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Medicine & Science in Sports & Exercise:
Roundtable Consensus Statement

Assessment of physical activity level in relation to obesity: current evidence and research issues

WESTERTERP, KLAAS R.

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Department of Human Biology, Maastricht University, Maastricht, THE NETHERLANDS

Address for correspondence: Klaas R. Westerterp, Department of Human Biology, Maastricht University, P.O. Box 616, 6200 MD Maastricht, The Netherlands; E-mail: K.Westerterp@HB.Unimaas.NL.

Roundtable held February 4–7, 1999, Indianapolis, IN.

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Abstract

WESTERTERP, K. R. Assessment of physical activity level in relation to obesity: current evidence and research issues. Med. Sci. Sports Exerc., Vol. 31, No. 11, Suppl., pp. S522–S525, 1999.

Purpose: Validations of methods for the assessment of physical activity and studies on the relation between energy expenditure of activity and obesity were reviewed, with suggestions for further research.

Design: Validation studies of field methods for the assessment of physical activity against doubly labeled water were evaluated, studies on the relation between doubly labeled water assessed energy expenditure of activity and obesity are discussed.

Methods: Three field methods for the assessment of physical activity, validated with doubly labeled water as a criterion method, were included: activity questionnaires, heart rate monitoring, and motion sensors.

Results: The triaxial accelerometer came out as the best field method for the assessment of physical activity, followed by the Baecke activity questionnaire. The majority of obese subjects are moderately active, and an increase in the activity level of obese subjects is limited by the ability to perform exercise of higher intensity.

Conclusions: Accelerometers are an objective tool for the assessment of physical activity in large populations over periods long enough to be representative for normal daily life and with minimal discomfort to the subjects. The accelerometer can be used to distinguish differences in activity levels between individuals and to assess the effect of interventions on physical activity within individuals.

To get more insight into the interaction between daily physical activity and health, an objective and reliable method for the assessment of physical activity in free-living subjects is required. The method should be suitable to measure physical activity in large populations over periods long enough to be representative for normal daily life and with minimal discomfort to the subjects. Presently there are a large number of techniques for the assessment of physical activity, which can be grouped into five general categories: behavioral observation; questionnaires (including diaries, recall questionnaires, and interviews); physiological markers like heart rate; calorimetry; and motion sensors. Validated techniques of estimating habitual physical activity are needed to study the relationship between physical activity and health. The greatest obstacle to validating field methods of assessing physical activity in humans has been the lack of an adequate criterion with which techniques may be compared. The interrelation of various field methods may be of some value, but because there are errors in all methods, it is impossible to determine the true validity of any one of them in doing so (17). However, calorimetry, more specifically the doubly labeled water method, is becoming a gold standard for the validation of field methods of assessing physical activity (16).

The doubly labeled water method allows accurate measurement of average daily metabolic rate (ADMR) under unrestricted conditions over 1- to 3-wk intervals (23). In combination with a measurement of basal metabolic rate (BMR), the activity level of a subject can be calculated. The physical activity level is calculated by expressing ADMR as a multiple of BMR (PAL = ADMR/BMR (8)) or by adjusting ADMR for BMR as suggested by Carpenter et al. (6). Doubly labeled water for the assessment of ADMR in man was first applied by Schoeller and Van Santen (25), and the technique has since then been evaluated in meetings of its users (19,24,28).

The current review comprises validations of methods for the assessment of physical activity against doubly labeled water, and studies on the relation between doubly labeled water assessed energy expenditure of activity and obesity. Finally, suggestions are made for further research.

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METHODS

So far, three field methods for the assessment of physical activity have been validated with the doubly labeled water technique as a criterion measure: questionnaires, heart rate monitoring, and motion sensors. A literature search yielded 14 papers including doubly labeled water in combination with one of the three methods mentioned above, four on questionnaires, eight on heart rate monitoring, and two on motion sensors.

