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John Durnin
The assessment of the physical activity of individuals or of groups of persons is distinct from the assessment of physical fitness; indeed, although the two are related, they are better considered as almost independent entities. Physical activity is important in that it determines, either by its duration or severity or both, the amount of energy expended per day by the individual over and above the resting energy metabolism. In a nutritional context, the daily energy expended, and therefore the daily energy requirements, are of the utmost relevance, because the level of energy expenditure sets the requirements for food.
However, energy expenditure, even of a relatively high level, is but one of the factors bearing on physical fitness. Physical fitness depends on genetic endowment, past and present nutritional state and level of habitual activity, and disease. For example, two individuals of the same ethnic group living in the same town and of comparable nutritional status could vary considerably in physical fitness because of differing genetic endowment, or because one of them had spent long periods in the past in habitual physical activity of a high degree, or because one of them had an occupation that required daily heavy physical exercise. One or all of these factors could be responsible for levels of physical fitness that could differ markedly in the two individuals.
It is worth emphasizing that the amount of physical activity required to cause a significant improvement in physical fitness in a well-nourished individual, and to maintain this state, need be no more than perhaps half an hour per day, if the exercise is sufficiently intense. Such an increase does not, therefore, necessarily have a large influence on levels of total daily energy expenditure. One example of this is the case of men doing extremely heavy labour in a steel mill, where work requirements were such that these men needed to be big, muscular, strong individuals. The total duration of this very heavy work however, was only about 30 to 40 minutes per day, and thus total daily energy expenditure of these men was no higher than that of men working at only low-to-moderate levels of activity in a conveyor-belt factory.
At the other extreme, many jobs necessitate continuous physical activity of moderate intensity for most of the working day, and the energy expended may be high even though the individuals do not need to have a high level of fitness to do this sort of work.
Physical activity is therefore of importance primarily because it is the most fundamental factor affecting energy expenditure. This chapter will examine ways in which energy expenditure can be measured, and why and when such measurements are useful. Occasionally physical activity may need careful examination in situations where it may appear so excessive as to shorten working life, or where the constant fatigue resulting from it may attenuate the enjoyment of leisure time.
The concept of "physical fitness" is quite broad; and an individual may be fit to perform one given physical task but not another. However, physiologists commonly limit the definition of physical fitness to mean the fitness to endure physical tasks of moderate to high intensity. Such tasks involve movements of large muscle masses, as in walking, running, cycling. In this context, physical fitness depends largely on the adequacy of the oxygen transport and oxygen utilization systems in the body. Thus it is also common to refer to physical fitness as "aerobic fitness" or "aerobic capacity"
While physical fitness is frequently measured in physiological and sport investigations, it has rarely been a part of studies on the investigation of nutritional status. Yet a measurement of physical fitness can provide very useful information on groups of individuals where ordinary criteria of nutritional status may be inadequate. For example, some populations (e.g., in New Guinea-cf. 2) with seemingly low nutritional intakes (especially of energy and protein) and a large proportion of very lean individuals have a high degree of fitness and ability to do hard physical exercise, relative to standards found in well-nourished populations. Such findings imply that low energy and protein intakes and low quantities of body fat probably are not indicative of significant nutritional deficiency.
For working adults, a measurement of exercise capacity or fitness is also useful in assessing the degree of physical stress on the individual imposed by any strenuous activity required by the occupation. Thus, there are physiologically acceptable standards within which an individual can be expected to work without suffering undue stress or fatigue, and these can be determined relatively easily when there may be concern about whether or not work is unjustifiably onerous.
Measurements of physical fitness can easily be carried out on children as young as five or six, as well as on adults, and the use of such measurements should be encouraged in intensive nutritional investigations of populations.
This chapter is divided into two separate sections: one on the measurement of energy expenditure, mainly from the aspect of the importance of physical activity, and the other on the assessment of physical fitness or physical working capacity.
Why Should Energy Expenditure Be Measured?
Measurements of energy expenditure in free-living populations are difficult, laborious, and expensive and can usually be done on only relatively small numbers of individuals. It is therefore necessary to be sure that it is essential, or at least highly desirable, to measure energy expenditure before embarking on this onerous task.
Energy expenditure has almost certainly not been measured often enough in field conditions when it would have provided very important additional information, and sometimes it has been measured in situations where its intrinsic value has been comparatively small. Thus it is the purpose of this section to provide some brief guidelines on the use and misuse of energy expenditure measurements. Methodological problems affecting its usage are dealt with in another section.
