Carbohydrate metabolism

Carbohydrate normally accounts for about 50% of total dietary energy intake, but the general recommendation is for an increased consumption of complex carbohydrates. After digestion and absorption, carbohydrate is metabolized to provide energy (4 kcal/g) or is stored in muscle and liver as glycogen. The body’s carbohydrate stores are normally about 400–500 g in the fed state. Six-carbon glucose molecules are degraded by a series of chemical reactions to three-carbon pyruvate by the reactions of glycolysis; pyruvate can be further metabolized to lactate. These reactions occur in the cell cytoplasm without the involvement of molecular oxygen, so are described as anaerobic. Pyruvate (and lactate) can be further oxidized to CO2 and water by the reactions of the Krebs’ (tricarboxylic acid) cycle that occur in the mitochondria. Glucose is an essential fuel for the brain and for some other cells, notably red blood cells. Because body carbohydrate reserves are limited, and also because stored fatty acids cannot be converted to carbohydrate, the metabolism of carbohydrates in different tissues is tightly regulated. Some de novo synthesis of glucose is possible from non-carbohydrate sources, including glycerol and the carbon skeletons of some amino acids. Excess dietary carbohydrate is generally oxidized rather than stored.