The sun is the ultimate source of all energy required for human life, as its thermonuclear energy is converted to chemical-bond energy in plants and animal tissues. When humans consume plant and animal tissues as food, the energy contained in chemical bonds is liberated and used immediately or stored. Free energy changes from the disruption of chemical bonds are used to perform all the tasks that require energy: e.g., circulating blood, moving ions across membranes, exercise, etc. In a sense, human energy metabolism is similar to an internal combustion engine in which energy substrates (food or gasoline) go in, are combusted, and heat and work are produced. Metabolic energy is expressed in kilocalories (kcal, 1000 calories) in the United States and kilojoules (kJ, 1000 joules) or megajoules (MJ, 1000 kJ = 239 kcal) everywhere else. Because energy can neither be created nor destroyed, the energy that is not immediately needed from food intake is stored as chemical bonds in triglyceride (fat), glycogen (carbohydrate) and, arguably, skeletal muscle (protein). The human system, as well as the systems of most other taxa, has devised mechanisms to efficiently store energy. The storage of energy frees the body from the demands of continuously adding energy to the system. Changes in storage are primarily reflected, at least over the long term, by increases or decreases in body fat. Of course, in reality, the flow of energy is a bit more complicated.
Nutrition Expenditure
Total daily energy expenditure (TDEE) can be roughly divided into three categories: resting metabolic rate (RMR), diet-induced thermogenesis (DIT), and physical activity (PA).Resting metabolism represents the energy demand during sitting or lying down and is used for maintenance of general body functions such as circulation, respiration, brain activity, etc. The RMR is largely determined by skeletal muscle mass, which represents a considerable source of energy demand because of energy consuming processes such as protein synthesis, ionic regulation, and heat generation. For non-athletes, RMR is often the largest component of energy expenditure (more than 50%), but for athletes in hard training, RMR may represent less than 1/3 of TDEE. This is not to say that RMR in non-athletes exceeds that of athletes though, as athletes have a higher mass-specific RMR.
Nutrition Expenditure
Total daily energy expenditure (TDEE) can be roughly divided into three categories: resting metabolic rate (RMR), diet-induced thermogenesis (DIT), and physical activity (PA).Resting metabolism represents the energy demand during sitting or lying down and is used for maintenance of general body functions such as circulation, respiration, brain activity, etc. The RMR is largely determined by skeletal muscle mass, which represents a considerable source of energy demand because of energy consuming processes such as protein synthesis, ionic regulation, and heat generation. For non-athletes, RMR is often the largest component of energy expenditure (more than 50%), but for athletes in hard training, RMR may represent less than 1/3 of TDEE. This is not to say that RMR in non-athletes exceeds that of athletes though, as athletes have a higher mass-specific RMR.