1. In general, if knowledge is needed about energy requirements of populations on which a certain amount of relevant information already exists; energy requirements can be measured more easily and possibly more accurately by measuring energy intake. In such a population, energy expenditure studies might indicate the exact ways in which alterations in energy requirements have occurred, but they are not essential for the primary purpose.
In most situations, energy intake measurements will also estimate energy expenditure because, other than for very brief periods of up to a few days, these two must be in balance or else body weight will be steadily increasing or decreasing. If body weight is approximately stable, energy intake must be roughly equal to energy expenditure.
This illustration applies especially to the developed countries.
2. By contrast, it can be argued that when energy requirements are being assessed in developing countries, it is essential that energy expenditure studies be attempted. The reasons are multiple.
For many population groups in developing countries, food in adequate quantities may not be available. If food intake alone is measured, true energy requirements may not be obtained. When energy expenditure is measured, better information is gathered; this is particularly important in providing essential "base-line" data that will later allow an evaluation to be made of the effect of food supplementation. For example, since for a large majority of people the work situation is the critical factor in maintaining their living standard, energy may well be expended to produce the necessary work output-in subsistence farming, paid labour, caring for the household, etc.-but there may not be sufficient energy left over for any desired leisure pursuits. In such cases, measuring the total daily energy expenditure will not necessarily provide much better information than measuring total daily energy intake: they will both represent a quantity less than the desirable energy needs for that group. On the other hand, the measurement of the energy expended in work will allow the calculation of an additional amount to cover an acceptable allowance for non-occupational activities. A total daily requirement can then be estimated that will be sufficient for a reasonable way of life for these populations. Only energy expenditure measurements will permit this situation to be properly assessed.
3. The assessment of the effects of food supplements can be considerably improved by measuring total daily energy expenditure. Such information can indicate not only whether total energy expenditure may have increased during work, but also whether changes have taken place in the type and duration of leisure activities.
4. As an extension of this, only by measuring energy expenditure. almost as a routine in populations potentially "at risk" nutritionally, is it possible to determine the type of alterations in physical activity-either at work or in leisure-that occur when food shortages arise or when famine situations develop. Such information will also add considerably to our knowledge of how life styles and activity patterns may be interpreted in populations where limited food availability may be almost the permanent state, but where obvious signs of malnutrition may not exist.
5. In the assessment of seasonal factors where food availability may vary from relative plenty to comparative scarcity, measuring energy expenditure as well as food intake will also indicate the degree of energy excess or deficit at different times of the year. This occurs in many communities where hard agricultural labour may be necessary at times of the year when food is not plentiful, while at other seasons food is more or less abundant when labour is not so intense.
6. Longitudinal studies of energy expenditure and energy balance are also necessary for assessing the real energy requirements for pregnancy and lactation. Only by measuring populations who differ in nutritional status and environmental situations can it be determined how much extra food is needed by pregnant and lactating women, and how much of the "extra" energy is compensated for by physiological change in the energy output of physical activity. In other words, the physiological or "natural" reduction in physical activity in the later stages of pregnancy and in lactation has to be measured before it is possible to form a reasonably accurate estimate of the real extra needs of the pregnant or lactating woman, and therefore of the likely effect of food aid to these groups.
7. A certain amount of sociologically important information can be obtained by measuring energy expenditure. The pattern of life and its energy variability can be measured.
8. Finally knowledge can be gained about the physiological stress-primarily in the work situation but also in certain leisure pursuits-of strenuous activity to the individual by measuring energy expenditure. Especially in developing countries, many aspects of work require considerable physical effort. Only by measuring these can the stress to the individual be assessed and allowances attempted for them.
Many of these suggestions imply large amounts of new knowledge, and many more measurements of energy expenditure in field conditions, particularly in the developing countries, are needed. It is critical that new, improved techniques for doing this be developed.
However, some caution is needed in attempting to use energy expenditure measurements alone to determine whether or not work productivity may have been improved by food aid programmes. Theoretically this might be possible; but in practice, in a changing situation such as when extra food is provided, work output is affected by so many factors that proper controls over a sufficiently long period become impossible.
The practical advice given below suggests various general viewpoints and describes the different techniques. It is addressed primarily to the investigator who is relatively inexperienced in measuring energy expenditure in man. Although the techniques are described and references are given to books or monographs that provide some detail of the methods, it is often desirable that experience should be gained initially under the guidance of an experienced scientist with extensive knowledge of field conditions.
