Dietary carbohydrates are digested in the gastrointestinal tract and converted into glucose, galactose and fructose. All are absorbed into the portal blood from intestine and reach in the liver where galactose and fructose are converted into glucose. The glucose then enters into systemic blood circulation where the blood glucose is controlled mainly by two hormones; insulin and glucagon. If the blood glucose level is high, insulin is secreted from the pancreas. Insulin stimulates the entry of glucose into the cells for metabolism. Within the cells, glucose is metabolized to pyruvate (in aerobic condition) or lactate (in anaerobic condition) and generates small amount of energy as ATP by the process of glycolysis. Then, pyruvate metabolize to Acetyl CoA that enters into the citric acid cycle for complete broken down into carbon dioxide and water linked to the production of large amount of energy as ATP in the process of oxidative phosphorylation.
After the meeting of body demand, extra blood glucose enters into the liver and muscle cells where glucose is metabolized into glycogen by the process of glycogenesis. This glycogen is reserved in the liver and muscle for future use. In the fasting state or during active exercise, blood glucose level becomes fall. Glucagon hormone is secreted from the pancreas to stimulate the conversion of glycogen into glucose by the process of glycogenolysis.
As a whole, carbohydrate metabolism can be divided into two categories:
A) Catabolic pathway of carbohydrate metabolism:
Glycolysis – Glucose is the major fuel of body. It is metabolized to pyruvate (in aerobic condition) or lactate (in anaerobic condition) and generates small amount of energy as ATP by the process of glycolysis.
Glycogenolysis – Sometimes the metabolic fuel of glucose arise from the liver or muscle glycogen by the process of glycogenolysis.
Pentose phosphate pathway – It is an alternative pathway for glucose metabolism. Here glucose is directly oxidized and supplies energy as NADPH.
Citric acid cycle – Pyruvate, the end product of glycolysis metabolize to Acetyl CoA that enters into the citric acid cycle, and broken down into carbon dioxide and water, and produces large amount of energy.
B) Anabolic pathway of carbohydrate metabolism:
Glycogenesis – Glucose store in the liver and skeletal muscle as glycogen by the process of glycogenesis.
Gluconeogenesis – Sometimes glucose is synthesized from non carbohydrate precursors such as pyruvate, lactate, glycerol, and glucogenic amino acids by the process of gluconeogenesis.