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Abstract:

The health benefits of physical activity are well established. However, several aspects of the association of physical activity and sedentary behavior with various health outcomes remain unclear, in part because of the challenges associated with measuring free-living activity. Up to now, questionnaires remain the most widely used method to evaluate activity patterns at the population level. The accuracy of these instruments is often limited due to recall errors, social desirability bias, misinterpretation of the questions and difficulties associated with capturing low-to-moderate routine activities of daily living. Over the past decades, significant advances have been made in the objective assessment of active and sedentary behavior, including the use of accelerometer-based monitors. Still, each of these techniques has important limitations. Recent monitors like the SenseWear Armband, that combine accelerometry with physiological measures, can address many of these limitations and improve the accuracy of measurement. However, no single technique can accurately assess the volume of physical activity and at the same time provide information about which activities are being performed. Therefore, to allow for a more comprehensive investigation of physical activity patterns, this study complemented the objective monitoring through the SenseWear Armband with a direct measure of the type of activity through an electronic diary. The combination of both techniques allowed for a better understanding of habitual physical activity, thereby contributing to systematic progress in physical activity and health-related research and providing essential information for policy guidance. The aim of this project was three-fold. First, methodological issues associated with the measurement of physical activity and sedentary behavior were examined. Secondly, current activity levels were described and compared among population subgroups. Finally, associations of physical activity and sedentary behavior with health outcomes were investigated. The first paper of the methodological chapter examined sources of variance in physical (in)activity measures obtained with the SenseWear Armband, and determined the number of monitoring days needed to quantify activity patterns reliably. The results indicated that at least three days of monitoring from Monday through Friday were required to obtain reliable estimates of physical activity patterns during weekdays. Moreover, because of the marked differences between weekdays, Saturday and Sunday, both Saturday and Sunday need to be monitored in addition to the three to five weekdays. These findings have important implications for study design and data reduction strategies. The second paper examined the agreement between intensity- and domain-specific physical (in)activity parameters obtained with the Flemish Physical Activity Computerized Questionnaire (FPACQ) with those obtained from the combination of the SenseWear Armband and the electronic diary (SWD). Despite themoderate correlations, significant differences between both methods were found. In general, physical activity was higher and sedentary behavior lower as calculated from the FPACQ, compared to SWD. In addition, correspondence varied with gender and age. Though, no clear patterns were observed. Results differed according to the specific intensity and domain of activity. In the chapter of descriptive epidemiology, current levels and patterns of physical (in)activity were described and compared among population subgroups. The first paper evaluated compliance with various physical activity guidelines and investigated the relationship between meeting the guidelines and socio-demographic characteristics. Approximately 87% of men and 68% of women achieved 150 min of moderate-to-vigorous physical activity (MVPA) per week, but only 58% and 37% accumulated this amount as 30 min/day on ≥5 days/week. With regard to vigorous physical activity, 28% of men and 16% of women achieved 75 min/week and 13% and 7% achieved 20 min/day on ≥3 days/week. Despite the high compliance with the guidelines for cardiovascular health, only 35% of men and 22% of women were sufficiently active to avoid excessive weight gain. The lowest compliance rates were observed for the goal of 10000 steps/day, with 16% of men and 14% of women attaining the minimum number of steps on seven consecutive days. The odds of meeting the recommendations was significantly lower among women and lower with higher age. Educational level was positively related with accumulating 75 min/week of vigorous activity, but negatively with taking 10000 steps/day. Smokers were 60% less likely to participate weekly in 150 min of MVPA compared to non-smokers. Marital status, on the other hand, was not related to meeting the different guidelines. These results highlight the need to further examine the nature of the different guidelines and determine the most appropriate way to communicate physical activity requirements to the general public. The second paper described a comprehensive physical activity and inactivity profile among body mass index (BMI) defined normal-weight, overweight and obese adults. Physical activity level was significantly lower with higher BMI. Sedentary time was higher in obese men compared to normal-weight men and overweight and obese women compared to normal-weight women. Light physical activity was higher in obese than normal-weight men, but was not significantly different between female subgroups. Overweight and obese adults had significantly lower levels of moderate and vigorous physical activity compared to their normal-weight peers. The average duration of a sedentary bout and the number of breaks in sedentary time were only different between female BMI groups. The difference in time spent in domain-specific activity was minimal, whereas the intensity of these activities was lower with higher BMI. Normal-weight subjects had the highest and obese subjects the lowest activity levels throughout the week, but patterns differed between subgroups, with the difference most pronounced on Saturday for the male BMI groups. This study provided a unique insight into current patterns of physical activity and sedentary behavior among adults with various degrees of overweight. The results could be used to develop programs towards those activities, intensity levels and periods of the week that are especially concerning in obese individuals. The first paper of the chapter on physical activity, sedentary behavior and health examined the independent associations of overall, intensity- and domain-specific physical activity and sedentary behavior with bioelectrical impedance determined percent body fat. Time spent in light activity was positively and moderate, vigorous and combined MVPA inversely associated with percent body fat, independent of total sedentary time and potential confounders. Furthermore, MVPA achieved during leisure time, but also during occupation and household chores contributed significantly to percent body fat. The positive associations with total and domain-specific sedentary behavior, on the other hand, diminished after MVPA was controlled for. Thus, engagement in MVPA may be important to successfully prevent fat gain. In addition, these findings indicate that lifestyle physical activity accumulated during occupational, domestic and recreational activities should be fully integrated into future programs. The second paper investigated the associations between subcomponents of physical activity and sedentary behavior and the presence of the metabolic syndrome (MetS) and its individual components. The MetS was observed in 11% of men and 6% of women. Adults who spent ≥60 min/day in MVPA were 54-82% less likely to have the MetS or individual risk factors, compared to those with < /30 min MVPA. Controlling for total sedentary time attenuated the associations, but subjects who engaged in ≥60 min/day still had a three-fold lower odds of having at-risk profiles for high-density lipoprotein cholesterol (HDL). Once total MVPA was controlled for, parameters of sedentary behavior were not associated with MetS or its components. Light-intensity activity, on the other hand, was beneficially related with abdominal obesity, independent of time spent in MVPA. Physical activity of at least moderate intensity was associated with a significantly reduced likelihood of the MetS and individual risk factors. The strongest associations were found for time spent at vigorous physical activity. Both total MVPA as well as MVPA accumulated in 10-min bouts were significantly inversely associated with individual and clustered metabolic risk. However, the lowest odds ratios were achieved with sustained bouts. Additional adjustment for total sedentary time revealed comparable results for low HDL and only partially attenuated the odds ratios for abdominal obesity. But, associations with hypertriglyceridemia, hypertension and the MetS were attenuated to a nonsignificant level. The findings of these studies have important implications for the assessment of physical activity in field-based research and the development of strategies to increase physical activity and promote health. Together, the results indicate that MVPA should be promoted as the key parameter for body composition and metabolic health. Lifestyle activities can be performed intermittently throughout the day, but to achieve additional benefits, adults should try to accumulate physical activity in sustained bouts. When feasible for the subjects, including some vigorous activities may be beneficial. Finally, it might be important to reduce sedentary behavior in addition to increasing physical activity.