|Bibliography of Studies of the Energy Cost of Physical Activity in Humans (London School of Hygiene & Tropical Medicine, 1997, 162 pages)|
The 1985 FAO/WHO/UNU Consultation adopted the principle of using measures of energy expenditure rather than energy intake to estimate energy requirements. Because data on habitual daily energy expenditure are required for this, the Consultation emphasized the importance of Basal Metabolic Rate (BMR) and suggested that the factorial approach be used to arrive at estimates of energy expenditure. The factor with which to multiply the BMR to estimate total energy expenditure would depend upon the level of physical activity of an individual. The factor may be used for single activities or may be used to obtain an integrated value for overall periods of work. James and Schofield (1990) have used the term "Physical Activity Ratio" (PAR) for single activities. This is a ratio which expresses the energy cost of an individual activity per minute as multiples of BMR per minute. For integrated activities over a period of time they have coined the term "Integrated Energy Index" (IEI) which is the energy cost of an activity or occupation, including the pauses of rest involved in conducting that activity, expressed on a minute or hourly basis and calculated again as a ratio of the BMR. The "Physical Activity Level" (PAL) expresses the total energy expenditure and hence requirement for a 24 hour period as a function of BMR. This enables categorization of daily levels of physical activity indulged in by the individual into light (sedentary), moderate or heavy activity levels in both males and females.
Different values representing the factor may be used depending upon whether it is obtained either by direct measurements of the energy cost of physical activity per se, or, whether the cost of a particular activity has been derived from data in the published literature. Potential sources of error are numerous when total energy expenditure is derived from the latter. These depend upon the method used to measure the energy cost of the activity, the corrections made for body weight differences and the variations with age and sex of subjects (although the assumption is that corrections of body weight normalizes for the gender differences). Another potential and major source of error when published data is used is the relationship of the energy cost of the activity to other related ancillary activities, such as rest pauses or other short term diversions related to the activity itself. Distinguishing whether an activity is "pure" or "adulterated" is not only arbitrary, but is very often guess work. If the activity is of a short term nature, it is likely to a "pure" one. If, on the other hand, the activity continues for a long enough period of time, it is certain to be "adulterated" by rest pauses, diversionary activity or secondary activities unrelated to the primary task. Hence, knowledge of the duration of the activity becomes essential to decide which factor to use in order to estimate 24-hour energy expenditure.
A range of physical activities are performed by an individual over a period of a day. The types of activity are likely to vary from day to day and over a period of time. Some activities are essential for the individual and community. Many of them may be categorized as occupational activities, which are life sustaining economic activities. Leisure time activities have been described as "discretionary" activities and are considered as desirable for the well-being of the community and health of the individual and population. This latter category includes optional household or domestic tasks, socially desirable activities and activities aimed at achieving physical fitness and the promotion of health. Estimates of energy costs of habitual activities of all types are essential to enable calculations of daily energy expenditure, which in turn are needed to calculate energy requirements.
The factor used to multiply BMR to estimate energy expenditure depends on the type, level and duration of physical activity. The different types of activity undertaken by an individual can be identified and the time spent on each activity measured. The energy cost of each activity can then be obtained by measuring the subject's oxygen uptake while performing the task or type of activity. There are numerous studies of the energy cost of different activities made by this method and, although they do not provide the net energy cost of each activity above the BMR, they do provide a measure of the total energy expended during the period of physical activity. The energy cost of an activity is usually expressed per minute and the total energy expended over a period of 24 hours is then calculated based on the duration of time spent on each activity.
The energy cost of a standardized form of physical activity or exercise performed in a laboratory or clinical setting is easy to measure and it is also possible to estimate the variability in the energy costs between individuals while performing the same task in such standardized conditions. This does not hold good in a field setting when the energy cost of habitual daily activities are measured in individuals and estimates of variabilities between and within individuals are also obtained. The difficulties in field measurement are compounded by the fact that it is difficult to obtain an accurate value for those tasks that combine a variety of movements, some of which demand the use of most parts of the body, while others may involve only small muscle groups without major weight bearing or movement of the body. The between individual variations in the energy costs of such habitual activities are also likely to be large because different individuals may perform the same type of task in quite different ways.
It is important to recognize that differences in body weight and body composition of individuals will influence the energy cost of activities. It is difficult to generalize on the extent to which differences in body weight affect the energy expenditure for a given type of physical activity. A relation between the energy cost and the body weight of an individual is to be expected when the task involves moving the body, but not when it involves work on external objects. Many types of daily activity are likely to be a mixture of both types. Using published data to generate generalizable energy costs for types of physical activity encountered in daily life needs to take this into consideration, while distinguishing from published data on energy expenditure measurements made during standardized activities in laboratory settings, which, in addition, may very often provide additional data on body composition and other parameters.
In order to convert the energy cost of different activities into estimates of total daily energy expenditure information is needed on the time spent by an individual on each type of activity conducted over a period of 24 hours. It is impractical to measure the cost of all activities carried out by an individual. Far more important than taking large numbers of measurement is the accurate recording of the time spent on each "important"¹ activity. Information on this can be obtained relatively easily and inexpensively by use of recalls, questionnaires or completion of activity diaries by the individual. However, more accurate estimates would require the use of "time and motion" studies using observers, which will make the study more difficult and expensive. It is important to remember that both these sets of methods are likely to influence the normal habitual activity of the individual and hence his/her total daily energy expenditure. It is also essential to recognize that the pattern of activity is meant to reflect habitual daily energy expenditure and that energy expenditure varies not only from day to day and week to week, but also over seasons. Substantial variations in the energy expenditure of individuals are known to occur over long periods and under different circumstances. This should be borne in mind when considering energy expenditure over long periods of time.
(¹ "Important" activity is defined as either occupying a significant period of time over the 24-hour period, or else involving considerable physical effort.)
Available data on the energy cost of human physical activities have been summarized in the past. The major publications include those by Orr and Leitch (1938) and by Durnin and Passmore (1955; 1967). Summary tabulations of the energy costs of activities based on these earlier data have been provided in the Technical Report based on the International Consultation (FAO/WHO/UNU 1985) and later by James and Schofield (1990). The information is still somewhat inadequate because for several types of activities the number of subjects on whom measurements have been made is very small. This bibliography has been systematically compiled and attempts to bring together all the published material in the literature in recent years. It is aimed at nutritionists, physiologists and other health professionals interested in the energy cost of physical activity and the estimation of daily energy expenditure and the energy requirements of individuals and population groups.