There are many pitfalls in the assessment of human energy expenditure in field situations. This section attempts to indicate these pitfalls and potential sources of error, to warn against unjustifiable conclusions from the results. and to give sufficient information so that the investigator may accumulate data and from them make deductions that will not, so far as possible, be open to damaging scientific criticism.
Need for an Energy Supply to the Body
Energy must always be available to the body if normal life is to continue. This is delivered in the form of food, of which the energy-providing nutrients are carbohydrate, fat, protein, and alcohol. The energy of these nutrients is liberated by chemical changes that require the supply of oxygen to the tissues. Energy is needed for:
There are considerable differences in the relative energy requirements, the four sets of factors mentioned above.
The resting metabolic rate or the "basal metabolic rate, " may represent a considerable proportion of the total energy expended by a man. For example, it may amount to a value of 1,600 kcal/day (6.7 MJ) for a man weighing 65 kg, or 1,300 kcal/day (5.4 MJ) for a 55 kg woman.
The energy required by physical activity or muscular work of any kind depends upon several variables, such as the type of movement, the muscle masses involved and the duration of the activity. If only energy expenditure is relevant, it may be very important to distinguish clearly and objectively between the different influences of a short strenuous burst of exercise and of long-continued mild activity. For example, 30 minutes of "very heavy" work would require an expenditure of perhaps an extra 300 kcal (1.3 MJ) above the resting level in a 65 kg man, whereas an 8-hour work shift at "light" levels would increase the resting metabolism by perhaps 1,000 kcal (4.2 MJ). The contrast between the vigorous, exhausting exercise on the one hand and the gentle, unhurried routine on the other may tend to deceive us in our assessment of their relative importance for energy expenditure.
Similarly, certain types of movement, if they involve relatively small muscle masses (for example, one arm), or if they are performed in an awkward or inefficient manner, may be very fatiguing for the individual but may require only small amounts of energy expenditure.
Therefore, objective measurement should be done wherever possible. If measurement is very difficult. the assessment of the energy required by physical activity should always be made carefully and critically. Occasionally, helpful information may be gleaned from reviews (see Spitzer and Hettinger or Durnin and
Passmore in the Bibliography) that have collected and tabulated data for many different forms of occupational and leisure activities. These values for energy expenditure may be applied, where appropriate, to individuals or population groups, but this requires fairly extensive knowledge and experience of the levels and the variability of energy expenditure.
Although it may appear at first sight improbable, the energy required for growth is comparatively small and may often be ignored as a significant factor in human energy expenditure: for example, normal growth in children from two to ten years of age accounts for only about 30 to 60 kcal per day (125-250 KJ), and the rapid period of growth at adolescence requires only about 60 to 90 kcal per day (250-375 KJ) out of a total expenditure of perhaps 2,500 kcal/day (see FAO/WHO report on Energy and Protein Requirements).
Pregnancy and lactation may be difficult conditions to assess in relation to energy expenditure. In pregnancy, the theoretical increase in energy intake needed to build up the maternal and foetal tissues may be of the order of 300 kcal/day (1.3 MJ) averaged throughout the whole period of pregnancy, but in practice this is often counterbalanced, to some extent, by a diminution in physical activity. This change in the pattern of daily life may also occur in lactation, so these are situations that are perplexing to evaluate in general terms and may pose considerable problems in measurement.
The other category of "growth" in this section is the increase in muscle mass that sometimes occurs in individuals who are engaged in recent hard physical exercise. The exercise has to be of comparatively recent inception or it must be of steadily increasing severity, or else the body reaches a state of equilibrium where no further addition to muscle mass occurs. Even where such exercise is exceptionally strenuous, and considerable increases in muscle are taking place, the extra energy required is only moderate. About 5 kcal (21 KJ) of energy are needed for every gram of tissue deposited. Thus, if 10 kg of muscle were to be added to the body during a period of 4 months (120 days)-and this is an increase that would be extremely improbable under "natural" conditions and would be seen only in men actively building-up their muscle mass by weightlifting-this amounts to 50,000 kcal (210 MJ) over the whole period, or about 400 kcal/day (1.7 MJ). In normal existence, it is unlikely that a value of even 100 kcal/day (0.4 MJ) would be encountered.