Studies on activity questionnaires comprised a 1-wk (5) or 2-wk activity diary (27), two 7-d activity recall questionnaires (21), the physical activity scale for the elderly (26), the Baecke questionnaire, the Five-City questionnaire, and an adapted version of the Tecumseh Community Health study questionnaire (18). The activity diary method was as described by Bouchard et al. (3). Subjects recorded every 15 min of the waking interval a number corresponding to one of a grouping of nine activity categories (5) or 12 activity categories (27), according to their average physical activity during that time period. Numbers were converted to ADMR by multiplying the integrated mean 24-h activity score with the measured BMR. The activity recall method included a standard questionnaire that categorizes activities by their intensity, using the compendium of Ainsworth et al. (1). Energy expenditure was then calculated by multiplying the amount of time spent in each activity by the corresponding number of MET. The physical activity scale for the elderly (PASE) was a brief questionnaire as described by Washburn et al. (30). It comprises activities commonly engaged in by elderly persons, and the reference period is 1 wk. The PASE-score is the sum of the time spent in each activity, multiplied by an item weight factor. The Baecke questionnaire is a brief questionnaire with three categories, work, sport and leisure time, adding up to a total activity index (2). The Five-City questionnaire asks the time spent in vigorous activities, moderate activities, light activities, and sleeping. Each category is multiplied by the reported hours and a weight factor to calculate an activity index. The Tecumseh questionnaire was an adapted version of the questionnaire of Reiff et al. (224). Subjects were interviewed on the estimated hours per week of sports participation, home repair and maintenance activities, sleeping and eating, quiet leisure time, and remaining activities. The hours per week were multiplied by the PAL values as listed by Ainsworth et al. (1) to get a figure for total activity.

Studies on heart rate monitoring generally included 2–4 d of continuous heart rate monitoring during the 7- to 14-d observation intervals with doubly labeled water. Observation days were weekdays as well as weekend days and results were weighted in a ratio according the doubly labeled water interval. All studies applied individually assessed heart-rate energy-expenditure calibration equations for the estimation of daily energy expenditure. The calibration equations comprised two linear regression lines, one for heart rates below and one for heart rates above the average value for sedentary activities; the so-called flex heart rate.

Studies on motion sensors included a single-axial accelerometer and an triaxial accelerometer for movement registration. Johnson et al. (10) monitored body movement with the single-axial Caltrac over 3 d of a 13- to 16-d observation interval with doubly labeled water. Bouten et al. (4) monitored body movement with a triaxial accelerometer over the first 7 d of a 14-d observation interval with doubly labeled water. Westerterp and Bouten (34) reported results of the same study with a different data analysis.

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RESULTS

Most studies on activity questionnaires showed an association between the derived activity score and the doubly labeled water assessed physical activity level (Table 1). The Baecke questionnaire tended to show the highest correlation, and next best was the physical activity scale for the elderly and the Tecumseh questionnaire. The index of the Five-City questionnaire was not related to the doubly labeled water assessed physical activity level. As expected, an activity diary was superior to activity recall.

Table 1
Table 1
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Total energy expenditure assessed with heart rate monitoring (TEEHR) was not different from total energy expenditure assessed with doubly labeled water (TEEDLW) at group level in all studies. However, individual differences were large as shown by the SD of the mean. The reported extremes ranged between −17% (13) and +52% (14).

Of the motion sensors, the single-axial Caltrac was not a meaningful predictor of physical activity (Table 3). The triaxial accelerometer for movement registration showed a close correlation with the doubly labeled water assessed PAL (4) as well as with ADMR adjusted for resting metabolic rate (34). The standard error of estimate of ADMR in the latter study was 0.9 MJ·d1.

