ABSTRACT
Every moment of every day the human body is engaged in a myriad of metabolic reactions in efforts to maintain homeostatic operations and other human functions and activities. The metabolic cost or energy expenditure is very large, easily 1500 to 3000 kcal or more in most adults. The total energy cost or total energy expenditure (TEE) is the sum total of all energy-releasing operations within the trillions of cells that comprise the human body. Adenosine triphosphate, along with other high-energy molecules, powers these opera tions. These molecules, principally ATP, are derived from the catabolism of energy sub strate molecules such as carbohydrates, proteins (amino acids), fat, ethanol, and their metabolic intermediates such as pyruvate and lactate. These substrates are initially pro vided by the diet whereby they can either be used immediately or incorporated into stor age forms such as glycogen, triglyceride, and even protein. All energy-requiring operations in human cells can fall into three broad groups: membrane transport, synthesis of mole cules, and mechanical work.
