Digestion and absorption

Digestion and absorption describe the breakdown of food into small particles and their movement from the gastrointestinal (GI) lumen into the body. The main groups of nutrients absorbed from food are carbohydrates, proteins, and lipids. Water, minerals, and vitamins are also absorbed by the GI tract along with orally administered drugs. Carbohydrates are enzymatically broken down into monosaccharides, which are absorbed across the small intestinal epithelium by passive (fructose) and active (glucose and galactose) mechanisms. Similarly, proteins are broken down from polypeptide chains into individual amino acids or dipeptides and tripeptides. Their absorption depends on their chemical properties and occurs via passive and active processes that utilize a variety of membrane transporters. Lipids and lipid-soluble vitamins require bile salts for emulsification to aid absorption. Water follows osmotic gradients generated by the absorption of nutrients. Since the processes of digestion and absorption require mechanical and chemical processes that can damage the mucosa, the GI tract has evolved several defence mechanisms, including a mucous barrier and regular renewal of the epithelial lining.