Table 3
Table 3
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DISCUSSION

Doubly labeled water is an accepted criterion measure for other methods for the assessment of physical activity level and energy expenditure. Unfortunately, the doubly labeled water method itself cannot be applied at a population level. Oxygen-18 water is expensive and not readily available. Of the three reviewed methods for the assessment of physical activity level, the triaxial accelerometer for movement registration showed the best result. A simple method like the Baecke questionnaire also came out surprisingly well. Heart rate monitoring showed large discrepancies for individual data. Unfortunately, studies on the comparison of total energy expenditure as assessed with heart rate monitoring and with doubly labeled water (Table 2) did not present a correlation between the derived activity score of both methods as presented for the questionnaires (Table 1) and motion sensors (Table 3). Some studies presented the correlation between total energy expenditure, as assessed with heart rate monitoring and with doubly labeled water. This correlation, however, is not primarily a function of physical activity but of resting metabolic rate, the largest component of total energy expenditure.

Table 2
Table 2
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Each of the three methods for the assessment of physical activity reviewed above has a number of positive and negative aspects. Positive aspects of questionnaires like the Baecke questionnaire are the short time needed for a subject to fill out the 21 questions, the simple scoring system for the calculation of an activity index, and the coverage of the normal daily life activity pattern of the subject. A disadvantage of questionnaires is the fact that subjects can easily overestimate or underestimate the time spent in activities, and most questionnaires are not applicable for all subject categories from children, people with and without jobs, to the elderly. Heart rate monitoring is an objective method. However, heart rate is affected by more factors than physical activity, data conversion needs individual measurements of heart rate in combination with oxygen consumption, and heart rate monitors are not tolerated by subjects for time intervals representative of daily life like 1 wk or more. Heart rate monitoring remains a proxy measure for physical activity (12). Motion sensors give objective information but the optimal instrument is not yet on the market. The triaxial accelerometer used in the studies of Bouten at al. (4) and of Westerterp and Bouten (34) was a self-made instrument, not yet commercially available. The commercial available triaxial accelerometer named Tritrac (Reining International, Ltd., Madison, WI) has not yet been validated with doubly labeled water. Additionally, the size of the validated triaxial accelerometer and of the Tritrac, respectively, 170 g, 238 cm3 and 275 g, 270 cm3, limits wearing comfort. We now use a miniaturized version of the validated triaxial accelerometer measuring 30 g, 11 cm3 (33).

A research issue for physical activity level and energy expenditure of activity in relation to obesity is the definition of risk groups. Successful intervention should start before obesity is manifest. Doubly labeled water studies show an increase of activity associated energy expenditure with body mass index, and the average PAL of lean and obese subjects is quite similar (20,32). The majority of obese subjects is moderately active and an increase in the activity level of obese subjects is limited by the ability to perform exercise of higher intensity.

Exercise training potentially increases energy expenditure and decreases body fat, a beneficial aspect for somebody with too much body fat. However, women tend to compensate for an exercise-induced increase in expenditure with an increased intake, resulting in a smaller effect on body mass and fat mass compared with men. Cross-sectional data confirm the evidence mentioned; a higher activity energy expenditure is related to a lower percent body fat in men, whereas no such relationship is apparent in women (35).

The combination of energy restriction with exercise training does not result in additional fat loss. Doubly labeled water studies have shown that training induced energy expenditure during dieting is compensated by a reduction of physical activity outside the training interval (32). However, there are indications that exercise training helps subjects to comply with energy restricted diet. Successful maintenance of body mass and body composition, after weight reduction, will be facilitated at a higher level of energy turnover (31).

Finally, there is an indication that the effect of overfeeding, i.e., a positive energy balance, on weight gain is a function of habitual physical activity. Levine at al. (11) recently showed, by measuring changes in ADMR with doubly labeled water after overfeeding, that resistance to fat gain in nonobese men and women was a function of what they called nonexercise activity thermogenesis (NEAT). An objective activity monitor like an triaxial accelerometer would allow more insight in the components of NEAT as an important potential determinant of obesity.

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Keywords:

DOUBLY LABELED WATER; ACTIVITY QUESTIONNAIRES; HEART RATE MONITORING; MOTION SENSORS; ACCELEROMETERS

© 1999 Lippincott Williams & Wilkins, Inc.

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