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Total energy expenditure (TEE) and physical activity levels (PAL) in adults: doubly-labelled water data

Over the past decade a new technique using stable isotopes has revolutionised the study of human energy expenditure. The doubly-labelled water (DLW) method permits determination of energy expenditure of free living individuals integrated over a period of, usually, between 7 and 20 days. The first data from humans were published in 1982 (Schoeller & van Santen, 1982). Since then sufficient data have accumulated to form a basis for establishing energy requirements. A database of 1614 measurements in 1123 individuals aged 2-90 years has been comprehensively analysed by Black et al (1996). Details of the methodologies employed, the database, studies included and excluded, and full references can be found in their paper.

Usage, validity and variability of the physical activity level (PAL) index

Total energy expenditure (TEE) is expressed as a multiple of BMR to determine the requirements of adults as recommended by the last FAO/WHO/UNU Expert Consultation Report (1985) on energy and protein requirements. These multiples of BMR are referred to as physical activity levels (PALs) and calculated by dividing TEE by BMR. The expression of energy expenditure (or requirements) of adults as PALs provides a convenient way of controlling for age, sex, weight and body composition and for expressing the energy needs of a wide range of people in shorthand form. The figures derived by the 1985 Consultation were based on theoretical factorial calculations, making assumptions about the energy cost and duration of day-to-day activities. The data in Table 8 on PAL values in adults are derived from actual measurements using the DLW technique. PAL provides a useful means of categorising energy requirements in a single number, taking into account differences in body size, as represented by BMR. However, the value of PAL depends both on BMR and TEE, and both have errors of measurement, so that PAL is only imprecisely estimated. The CV of BMRs, when actually measured, is very small, as described earlier, while the CV of BMRs predicted using the Schofield equations for given body weights is of the order of about 8% (Schofield, 1985). For TEE, the within-subject CV can be obtained from studies with repeated DLW measurements in persons with stable weight, activity and physiological state. Data from nine such studies, collated by Black et al (1996), have shown that the mean within-individual CV for 79 subjects was 8.9%; this includes methodological error as well as variations in activity levels. Thus, the 95% confidence limits on PALs at the individual level, assuming a measured BMR and no change in body weight or physical activity, is of the order of ± 18.5% representing about ± 0.3 PAL units on a mean PAL value of 1.65.

Table 8 also presents TEE, BMR and energy expenditure for activity (AEE) derived as TEE minus BMR. The latter expression has hitherto not been used, although a related expression, i.e. Physical Activity Ratio (PAR), has been in vogue. PAR is used as an abbreviation for multiple of BMR for an activity, and is used to provide an energy cost for a specific activity, such as sitting down, walking, etc. On the other hand, AEE represents the energy expended by an individual over and above BMR, and includes requirements for thermogenesis, including diet-induced termogenesis (DIT), and for physical activity. The usage of PAL treats it as an index of TEE adjusted for BMR. Since the PAL is a multiple of the BMR and BMR is related to body weight, it is implied that the component of TEE that represents physical activity must also be related to body weight. In theory this would only be true for those activities that involve movement of the body. The relationship of TEE to body weight suggests that most human activities do fall into this category, and the occasion on which significant physical work is done without much movement of the body, e.g. lifting heavy sacks, are relatively rare.

Table 8 Subject characteristics and energy expenditure (obtained by DLW) in different age and sex groups

The limits of human energy expenditure

Studies carried out under special conditions provide information on energy expenditure at the extremes of physical activity levels in adults and thus provide a frame of reference for evaluating values of TEE and PAL from the general population. These studies of TEE measurements using the DLW technique have been summarised by Black et al (1996). At the lower limit of physical activity, studies in non-ambulatory, chair bound subjects and in individuals confined to a calorimeter and apparently not exercising, provide a mean PAL of 1.21. This is slightly lower than the value of 1.27 suggested by FAO/WHO/UNU (1985) as the survival requirement. At the upper limit of physical activity there is a distinction to be drawn between the maximum achievable over a limited period of time and the maximum sustainable as a long-term way of life, given physical fitness and adequate food. The maximum achieved over limited periods of time was a PAL of >4.0 and TEE of 33 MJ/d in a bicycle race and a polar exploration. The maximum for a sustainable way of life may be that represented by soldiers on active service, with a mean PAL of 2.4 and TEE of 18 MJ/d. In support of this, energy intakes of 19.5 MJ/d have been recorded in colliers (Moss, 1923) and in lumberjacks (Karvonen et al, 1961). Among athletes in training, mean PALs of 2-3.5 were found, with TEE ranging from 11 to 18 MJ/d in women, and from 15 to 30 MJ/d in men. PALs greater than 2.4 were obtained in periods of 'rigorous training', which is unlikely to be a sustained lifestyle. The lower values for PAL, 2.0-2.3, were obtained in periods of apparently routine training and may well be sustained for extended periods of time. Similar values have been observed in Gambian women during the farming season (Singh et al, 1989). These data suggest a PAL range of 1.2-2.5 for sustainable lifestyles, where 1.2 is indicative of a non-ambulant life style and 2.5 represents a very physically active lifestyle (Table 9).

