|Das, Sai Krupa -|
|Roberts, Susan -|
Submitted to: Present Knowledge in Nutrition Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: April 14, 2012
Publication Date: July 20, 2012
Citation: Das, S., Roberts, S.B. 2012. Chapter 05: energy metabolism in fasting, fed, exercise and re-feeding states. In: Erdman Jr., J.W., MacDonald, I.A., Zeisel, S.H., editors. Present Knowledge in Nutrition Book Chapter. 10th edition. Hoboken, NJ: Wiley-Blackwell. p. 58-68. Technical Abstract: Energy is expended by the body to maintain electrochemical gradients, transport molecules, support biosynthetic processes, produce the mechanical work required for respiration and blood circulation, and generate muscle contraction. Most of these biological processes cannot directly harness energy from the oxidation of energy-containing substrates (primarily carbohydrate and fat from food and body energy stores). Instead, the resulting energy from the oxidation of metabolic fuels is captured by adenosine triphosphate (ATP) in the form of high-energy bonds. ATP is the major energy carrier to body sites and releases the energy required for chemical and mechanical work. Use of that energy results in the production of heat, carbon dioxide and water, which are eliminated from the body. Nutrient degradation pathways are linked to the formation of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi) and are often referred to as ATP generation or energy generation pathways. Likewise, the term energy expenditure is used to describe the degradation of ATP to ADP and Pi. In a resting adult, ˜25–35 g ATP is used every minute to drive the life-sustaining processes, and this is approximately the total amount contained in the body at any one time. The close relationship between energy metabolism and oxygen consumption stems from the fact that oxygen is required to transform food to a usable source of energy. Because the ratio of carbon dioxide produced to oxygen consumed (i.e., the respiratory quotient) varies with nutrient type, it can be used to predict the ratio of metabolic fuels being oxidized provided that additional information on urinary nitrogen excretion is also available. Simple equations can also be used to predict energy expenditure from oxygen consumption and carbon dioxide production, of which de Weir's is probably most widely used today (2). The following sections discuss the components of total energy expenditure in relation to the fasting state, namely basal metabolic rate (BMR) or resting metabolic rate (RMR); the fed state or the thermic effect of feeding (TEF); and the activity and exercise-related energy expenditure which is termed energy expenditure for physical activity and arousal (EEPAA). Combined, these three components are equivalent to total energy expenditure, which in turn is equivalent to dietary energy requirements for non-growing individuals. In addition, the effects of changes in energy balance on energy expenditure are considered.