Deposition of adipose tissue will also require extra energy but again will be of small consequence in relation to the total daily energy expenditure. An increase in body fat during adulthood is almost the norm in all societies whose way of life is largely sedentary and where food availability is not unduly restricted. Usually, the increase in adipose tissue is gradual, even though it may sometimes reach considerable proportions. However, even an increase of 25 kg (55 lb) of extra adipose tissue acquired over a five-year period represents only an extra 140 kcal/day (570 KJ) (1 kg adipose tissue requires about 10,000 kcal (42 MJ) for its deposition).
The conclusion that may reasonably be derived from the above account is that the most important factors in determining the mean energy expenditures of an individual during an average day are his resting metabolic rate and the type and duration of physical activity. If the occupation alone is being examined, or if specific periods of daily life are to be evaluated, again, the type and duration of physical activity are the most relevant factors.
Change in body weight, due either to growth or other causes, is likely to be quite unimportant and may usually be ignored unless it occurs over a relatively short period of time.
Implications of the Levels of Accuracy Attainable in Measuring Energy Expenditure in Man
In attempting to measure energy expenditure in man, we are often placed in a dilemma because of the potential importance of small quantities of total daily energy. With present techniques, it is difficult to make measurements of daily energy expenditure where the end-result will have an accuracy of less than 200 kcal (0.84 MJ). Nevertheless, if this standard is not attained, the implications should be clearly understood when deductions from the results are to be applied either to large communities or to the individual. Two examples demonstrate this:
1. The reduction in the daily energy requirements of the "reference man" from 3,200 to 3,000 kcal/day (13.4 to 12.6 MJ), which is the difference between the FAO/WHO standard of 1973 (6) compared to the previous 1957 standard (7), meant that the official WHO calculation of the number of people in the world who were undernourished was reduced by some hundreds of millions.
2. A difference of 200 kcal/day (840 KJ) in the energy balance of an individual would represent a gain or loss of tissue equivalent to almost the total body weight of an average woman over a five-year period.
These examples of course oversimplify the situation, but they illustrate the importance of understanding the level of accuracy required when certain conclusions need to be drawn from the data.
Also, it is not possible to rely on a randomization of error if an inaccurate technique is used. There is much evidence to show that errors frequently cause a bias in one direction so that, for example, the correct level of physical activity or of total daily energy expenditure may be grossly overestimated. If only a comparison between two points in time (say, before and after a food aid programme) is needed, this error may be partially ignored, although it should be remembered that the actual data may be misleading.
Therefore, the objective of the study should be clearly defined beforehand and the method chosen should be capable of providing results of the required accuracy. If this is technically impracticable, at least the investigator should be aware of the extent of his possible errors.
Within this context, certain other practical points need to be made. In field conditions, it is virtually impossible to be sure, by measurement, that an individual is in energy balance. The techniques available for measurement of both energy intake and expenditure, and of changes in body composition, are not sufficiently precise to allow any exact conclusions about small changes in energy balance or in body weight. For example, it may be possible to show, perhaps at different seasons of the year. that the energy intake or the energy expenditure of an individual may have been altered. However, differences between energy intake and expenditure that theoretically would have large quantitative implications are very common in people who are manifestly in energy balance, even when these measurements are made during a time interval of seven consecutive days (see Edholm et al. and Norgan et. al. in the Bibliography).
In most field situations, as a result of technical inadequacies and biological fluctuations, it is usually not worth searching for short-term variations in energy balance in an individual. Where this is thought to be occuring in a biologically significant way, it will probably have to be deduced from indirect observations.
As a corollary to the above, it is usually justifiable to assume that, in most situations, an accurate measurement of expenditure alone will also equal the energy intake, and vice versa. Measurement of both together, which may sometimes be desirable, gives added confidence in the respective results but should preferably be done principally to investigate separate components of the problem.
A more serious problem arises when the body weight of either an individual or a group is used as the basis for calculations of energy expenditure. When this is done, it should be remembered that in many human populations the correlation of body weight and mean total daily energy expenditure rarely has a value for r of more than + 0.4 and often it is about 0.2 or 0.3. Thus, too much reliance should not be put on extrapolations of total energy output derived on a body weight basis.
Environmental and Seasonal Effects on Energy Expenditure
There may be many situations where obvious alterations in the pattern of daily life occur at different periods of the year. However, these alterations may not necessarily have a large influence on changing the levels of energy expenditure. Sufficient guidelines to indicate whether or not a more extensive investigation is required may be provided by studies on a few selected "typical" individuals at different periods.
Climatic influences are sometimes misleading since man usually maintains, in a very large variety of circumstances, a relatively constant micro-climate over much of his body by the appropriate use of clothing and housing. Nevertheless, climate may have a marked effect on the rate of physical work or on its timing, and this may equally apply to physically-active leisure.
