Methods Of Measurement

Many methods of measuring energy expenditure have become available over the years, and they vary in complexity, cost, and accuracy (12). It is important to gain an appreciation of the differences in the methods and of their applications in laboratory and other settings. The techniques used to measure total daily energy expenditure and its components are briefly described below. A more detailed explanation of the laboratory methods of measuring energy expenditure has been published (13).

The most widely used methods for measuring the energy expenditure involve indirect calorimetry. Direct calorimetry (measurement of heat loss from a subject) has been used to measure energy expenditure, but the high cost and complicated engineering of this method have discouraged investigators from using this approach.

Indirect Calorimetry

The term indirect refers to the estimation of energy production by measuring O2 consumption and CO2 production rather than by directly measuring heat transfer. This method requires a steady state of CO2 production and respiratory exchange and subjects with a normal acid-base balance. To determine the RMR, measurements are usually taken with the subject in a supine or semireclined position after a 10- to 12-hour fast. Depending on the equipment, the subject typically breathes through a mouthpiece, face mask, or ventilated hood or is placed in a room calorimeter in which expired gases are collected. Typical RMR values range from 0.7 to 1.6 kcal/min, depending on the subject's body size, body composition, level of physical training, and gender. The room in which the measurements are made is usually darkened and quiet, and the volunteer remains undisturbed during the measurement process. Measurement of RMR typically lasts 30 minutes to 1 hour, whereas postprandial measurements frequently take 3 to 8 hours. These measurements are generally easily reproducible (with a coefficient of variation below 5%).

Several methods have been used to measure O2 consumption and CO2 production at rest. Generally, an "open circuit" method is used in which both ends of the system are open to atmospheric pressure and the subject's inspired and expired air are kept separate by means of a three-way respiratory valve or nonrebreathing mask. The expired gases are usually collected in a Douglas bag or Tissot respirometer for measurement of O 2 and CO2 content. Hyperventilation may occur in subjects who are not well adapted to a mouthpiece and may result in inappropriately high levels of O 2 consumption and CO2 production. When a mask is used, it is frequently difficult to obtain an airtight seal around the subject's nose and mouth.

To circumvent some of these problems, ventilated hoods have been developed in which the subject is fitted with a transparent hood equipped with a snugly fitting collar. Fresh air is drawn into the hood via an intake port, and expired air is drawn out of the hood by a motorized fan. The flow rate is measured by a pneumotachograph, and aliquots of the outflowing air are analyzed for O 2 consumption and CO2 production after temperature and water vapor content have been adjusted. O2 consumption and CO2 production are calculated from the differences in their concentrations in the inflowing and outflowing air and the flow rate. Ventilated hoods are excellent for both short- and long-term measurements but are less useful in measuring the energy expenditure of physical activity; in the latter case the subject may find the hood uncomfortable, and there is a problem with dissipation of perspiration and water vapor.

Measurement of the energy expenditure of physical activity has traditionally presented several methodological challenges. Indirect calorimetry using a mouthpiece or face mask has been used to assess O2 consumption and CO2 production. This method generally yields reliable and accurate measurements of the energy cost of physical activity in a laboratory setting but provides no information about the energy cost of physical activity under free-living conditions because of the stationary nature of the equipment. Portable respirometers use a face mask with valves that direct expired air through collection tubes to a respirometer carried on the subject's back. The respirometer contains a flowmeter and a sampling device that collects an aliquot of expired gases for analysis at a later time. There are drawbacks to this method: first, there is an inherent delay in obtaining results, and second, the rate of energy expended during work performance is integrated over the entire period of gas collection.

In an attempt to avoid some of the problems associated with measurement of free-living physical activity, several less complicated (and less accurate) methods have been devised. These methods use physiologic measurements, observation, and records of physical activity, as well as activity diaries or recall. Heart-rate recording, used to measure energy expenditure, is based on the correlation between heart rate and oxygen consumption during moderate to heavy exercise ( 13, 14). The correlation, however, is much poorer at lower levels of physical activity, and a subject's heart rate may be altered by such events as anxiety or change in posture without significant changes in oxygen consumption.

It is possible to estimate energy expenditure over relatively long periods of time by measuring energy intake and changes in body composition. However, there are errors inherent in attempting an accurate determination of energy intake over several days, weeks, or months, as well as in the methods available for determination of body composition.

Time-motion studies have also been used to estimate the energy expenditure of physical activity in real-life situations. In time-motion studies, detailed records of physical activity are kept by an observer, and energy expenditure is estimated from the duration and intensity of the work performed. The major problem with this method is the marked individual variations in the energy costs of doing a particular task.

Physical activity diaries and physical activity recall instruments have been used to quantify the energy costs of different activities over a representative period of time. Record keeping is often inaccurate and may interfere with the subject's normal activities. Furthermore, the subject's recall of physical activity depends on his or her memory, which may not always be reliable. Measuring motion by devices such as a pedometer or an accelerometer may provide an index of physical activity (i.e., counts) but does not quantitate energy expenditure. In summary, measurement of free-living physical activity continues to be the most significant challenge in the field of energy metabolism.

In recent years, large respiration chambers have been built in laboratories. Such a chamber operates on the same principle as the ventilated hood system: it is essentially a large, airtight room in which temperature and humidity are controlled. Fresh air is drawn into the chamber and allowed to mix. Simultaneously, air is drawn from the chamber, and the flow rate is measured and analyzed continuously for O2 and CO2 content. The size of the room affords the subject sufficient mobility to sleep, eat, exercise, and perform normal daily routines, making detailed measurements of energy expenditure possible over a period of several hours or days. Room calorimeters are probably the best method currently available for conducting short-term studies (several days) of energy expenditure in humans when the object is to measure RMR, TEF, and the energy expenditure of physical activity. Physical activity level is quantified by a radar system that is activated by the subject's movement within the chamber. As with other movement devices, the radar system does not quantitate the intensity of activity. It is also likely, however, that free-living physical activity is blunted in the room calorimeter because of its confining nature. Thus room calorimeters do not offer the best model for examining adaptations in free-living physical activity. Although room calorimeters are moderately expensive to construct, they provide reliable information on daily energy expenditure and substrate oxidation.

Substrate Oxidation

The assessment of nutrient use is frequently used in combination with the assessment of energy expenditure. This area has been previously reviewed ( 14) and is

briefly summarized in this chapter. When the measurement of V02 is available (in liters of 02 STPD [standard temperature (0°C), pressure (760 mm Hg), and dry] per minute), metabolic rate (^i), which corresponds to energy expenditure, can be calculated (in kJ/min) as follows:

where 20.3 is the mean value (in kJ/L) of the energy equivalent for the consumption of 1 L (STPD) of O 2. To take into account the heat generated by the oxidation of t t the three macronutrients (carbohydrates, fats, and proteins), three measurements must be performed: oxygen consumption (V02), carbon dioxide production (V VCO2), and urinary excretion (N). Simple equations for computing metabolic rate (or energy expenditure) from these three determinations are written in the following form:

The factors a, b, and c depend on the respective constants for the amount of O2 used and the amount of CO2 produced during oxidation of the three classes of nutrients (TableA,!,,). An example of such a formula is given below:

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Table 5.1 Energy Equivalent from Oxidation of Substrates

Table 5.1 Energy Equivalent from Oxidation of Substrates

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Responses

  • prospero
    What are the modern method of measurement?
    8 years ago

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