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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