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Measuring energy expenditure rates

Two general methods exist for measuring an individual's rate of energy expenditure: direct and indirect calorimetry. Direct calorimetry is based on the principle that energy utilized is ultimately degraded into heat, and that the amount of heat output from the body, therefore, provides a direct measure of metabolic rate. However, since techniques for collecting such data are unsuitable for field conditions, indirect measures of metabolism are employed (see Durnin and Brockway, 1959). Indirect calorimetry is based on a knowledge of the oxidation rate of food energy which is, in turn, dependent upon oxygen utilization by the metabolizing tissue, such that a litre of oxygen will yield 4.60, 4.69, and 5.05 kilocalories for each gram of protein, animal fat, and starch respectively.

For the purpose of most energy-expenditure studies, indirect calorimetry consists of measuring the volume of expired air per unit of time and determining the percentage of oxygen utilized (Weir, 1949). By calculating the difference between the percentage of oxygen in inspired and expired air, the percentage utilized is arrived at. This value is then multiplied by the volume of expired air and corrected to standard temperature and atmospheric pressure conditions (STP) to determine the amount of oxygen consumed. Consequently, if: (a) V equals the corrected volume of expired air per minute, (b) two per cent equals the oxygen content of expired air expressed as a percentage, and (c) 20.93 is the constant per cent of oxygen in inspired air, then

Energy expenditure (kcal/min) =

A conversion factor of 4.92 kcals per litre of oxygen consumed is used for low to moderate work-levels, where it is assumed a mixed food substrate is being oxidized. At levels approaching maximum working capacity, however, a higher proportion of carbohydrate food is utilized, and the conversion factor becomes 5. Standardization of expired air volumes to 0°C and 760 mmHg pressure permits comparison of gases collected under different ambient conditions. Data derived from indirect calorimetry are presented in terms of litres of oxygen consumed per minute or kcal/min. When comparisons among individuals of different body weights are desired, data are presented in terms of kilocalories expended per minute per kilogram of body weight. For a discussion of indirect calorimetric techniques refer to Consolazio et al. (1963) or Durnin and Passmore (1967).

Although measurement of carbon dioxide production is necessary for a more precise determination of oxygen extraction rates than is possible with oxygen measurement alone, justification for omitting this measurement in energy expenditure studies is provided by Weir (1949). The Weir method reviewed above is considerably simpler and less expensive and timeconsuming, while error involved in not measuring carbon dioxide is no greater than + 0.5 per cent. For most energy-expenditure studies this error rate is of little or no significance. In order to assist the reader in calculating the energy-expenditure rate, table 4 provides a computational data-recording sheet and an example of data collected during rest. Although sampling periods should be minimized, they need to be frequent enough to track significant alterations in metabolic rate throughout the performance of an activity.

As in the use of any set of procedures, the investigator must decide the degree of precision, realism, and generality his/her measurements need to represent. Maximizing all three is frequently not possible. Thus, for many research situations, where only a general impression of energy expenditure flow is desired or where a broad comparison among different expenditure patterns is sought, reference to standardized energy costs of various behaviours found in the published literature may be sufficient. On the other hand, if one is comparing behavioural responses without clear correlates in the published values, then actual measurements of expenditure rate need to be carried out. The same is true in trying to assess and compare expenditure rates of different biological types, body sizes, or sex-age groups. In many instances, achieving a small increase in the accuracy of results may be far outweighed by the limited sampling and sizeable time commitment attendant upon the use of more precise methods.

Equipment and techniques available to measure the volume of expired air from a subject and to collect a sample of expired air for gas analysis include the Douglas bag method, the Max Planck or Kofranyi-Michaelis (K-M) respirometer, and the Oxylog. All require use of either a respiratory valve placed in the mouth or a face mask for sample collections. Summary descriptions of this equipment can be found in Durnin and Passmore (1967), Weiner and Lourie (1969, 1981), and Bassey and Fentem (1981). For the present discussion, we will simply note some of the strengths and weaknesses of each method under actual field conditions.

Table 4. Computation form for calculating energy expenditure from an expired air sample a

a. Equipment needed: stop-watch, barometer, STP correction factor table, K-M respirometer and accessories, oxygen analyser.

The Douglas bag method is both simple and reliable for collecting expired air samples over periods of from 5 to 15 minutes (Consolazio et al., 1963). The limitations of this method result from interference with locomotive activity caused by the need to carry a cumbersome 100-200 litre bag, and the limited duration of the collection or sampling period because of bag capacity. Once an expired air sample is collected, its percentage of oxygen is analysed and the volume is then recorded using a dry gas meter.

