Introduction

Digestion is the process of mechanically and enzymatically breaking down food into substances for absorption into the bloodstream. The food contains 3 macronutrients that require digestion before they can be absorbed: fats, carbohydrates, and proteins. These macronutrients are broken down through digestion into molecules that can traverse the intestinal epithelium and enter the bloodstream for use in the body. Digestion is a form of catabolism or breaking down of substances that involves 2 separate processes: mechanical digestion and chemical digestion. Mechanical digestion involves physically breaking down food substances into smaller particles to more efficiently undergo chemical digestion. The role of chemical digestion is to further degrade the molecular structure of the ingested compounds by digestive enzymes into a form that is absorbable into the bloodstream. Effective digestion involves both processes and mechanical or chemical digestion defects can lead to nutritional deficiencies and gastrointestinal pathologies.

The nutritional substances, minerals, vitamins, and fluids enter the body through the gastrointestinal system. Lipids, proteins, and complex carbohydrates are broken down into small and absorbable units (digested), principally in the small intestine. The products of digestion, including vitamins, minerals, and water, cross the mucosa and enter the lymph or the blood (Absorption).

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Digestion of the major food macronutrients is an orderly process involving the action of a large number of digestive enzymes. Enzymes from the salivary and the lingual glands digest carbohydrates and fats, enzymes from the stomach digest proteins, and enzymes from the exocrine glands of the pancreas digest carbohydrates, proteins, lipids, RNA, and DNA. Other enzymes that help in the digestive process are found in the luminal membranes and the cytoplasm of the cells that line the small intestine. The action of the enzymes is promoted by the hydrochloric acid (HCl), which is secreted by the stomach, and bile from the liver.

The mucosal cells in the small intestines are called enterocytes. The small intestines have a brush border made up of numerous microvilli lining their apical surface. This border is rich in enzymes. The glycocalyx is lined on its luminal side by a layer rich in neutral and amino sugars. The membranes of the mucosal cells contain the glycoprotein enzymes that hydrolyze carbohydrates and peptides, and glycocalyx is part of the carbohydrate portion of these glycoproteins that extend into the lumen of the intestine. Following the brush border and the glycocalyx is an unstirred layer similar to the layer adjacent to the biologic membrane. Solutes must diffuse across this layer to reach the mucosal cells. The mucous coat overlying the cells also continues a significant barrier to diffusion. Most substances pass from the lumen of the intestines into the enterocytes and then out of the enterocytes to the interstitial fluids.

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