![]() ![]() The principle of the calculation consists of three equations with the mentioned three measured variables: This method is known as gradient layer calorimetry.īrouwer ( 2) drew up simple formulae for calculating the heat production and the quantities of carbohydrate (C), protein (P) and fat (F) oxidized from oxygen consumption, carbon dioxide production and urine-nitrogen loss. In the latter case instead of preventing heat flow through the wall, the rate of this flow is measured from differences in temperature over the wall. In the first case, heat conduction through the wall of the calorimeter is prevented and the flow of heat is measured by the product of temperature difference between inflow and outflow and the rate of flow of the cooling medium. Today, heat loss is measured in a calorimeter by removing the heat with a cooling stream of air or water or measuring the heat flow through the wall. Thus, no heat could dissipate from the surroundings to the inner ice jacket. The ice cavity was surrounded by a space filled with snow to maintain a constant temperature. As the ice melted from the animal's body heat, the water was collected in a container, and weighed. The surrounding space was filled with chunks of ice ( Figure 1). The animal was placed in a wire cage, which occupied the center of an apparatus. At the end of the 18th century Lavoisier constructed one of the first calorimeters, measuring energy expenditure in a guinea pig. The measurement of heat production by measuring oxygen consumption and/or carbon dioxide production is called indirect calorimetry.Įarly calorimeters for the measurement of energy expenditure were direct calorimeters. Measuring energy expenditure means measuring heat production or heat loss, and this is known as direct calorimetry. In this process oxygen is consumed and carbon dioxide produced. The metabolism of an organism requires energy production by the combustion of fuel in the form of carbohydrate, protein, fat, or alcohol. Living can be regarded as a combustion process. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, MEASURING ENERGY EXPENDITURE Thus, preventing weight gain is more effectively reached by eating less than by moving more. An activity-induced increase in energy requirement is typically followed by an increase in energy intake, whereas a reduction in physical activity does not result in an equivalent reduction of energy intake. Consumed fat is mostly stored before oxidation, especially in heavier people, increasing the likelihood of creating a positive energy balance. As a substrate for energy metabolism, fat is less likely to be oxidized for fuel than carbohydrate or protein. When intake exceeds energy requirements, the excess is primarily stored as body fat. Additionally, underfeeding causes a metabolic adaptation as reflected in a reduction of maintenance energy expenditure below predicted values and defined as adaptive thermogenesis. Over- and underfeeding induces changes in activity-induced and maintenance energy expenditure as a function of changes in body weight and body composition. Energy balance in humans is maintained by adjusting energy intake to energy expenditure. Thus, energy expenditure can be calculated with a prediction equation for resting energy expenditure, based on height, age, weight and sex, in combination with the measurement of the physical activity level of a subject with a doubly labelled water validated accelerometer for movement registration. Variation in energy expenditure is mainly a function of body size and composition (resting energy expenditure) and of physical activity (activity energy expenditure). The indicated method to assess free-living energy expenditure is the doubly labelled water technique. Resting and meal-related energy requirements can be assessed by measuring energy expenditure with indirect calorimetry. ![]()
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