Carbohydrates

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, typically with a hydrogen to oxygen ratio of 2:1 as in (CH2O)n. They are a major energy source for many organisms and play structural roles in plants and some microbes.

They are classified by the number of sugar units: monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, lactose, maltose), oligosaccharides (short chains of a few sugars), and polysaccharides (long chains such as starch, glycogen, cellulose). Simple carbohydrates include monosaccharides and disaccharides; complex carbohydrates include starch, glycogen, and dietary fiber.

Dietary sources include grains, fruits, vegetables, legumes, and dairy. Dietary fiber, comprising non-digestible polysaccharides such as cellulose, hemicellulose, and pectins, supports gut health; soluble fibers can slow glucose absorption.

Digestion begins with salivary amylase and continues in the small intestine with pancreatic amylase and brush-border enzymes, breaking carbohydrates down to monosaccharides for absorption. Glucose and other monosaccharides enter the bloodstream and are used for energy or stored as glycogen in the liver and muscles; excess carbohydrate can be converted to fat.

Health considerations include effects on blood sugar, described by glycemic index and glycemic load. Dietary guidelines generally recommend choosing minimally processed carbohydrates and adequate fiber while limiting added sugars. In nature, carbohydrates provide structural roles, with cellulose forming plant cell walls and starch serving as a major plant energy reserve.