The K-M respirometer, carried in knapsack fashion, is much smaller (3 kg) than the Douglas bag so that interference with normal activities is far less severe. Because it has a built-in gas meter, it can measure consumption over extended periods at low metabolic rates, although the sampling bladder generally fills after only 10 minutes during moderate work. The major limitation of the K-M respirometer is that its design causes the equipment to begin to resist the air flow at high ventilation rates. Consequently, oxygen consumption levels tend to be underrecorded above 60 litres of expired air per minute. Nevertheless, this method has proved highly reliable for normal work activities. lts overall practicability for use under field conditions has made it the most widely employed method. A complete description of this apparatus and the sources of inaccuracy are provided in Consolazio et al. (1963) and Consolazio (1971).

The Oxylog is a lightweight, battery-driven instrument for measuring oxygen consumption. It weighs 2.5 kg and provides both a cumulative total oxygen volume and minute oxygen volume using a Medilog tape-recorder. This recorder is particularly versatile, being able to record four digital signals (e.g. oxygen, consumption, heart rate, and breath rate) for 24 hours. The equipment is well-suited for studies in which duration and intensity of activities need to be well-defined (Bassey and Fentem, 1981).

An alternative to indirect calorimetry is the monitoring of heart rates. The use of this technique is based on an association between heart rate and oxygen consumption or energy expenditure. This relationship, however, does not hold for either sedentary activities or very high levels of exertion. Moreover, although the relationship of heart rate to energy expenditure may be highly significant in a single subject at any one time, it can vary quite considerably both between individuals and within one individual under different conditions. In order to deal with these problems and increase the precision of the method, it is necessary first to establish heart rates and associated oxygen consumption rates for each subject for various levels of physical exertion. One must then develop a regression equation for each subject in order to estimate energy expenditure for monitored activities.

Detailed descriptions of problems involved in oxygen consumption estimation from heart-rate data can be found in Edholm and Fletcher (1955), Bradfield (1971), Poleman (1972), Dauncey and James (1978), Blair (1980) and Nelms (1982). Most important, heart rate shows especially high variation relative to oxygen consumption at low activity levels, and thus has a particularly poor correlation with energy expenditures in this range. In an excellent review of this literature, Blair (1980, pp. 32-33) notes: "Errors in estimation in the sedentary range are too large to be acceptable. As even very active persons spend two-thirds of their day in sedentary pursuits, the heart-rate technique alone cannot be used to estimate daily energy expenditure." In order to overcome this difficulty, Blair constructed three regression equations predicting energy expenditure from heart rate for sedentary, intermittent, and rhythmic activities. This technique resulted in estimates of average daily energy expenditure similar to those derived from accumulated time spent in each activity and the measured energy cost per minute of each activity. These values, in turn, were significantly below those calculated using a single regression equation. It therefore appears that long-term heart-rate monitoring can yield acceptable values of energy expenditure, if the investigator and subjects are willing to undergo preparatory testing in order to establish heart-kcal relationships. This, of course, is not necessary using indirect calorimetric techniques where oxygen consumption is directly measured as the activity takes place.

Despite these methodological limitations, some researchers have found heart-rate monitoring to be a useful and necessary procedure. In contrast to indirect calorimetric methods, heart-rate recorders are lightweight and capable of continuous, long-term recording of data. In addition, the equipment is less bothersome to subjects, tends to cause less subject anxiety, and may be easily used for measurements on children. Two types of instruments commonly used for recording heart rate are a heart-rate integrator, which averages heart rate within set ranges over a time period, and a continuous monitor of heart rate. The former, referred to as a "socially acceptable monitoring instrument" (SAMI) is a cigarette-pack size instrument (Baker et al., 1967). Because data are averaged, it lacks the versatility of the somewhat heavier longterm tape recorder such as the Medilog, which continuously monitors EGG. These instruments are described in Weiner and Lourie (1969), Bassey and Fentem (1981), and Spurr (1984).

In field measurements, there is a great deal of variability in testing conditions. Apart from inter-subject variation in the performance of the activity itself, values also reflect variability in conditions including weather, terrain, and the amount of time an individual has been performing the task before testing commences. When it is desirable to make comparisons in energy-expenditure rate between sex-age groups and body types, then more controlled testing conditions must be established.

In the Nuñoa study (Thomas, 1973a), typical work patterns and expenditure rates derived from field measurements were used to design standardized testing conditions for a given activity. Critical activities were performed at fixed rates, and data was collected at designated intervals during the testing period. These included data collection during a pre-test rest as well as a recovery period, which permitted a more accurate basis for making comparisons.

Table 5. Physiological characteristics of males performing a five-mile walk at 5 kph^a

a. Age, oxygen consumption (VO2), oxygen consumption per kilogram body weight, oxygen consumption as a percentage of maximal oxygen consumption, heart rate. breathing rate.

Source: Thomas, 1973a, p. 87.

Table 5 presents oxygen consumption rates and heart and breath rates for four age groups of Nuñoa males performing a standardized five-mile walking test. "Per cent maximal" refers to the oxygen-utilization rate of the activity relative to maximal oxygen consumption or working capacity. This rate provides a basis for determining the ability to sustain strenuous activity for long periods. If sex-age or body-type comparisons are to be made between activities it is, of course, best to use the same sample in all tests.

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