Introduction

It is the purpose of this paper to review the relative merits of approaches used in collecting and analysing human energy expenditure data from an anthropological and population perspective. As shown diagramatically in figure 1, expenditure patterns of population segments result from (a) the nature of local energy acquisition, including relevant environmental, technological, and exchange factors, and (b) the quantity and patterns of energy consumption. Total energy expenditure, in turn, consists of energy spent carrying out any number of functions, including those necessary to ensure the continued well-being of a population. Such functions involve individual biological as well as group economic maintenance and reproduction.

In reference to figure 1, it should be noted that expenditure is not simply a dependent variable. It can assume a determinant role in limiting and channelling a population's energy acquisition, principally through its potential influence on group work capacity. In this way, the three components of generalized energy flow - energy acquisition, consumption, and expenditure - together comprise the elements of a complex feedback system.

In this chapter we are concerned with methods for estimating the overall time and energy flows from which a human system can be described and adaptive behavioural patterns assessed. These considerations can suggest to researchers locally relevant, nutrition-related problems of particular population segments, or particular types of activities that warrant more detailed examination and possible intervention.

Procedures for collecting and analysing data pertaining to all three components of a flow study are outlined in table 1. In the sections that follow, those procedures related to energy expenditure will be considered in detail, with major emphasis placed specifically on the measurement of economically related activities and labour patterns. Because of the difficulty of obtaining accurate measures under field conditions, less attention will be given to a consideration of biological factors that influence individual variations in expenditure (e.g. the energy costs associated with physiological maintenance, pregnancy, growth and development. The reader is referred to the following for a summary of these data: Brody (1945), Durnin and Passmore (1967), Consolazio (1971), Garrow (1974, 1978), FAO/WHO/UNU (1985), and Durnin (1987).

Fig. 1. Factors affecting human energy flow

Table 1. Methods and procedures used in measuring and analysing energy flow

1. Energy acquisition

Collection of data

Regional survey of production
Questionnaires and conversations concerning production techniques
Direct measurement

• of production per land unit or animal, per time unit
• of food stores
• of wastage and loss preceding consumption
• in experimentation with productive techniques

Conversion to caloric values
Evaluation of the measurement period
Representativeness of annual production
Annual variability in production

2. Energy consumption

Collection of data
Weighing of food consumed
Questionnaires concerning food use
Daily recording of food items consumed
Conversion to caloric values
Establishment of consumption by sex-age group
Evaluation of the adequacy of energy consumption
Comparison with international standards
Presence of deficiency-related symptoms
Anthropometric indicators of caloric balance

3. Energy expenditure

Survey of habitual activity
Determination of critical activities
Importance in subsistence pattern
Extent to which activity is relied on
Performance effort necessary
Determination of participants in activities
Measuring energy expenditure
In field measurements
Under standardized testing conditions
Estimation of the energy cost of activities not examined
Time-motion studies
Assessment of endurance capacity in performing activities

We are assuming that a general methodological review oriented toward individuals who have not previously worked with human energetic analytical techniques would be most useful. Furthermore, it is assumed that field technicians or investigators just becoming acquainted with time-energy expenditure analysis will not have the advantage of a large research team or expensive equipment. Recommendations are, therefore, made with these points in mind.

In order to maintain continuity, many of the examples used will be drawn from research carried out by Brooke Thomas (1973a) on a Quechua Indian population. Despite its atypical setting in the high Andes of southern Peru, the Nuñoa population shares a number of characteristics with other third-world rural groups. It is reliant on both plant (hardy tubers and cereals) and animal resources (alpaca, llama, sheep) which provide marginal caloric intake. Labour is carried out by most family members without the use of machines. Finally, the group is only loosely tied to the national economy, hence many productive and distributive activities have no monetary consequences. Since labour is not sold nor is a cash value ascribed to many items exchanged, utilizing a standard economic analysis based on cash flow is largely precluded.

Beginning with an overview, figure 2 presents a schematic representation of factors influencing both individual and group work-performance requirements. Determinants of an individual's biological work potential, indicated on the left-hand side of the figure, and reviewed in comprehensive texts on work physiology (Durnin and Passmore, 1967; Edholm, 1967; LangeAnderson et al., 1971; Shephard, 1978; Astrand and Rodahl, 1986), and human environmental physiology (Folk, 1974; Damon, 1975; Frisancho, 1979; Edholm and Weiner, 1981). Individual variations in performance become meaningful in the context of their combined effects in meeting group performance requirements (right side of figure 2). Depending upon behavioural, demographic, and environmental conditions, essential tasks vary in terms of their frequency of occurrence, duration, and intensity of work (Thomas, 1973b). From an energetic point of view, power requirements or rates of expenditure per unit time and total energy input are most relevant. Energy expenditure measures, however, need to be accompanied by some assessment of an individual's ability to complete a task. This item will be discussed under "endurance capacity" in a later section.

Fig. 2. Factors influencing individual work performance and group performance requirements (after Thomas, 1973b, p. 65).

Two pieces of information are required to determine total energy expended for a given task: the rate of energy expenditure, usually expressed as kilocalories per minute (kcal/min), and the amount of time spent performing the task. Thus,

Total energy expenditure = Energy cost of activity (kcal/min) x time spent in activity (min)

A number of techniques, which vary in accuracy and practicality under field conditions, are available for estimating these two variables. The sections that follow briefly review (a) survey techniques used for determining "critical activities," defined as those activities for which actual measurement of expenditure rates is desirable; (b) methods of indirect calorimetry and derived techniques for measuring expenditure rates of activities deemed critical; and (c) time-allocation methods and alternative methods of estimating energy expenditure rates of activities not measured.