If environmental factors appear to have a significant influence on energy expenditure, it should be borne in mind that there is probably no simple allowance that can be made for these factors, and they may need careful investigation.
"Leisure" as an Influence on Energy Expenditure
The importance of "leisure" as an influence on patterns of energy expenditures will vary considerably among individuals and in different societies. If its investigation is important, leisure should be studied carefully since many misleading impressions may be gained from non-specific information on "active" leisure. For example, it is frequently found, among populations where physically active leisure pursuits do not occur regularly and frequently, that the significance of exercise may be much overestimated; general impressions, even of a semi-quantitative nature (such as by retrospective questionnaire), may considerably exaggerate the duration of the physical activity.
Some interesting relationships have been shown to exist between energy expended in work and in leisure in an analysis of studies extending over the complete 24-hour period of each of seven consecutive days on 492 men and women working in a variety of occupations. For many of these individuals, the energy expended in work accounted for only 30-40 per cent of the total day's energy. However, in relation to physical activity, there was a correlation between the physical activity required by work and total daily energy expenditure of +0.7, whereas this value was only +0.07 for physical activity in the "leisure" period and total daily energy.
There was also no significant relationship (-0.11) between physical activity at work and during non-working or leisure time. Some of these groups of men were engaged in occupations that required heavy physical activity, but there was no indication that they were less physically active in their leisure time than men working in light occupations. Sitting both at work and at leisure occupied much of the day and even a considerable part of the working time in groups involved in "moderately heavy" or "heavy" occupations; it also accounted for between 15 per cent up to 37 per cent of the total day's energy.
However, these results apply to well-fed populations, and comparable data on populations on marginal intakes of food may well show quite different conclusions.
In general, unless the work situation alone is being studied, leisure may exert an important influence on energy expenditure and may also indicate the basic nutritional state so it will require careful and critical examination.
Children and Infants
Making accurate measurements of energy expenditure on children under the age of about ten years is difficult, and no really satisfactory techniques have been evolved for use with very young children or with infants of more than a few weeks of age. Infants who are so young that they still spend most of their time lying asleep may be measured in one of the calorimeters designed for this purpose.
If some estimate of energy expenditure of children is necessary, an approximation of variable accuracy may be obtained by collecting data on the apparent duration and degree of their physical activity.
One way in which reasonably reliable and representative information can be collected is for some of the children from the group to be watched for perhaps two or three hours and their activities recorded. If some individual children from the group are randomly selected and watched for short periods at different parts of the day, during the space of a few days it should be possible to obtain some reasonable knowledge of the general level of activity of the group, although it may not be justifiable to apply this to any particular individuals.
If no personnel are available for observational studies, some limited and mainly qualitative information may be gathered from a retrospective questionnaire, adressed to the mother in the case of young children, and perhaps covering 24 hours on two or three separate occasions.
Use of the Individual as the Population Unit
Many studies concerned with nutritional status, either as a follow-up to intervention programmes or to assess the basic state, are primarily relevant to comparatively large population groups. Nevertheless, there are at least two reasons why these studies frequently may have to be carried out in some detail on the individual. The first and obvious reason is that populations are made up of individuals who will show a diversity in almost any variable. Thus, for example, an adequate overall supply of nutritional energy for a population is no guarantee that all the individuals within the population will receive a sufficient supply of energy. Some may receive an excess and others may receive inadequate quantities and be undernourished. This is partly, but not entirely, a problem of distribution.
Second, there are methodological advantages in the investigation of individuals where we may demonstrate, for example, the variability due to age, sex, body weight, occupation, or the environment. These studies will allow results from certain groups of individuals to be extracted and applied to other comparable groups. In addition, the relevance of energy availability to the health of the individual may show marked variability within the population. We know that individuals do not appear to adapt uniformly to variations in the supply of nutritional energy, yet at present we do not understand the extent of, or the reasons for, this individual variability. We will probably still have to rely to a considerable extent on studies of individuals although, with proper selection of subjects, the results should be relevant to large populations.
Lastly, many food aid programmes are aimed at individuals: pre-school children, pregnant and lactating mothers, or the wage-earners in the household. Great care should be exerted in the selection of the population to be studied so that correct principles are used and no avoidable bias results. There are probably many reference sources of assistance to the investigator but the help of an experienced demographer or other allied field worker may also be necessary.