by denaturation;
  • describe and explain the digestion and absorption of proteins;
  • describe the absorption of minerals, especially iron.
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