Energy expenditure of free-living adults with normally active daily life

A total of 319 adults (212 non-pregnant non-lactating (NPNL) females and 107 males aged 18-64 years) were identified as healthy, free-living, leading a normal daily life, not recruited as having specific and special circumstances, occupations or activities, and in whom BMR had been measured. Table 8 summarises the anthropometric characteristics of the sample, TEE, BMR, AEE and PAL by age and by sex. The data fully encompassed the PAL range from :1.2-2.5, established above as the likely range of sustainable energy expenditures (Table 9). The wide range of expenditures at any age was notable. Regression analysis of the entire data set accumulated by Black et al (1996), which included a total of 574 subjects aged 2-90 years on whom DLW data and BMR measurements were available indicated that equations based on weight, height, age and sex can explain 77% and 86% of the variance in TEE and BMR, and 41% of the variance in AEE. The latter, i.e AEE, was found to be much more sensitive to individual behavioural choices and therefore less definable using purely physiological measures. Age was a negative predictor of energy expenditure, particularly of the activity component (AEE), and remained so when TEE was expressed as PAL. Taken together with the regression analysis, the following key features seem to emerge from the analysis of Black et al (1996):

Table 9 Physical activity levels (PALs) based on doubly-labelled water (DLW) studies

(1) In early life, absolute levels of energy expenditure, whether expressed as TEE, BMR or AEE, rise with increasing body size, peak in the young adult years and decline thereafter. Adjusted for body size, TEE declines with age throughout life.
(2) Adjusted for body size, males have 11 % greater TEE than females.
(3) Expressed as PAL, differences with age remain significant. For females PAL is fairly constant during the adult years, and lower at younger and older ages. For males PAL rises to a peak at 18-29 years and declines thereafter.
(4) Differences in expenditure between the sexes are not completely removed by adjusting for body size using PAL, although the sex effect is confounded with height to some extent.
(5) As expected, mean TEE in the free-living population, however expressed, is well below that of the athletes in training and soldiers on exercises. It is important to note that this sample of 319 adults is from affluent societies and contains very few manual workers with no data from developing country individuals with a high level of physical activity.

PAL values from DLW data on free-living adults in developed societies

Figure 2 (A-D) shows the distribution of energy expenditure of adults aged 18-64 years. The distributions of PAL for both men and women have a modal value at 1.6 (encompassing 1.55-1.65). The distribution for men has a shoulder to the right suggesting the existence of two populations: active and inactive. This could be either real or an artifact of the sample. Many of the authors gave no information about the subjects beyond sex, age and body weight. Subjects designated as free living were typically recruited from among colleagues, from employees in research centres, universities or hospitals, or were volunteers responding to advertising in the local media. Occupations were typically student, housewife, white collar or professional occupation, unemployed or retired. Only three individuals were specifically identified as manual workers. This suggests a predominantly 'sedentary' population. However, some individuals had PALs at a level associated with athletes or soldiers in training, and the limited information on occupation or activity usually suggested plausible reasons for these high values. Among the twenty highest values were three manual workers, six out of thirteen 'university students and laboratory technicians' with an average of 34 min 'strenuous activity' per day and with several active sports specifically mentioned, while five 18 year old college students and two professionals were known to cycle or walk as a primary means of transport. Women were not well represented in the data set at higher PAL levels. Whether this reflects an absence of subjects recruited from more active groups or a general tendency for women to be less involved in strenuous activities is not known.

