Energy Production

The body derives energy from the metabolism of carbohydrate, fat, and protein provided exogenously in the fed state and endogenously in the postabsorp-tive state. A mixture of metabolic fuels, including glucose, triacylglycerols, ketone bodies, nonesteri-fied fatty acids, alcohol, and amino acids, are present in the circulation. The proportion of these energy substrates in the blood at any one time depends on the fed or fasting state of the individual, the extent of fuel stores, and recent or current metabolic demand. In a normal, nonobese, 70-kg adult there are approximately 500 MJ (120 000kcal) contained in adipose tissue, 100 MJ (24 000kcal) stored in muscle and visceral proteins, and 4.2 MJ (1000 kcal) stored as liver and muscle glycogen. During a normal day, half of the total energy requirement is met by carbohydrate metabolism. At this rate, glycogen stores would be exhausted after 1 or 2 days of fasting. However, glycogen stores are maintained for a longer period due to the production of glucose from gluconeogenesis (Figure 1).

Carbohydrate metabolism Glucose plays a key role in body metabolism. It is the preferred metabolic fuel for many tissues and is an essential fuel for the retina, red blood cells, the renal medulla, and the brain under normal conditions. In the fed state, glucose is derived from the digestion and absorption of carbohydrates provided in the meal. To produce energy from glucose, three metabolic pathways are involved (Figure 2). Glucose is first oxidized to form pyruvate via the glycolytic pathway. Pyruvate then enters the Krebs cycle and is completely oxidized to form NADH + H, FADH2, and carbon dioxide. The NADH + H transports hydrogen to the respiratory chain, where it is used to reduce oxygen to water. The net yield of energy from the metabolism of 1 molecule of glucose is 38 molecules of ATP.

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