Energy Expenditure Technologies

The measurement of energy expenditure (EE) is valuable in understanding the etiology of obesity and in the prevention and treatment of excess body weight. There are several methods that have been developed to quantify the various components of EE in as precise and accurate a manner as possible.

Two fundamental approaches to the objective measurement of EE are direct and indirect calorimetry. Direct calorimetry measures EE as the rate at which heat is lost from the body to the environment, including the nonevaporative heat losses (conduction, convection, and radiation) and evaporative heat loss in the form of water vapor.

Direct calorimetry usually involves whole body measurements in an enclosed chamber. Today, very few calorimetric chambers work on the principle of direct calorimetry because it is technically much more difficult than indirect calorimetry. Indirect calorimetry predicts heat production (EE) from rates of respiratory gas exchange, that is, oxygen (O2) consumption and carbon dioxide (CO2) production. There are several indirect calorimetric methods to measure EE. In the closed-circuit method, the subject is kept in a sealed room (calorimeter room and respiration chamber), and this chamber is ventilated with a constant supply of fresh air. The subject's respiratory gas exchange is measured by comparing the composition of well-mixed air in the chamber with the composition of air entering the chamber, together with the flow rate of air. Most chamber calorimeters are furnished and include television, radio, telephone, some exercise equipment, and toilet and washing facilities, thus permitting measurements that approximate sedentary existence with tight control on intake and activity.

Careful monitoring of the chamber and gases is required to ensure accurate measurements. In the open circuit system, a hood or canopy is worn over the subject's head and is ventilated with room air that enters the hood. As the subjects breathes under this hood or canopy, the airflow and the percentage of O2 and CO2 are measured using a metabolic cart consisting of gas analyzers for O2 and CO2. The openness and portability of the metabolic cart make it the method of choice for measuring basal EE and particularly for exercise-related EE. The doubly labeled water method is a form of indirect calorimetry because it measures CO2 production. One advantage of this method over the chamber is that it does not restrict physical activity and is currently the best objective method to measure free-living EE over a longer period of time, for example, 7 to 14 days.

There are several technologically advanced devices that allow continuous monitoring of the various components of EE with little interference to the subject's activity. These include movement assessment devices such as pedometers, which are either clipped to a belt or worn on the ankle and are designed primarily to count specific movements such as steps while walking or running. Some pedometers adjust for stride length to estimate the distance walked while the more sophisticated battery-operated ones also have a sensitivity adjustment.

Portable accelerometers work on the principle that when an individual moves, the limbs and body are accelerated, theoretically in proportion to the muscular forces responsible for the accelerations and thus to EE. Advances in global positioning systems and radar technology have provided opportunities for remote monitoring of activity EE, and devices have now [Page 228]been developed to map and measure this component of EE. Physiological measurements such as heart rate, core body temperature, blood pressure, oxygen uptake while performing an activity, or a combination of such measures can also be obtained using various modern-day monitors. These measures are subject to limitations and should be interpreted with caution when used to quantify EE.

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