Figure 2 Data on (A) total energy expenditure (TEE) by doubly labelled water, (B) BMR, (C) energy expenditure for activity and thermogenesis (AEE) and (D) physical activity levels (PAL) of male and female subjects compared with extreme levels of physical activity.

Western lifestyle is commonly referred to as 'sedentary', and the recommendation of FAO/WHO/UNU (1985) for light activity (1.55 × BMR) is frequently interpreted as 'sedentary' and taken as applying to this whole population. However, many desk jobs involve frequent moving around. Other occupations, not necessarily strenuous, require persons to be on their feet all day (e.g. housewives, shop assistants, nurses, storekeepers). Thus a PAL of 1.55-1.65 appears to represent the average for the so-called sedentary lifestyle. There are also data to suggest that activities do not have to be obviously strenuous for relatively high PAL values to be achieved. Calorimetry studies allowing 'free activity' provide mean PALs ranging from 1.50-1.75, and individual PALs from 1.39-2.04. A factorial calculation based on 8 h sleep (PAL 0.95), 4 h sitting (PAL 1.2) and 12 h walking around (PAL 2.5) might represent the lifestyle of a housewife and yields a PAL of 1.8 × BMR.

A PAL of 1.35 has been suggested as the lowest PAL compatible with long-term weight maintenance in persons other than the completely chair- or bed-bound; this was the mean PAL in nine calorimeter studies (n = 207) with controlled, limited activity (Goldberg et al, 1991). Figure 2D shows 7.5% of men and 10.9% of women below a PAL of 1.35. However these may not represent the true long-term energy expenditure due to inaccuracies in the methods. As mentioned before, the coefficient of variation on repeat DLW measurements was 8.9% from nine studies (n=79) on subjects with no change in activity, weight or physiological status; while the CV on measured BMR can be as low as 2.5% under the rigorously controlled conditions of a calorimeter, many studies employed less rigorous conditions. The combined error for PAL is equal to at least ± 9.2%; while the FAO/WHO/UNU Report of 1985 suggested that the inter-individual variability in TEE in a specified group of individuals in whom energy expenditure measurements have been made over a week has a coefficient of variation of ± 12.5% on a body weight basis (Edholm, 1973).

The effect of moderate sport on energy expenditure can be gauged from three studies (n = 28) that imposed a programme of exercise on free-living people normally undertaking very little strenuous activity. The mean sedentary and exercising PALs were 1.63 (s.d. 0.16) and 1.99 (s.d. 0.19), respectively. The mean sedentary and exercising TEEs were 10.53 MJ (s.d. 1.67) and 12.54 MJ (s.d. 2.14) respectively. These figures lend support to the mode of 1.6 for 'sedentary' lifestyles, and show that 30-60 min of active sport, 4-5 times per week, can raise PAL by 0.3 units, but need not necessarily be reflected in a PAL above 2.0.

The relationships between lifestyle, activity and PAL suggested by a careful analysis of the measurements by DLW in adults in developed countries are summarised by Black et al (1996). The data provide little evidence to quantify the energy cost of manual occupations with fairly strenuous physical activity levels which are occupation-related, or to make recommendations about PALs. The range of PAL values which are considered as the maximum for a sustainable lifestyle appears to be between 2.0 and 2.4. The higher energy expenditures, seen in adults in the analysis by Black et al (1996), appear to be due to recourse to active means of transportation such as that resulting from cycling or walking, or to regular participation in active sports. This emphasises the importance of sport or active leisure pursuits in raising energy expenditure in sedentary Western populations, which may provide both for socially desirable activities and for increasing physical fitness and the promotion of health.

The FAO/WHO/UNU Expert Consultation (1985) suggested the average daily energy requirement of adults whose occupational work is classified as light, moderate, or heavy, expressed as a multiple of BMR, to be as follows:

It is obviously difficult to relate these categories to the data in the analysis of DLW studies (Black et al, 1996), as the information on occupations was limited and the categories do not take active leisure into account. The modal value of 1.55-1.65 for adults in the analysis falls between the light and moderate categories. The suggested range for strenuous occupation of 2.0-2.4 is compatible with the recommendation of 2.10 for heavy occupations. DLW data from adults do not appear seriously at variance with the recommendations made by the Expert Consultation.

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