Carbohydrate Digestion

Carbohydrate digestion begins in the mouth during mastication, or chewing. Salivary glands release the enzyme amylase, which breaks down starches into smaller sugars called di- and trisaccharides. This initial enzymatic breakdown is the first step in converting complex carbohydrates into forms the body can absorb and use for energy.

Carbohydrate digestion halts in the stomach because the enzyme amylase is inactivated by the stomach's highly acidic environment. The masticated carbohydrates remain largely intact until they enter the duodenum of the small intestine, where digestion resumes with pancreatic enzymes and brush border enzymes that can function in less acidic conditions.

Brush border enzymes of the intestinal microvilli break down di- and trisaccharides into monosaccharides in the small intestine. For example, the enzyme sucrase breaks down the disaccharide sucrose into glucose and fructose. These monosaccharides are then absorbed by the intestinal epithelium for energy usage throughout the body.

Pancreatic alpha-amylase is released into the duodenum to digest remaining complex carbohydrates that were not broken down by salivary amylase. This enzyme continues the breakdown of starches into smaller sugar molecules, preparing them for further digestion by brush border enzymes and eventual absorption as monosaccharides.

Simple carbohydrates are monosaccharide molecules such as glucose, fructose, and galactose. Complex carbohydrates, by contrast, are polysaccharides composed of long chains of glucose. Both types are broken down during digestion, though complex carbohydrates require more enzymatic processing to be converted into monosaccharides for absorption.

Dietary fiber is a carbohydrate found in the cellulose of plant-based foods that cannot be broken down by the body for energy. Instead, it plays a vital role in healthy digestion by helping ingested food move along the digestive system, supporting overall digestive function and health.

After carbohydrate digestion is complete, monosaccharides are absorbed by the intestinal epithelium in the small intestine. Once absorbed, these simple sugars enter the bloodstream and are transported throughout the body to be used as energy by cells and tissues.