For Losing Weight While Pregnancy Measuring Energy Expenditure
Just as getting a precise value for energy intake in individuals is quite challenging, measuring human energy expenditure is neither an exact nor a straightforward process. Direct and indirect energy expenditure methods are described as follows.
Direct calorimetry measures energy expenditure in humans based on the same principles that measure the calorie content of food. That is, the heat an individual produces is proportional to the energy one is expending. A bomb calorimeter, which, in this case is an enclosed chamber large enough to hold an individual, has specialized sensors that measure the change in temperature when the individual is engaging in activity. This change in temperature measures the heat being produced and thus assesses the amount of energy being expended. Because of the advanced technology and equipment required, direct calorimetry is an expensive assessment and not without limitations. Exercise machines housed within the chamber may produce their own heat during use, resulting in an overestimation of heat produced by the exercising individual.
Further, not all heat produced from the body is “liberated” and thus may not be measured; rather, in the process known as excess postexercise oxygen consumption (EPOC), the body expends additional energy in efforts to restore homeostasis. Processes of EPOC include replenishing the energy resources, such as phosphocreatine and muscle glycogen, utilized during exercise; reoxygenating blood and restoring hormonal balance; and resuming normal ventilation and heart rate. EPOC also includes expending energy to restore normalcy of the body temperature that was elevated due to the metabolic heat produced from working muscle. The result is that not all heat is “liberated” during exercise but continues to be released after exercise is completed. Thus, detection of heat changes during exercise will fail to capture all of the heat produced in that time. These limitations must be taken into consideration when interpreting energy expenditure results; however, in comparison to other measures of energy expenditure, direct calorimetry is the most accurate method and is still utilized in research settings.
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The equipment and technology required for direct calorimetry do not make it a feasible measurement of energy expenditure for most practical applications. The small enclosed chamber is also not very representative of the environment in which “real-life” athletes perform and thus limits the types of activities that can be measured. Thus, more practical methods have been developed and are more commonly used to measure energy expended during physical activity and exercise.
Indirect calorimetry. Indirect calorimetry is based on the assumption that heat produced during activity can be assessed by measuring the volume of oxygen (O2) consumed (VO2) and carbon dioxide (CO2) expired. As energy expenditure increases, the amount of oxygen consumed and carbon dioxide expired increase proportionately. Indirect calorimetry thus allows for the calculation of the respiratory quotient, or RQ. The RQ
Volumes of O2 consumed and CO2 produced depend upon which fuel source is being metabolized by the individual. An RQ of 1.0 indicates that the individual is metabolizing pure carbohydrate; an RQ of 0.7 represents pure fat oxidation. Values in between represent oxidation of carbohydrate and fat and depend upon the exact number to estimate the contribution of each macronutrient. This measurement of O2 and CO2 gas exchange also allows the calculation of energy expenditure indirectly and thus is a measure of indirect calorimetry since the RQ value corresponds to a caloric value for each liter of CO2 produced. This technology also has the capacity to measure maximal oxygen consumption, or VO2 max. VO2 max is the maximal rate of oxygen consumed during incremental exercise, measured as mL/kg/min and is often used as a measurement of aerobic capacity. Application of VO2 max will be discussed later in this chapter.
VO2 can be measured in liters per minute (L/min), or as an adjusted measurement relative to body weight, as milliliters of oxygen consumed per kilogram of body weight per minute (mL/kg/min). One liter of oxygen consumed approximates 5 kcal of energy expended, and total oxygen consumption of a given activity can be used to calculate energy expenditure (total liters consumed x 5 kcal). Energy expenditure of a given activity is typically expressed in terms of kilocalories expended per minute (expressed as kcal/min). It is important to understand this is a measurement of total kilocalories expended during the time frame and not just the kilocalories used for the specific activity, because the measurement includes calories expended for other metabolic functions. Thus, when athletes see their caloric expenditure for specific activities based on indirect calorimetry, this is total energy expended for all physiological processes during that time and not just the calories used for the activity.
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