Carbohydrates are predominantly obtained from plant sources. With the exception of lactose found in milk and insignificant glycogen amounts in meat, most consumed carbohydrates have plant origins. Monosaccharides and disaccharides, or sugars, can be sourced from fruits, honey, milk, sugar cane, and sugar beets. Grains and vegetables are rich in the polysaccharide starch. Two types of polysaccharides provide fiber: cellulose, which is abundant in many vegetables, forms undigestible roughage or insoluble fiber, while soluble fiber, like pectin, found in apples and citrus fruits, contributes to lowering blood cholesterol levels.

Cells in the human body primarily use the monosaccharide glucose as fuel to produce ATP. Although carbohydrate digestion produces fructose and galactose, these are converted into glucose by the liver before joining the bloodstream. Neurons and red blood cells depend heavily on glucose for energy and The body carefully regulates blood glucose levels, as sustained or extreme deficits can impair brain function and, in severe cases, may harm neurons. However, moderate reductions in glucose can often be tolerated without lasting damage. Extra glucose, beyond what is required for ATP synthesis, is stored as glycogen or fat.

Carbohydrates also have minor applications; pentose sugars are used in nucleic acid synthesis, and a range of sugars are attached to proteins and lipids on the external side of the plasma membrane.

Across the globe, there is considerable variation in carbohydrate intake. There are several reasons for such variation in the carbohydrate percentage in the diet, including availability, abundance, or scarcity of a specific food type, culture, and lifestyle choices.

The recommended carbohydrate intake for health maintenance is 45-65% of total calorie intake, with a focus on complex carbohydrates sourced from whole grains and vegetables as opposed to simple carbohydrates. However, overconsumption of refined carbohydrates in candy and soft drinks can lead to nutritional imbalances and obesity.