Physical activity (PA) has been consistently implicated in the etiology of obesity, whereas recent evidence on the importance of sedentary time remains inconsistent. Understanding of dose–response associations of PA and sedentary time with overweight and obesity in adults can be improved with large-scale studies using objective measures of PA and sedentary time. The purpose of this study was to examine the strength, direction and shape of dose–response associations of accelerometer-based PA and sedentary time with body mass index (BMI) and weight status in 10 countries, and the moderating effects of study site and gender.
Data from the International Physical activity and the Environment Network (IPEN) Adult study were used. IPEN Adult is an observational multi-country cross-sectional study, and 12 sites in 10 countries are included. Participants wore an accelerometer for seven consecutive days, completed a socio-demographic questionnaire and reported height and weight. In total, 5712 adults (18–65 years) were included in the analyses. Generalized additive mixed models, conducted in R, were used to estimate the strength and shape of the associations.
A curvilinear relationship of accelerometer-based moderate-to-vigorous PA and total counts per minute with BMI and the probability of being overweight/obese was identified. The associations were negative, but weakened at higher levels of moderate-to-vigorous PA (>50 min per day) and higher counts per minute. No associations between sedentary time and weight outcomes were found. Complex site- and gender-specific findings were revealed for BMI, but not for weight status.
On the basis of these results, the current Institute of Medicine recommendation of 60 min per day of moderate-to-vigorous PA to prevent weight gain in normal-weight adults was supported. No relationship between sedentary time and the weight outcomes was present, calling for further examination. If moderator findings are confirmed, the relationship between PA and BMI may be country- and gender-dependent, which could have important implications for country-specific health guidelines.
In recent decades, the prevalence of overweight and obesity has increased in developed and most developing countries.1 It has been argued that this represents an ‘obesity pandemic’, which may be responsible for serious medical, psychological, social and economic consequences, including increased population rates of hypertension, type 2 diabetes and dyslipidemia, decreased quality of life, higher rates of depression and low self-esteem and higher health care utilization and costs.2
Physical activity (PA) is an important contributor to energy expenditure and a major pillar for population-wide weight control strategies.3 It has been suggested that high volumes of sedentary time may be associated with increased risk of overweight and obesity, independently of PA4,5 but the currently available study results are inconsistent and more high-quality studies are needed to confirm the importance of sedentary time for weight control.6,7 Maintaining PA, limiting sedentary time and having a normal weight can jointly affect other health outcomes, including cardiovascular diseases, type 2 diabetes and some cancers.3,5,8
Next to the need for more evidence on the relationship between sedentary time and weight outcomes, the specific dose–response associations of PA and sedentary time with overweight and obesity remain to be determined. To address this issue, international PA and sedentary time data are needed, preferably employing objective exposure measures.9 Although many countries conduct population-based surveys as part of health surveillance systems, sedentary time is usually not included, and the use of different assessment methods across studies makes it difficult to compare results worldwide.9,10 Most population-based and epidemiological studies have used self-report questionnaires to assess PA.9,10 Some of these questionnaires have been extensively validated, particularly the International Physical Activity Questionnaire,11, 12, 13 but objective measurements using small, wearable devices (accelerometers) are needed to more accurately capture volumes and intensities of PA and sedentary time. The International Physical Activity and the Environment Network (IPEN) Adult study was conducted in 12 countries worldwide, using a comparable study design14 and can address some of the shortcomings of prior studies.
The first aim was to examine the strength, direction and shape of the dose–response associations of objectively assessed PA and sedentary time with body mass index (BMI) and weight status. Second, because associations between PA, sedentary time and overweight/obesity can be culture- and gender-dependent,15 the moderating effects of study site and gender were examined.
Materials and Methods
IPEN Adult is an observational epidemiologic multi-country cross-sectional study, including 17 city-regions (hereafter, sites) located within 12 countries: Australia (Adelaide), Belgium (Ghent), Brazil (Curitiba), Colombia (Bogota), Czech Republic (Olomouc, Hradec Kralove), Denmark (Aarhus), China (Hong Kong), Mexico (Cuernavaca), New Zealand (North Shore, Waitakere, Wellington, Christchurch), Spain (Pamplona), the United Kingdom (Stoke-on-Trent) and the United States (Seattle, Baltimore). For the present analyses, 10 countries (12 sites) that collected objective data using Actigraph accelerometers were included, as no accelerometer data were collected in Australia and a different accelerometer type that provided incompatible data was used in New Zealand.
Study participants were recruited in neighborhoods chosen to maximize variance in neighborhood walkability and income. For selection of neighborhoods, all countries but one (Spain) used a neighborhood walkability index that was measured objectively with GIS data. Further details for each country can be found elsewhere.14 The walkability index was derived as a function of at least two of these variables: net residential density, land use mix and intersection density. In four countries, retail floor area ratio was also included as a proxy for pedestrian-oriented design. The method used to create the walkability index is described in more detail elsewhere.16,17 Each country used the walkability index to select higher- and lower-walkability areas and household-level income data from the census to select higher- and lower-income areas. The neighborhood-selection techniques employed in each country can be found in Table 1 and elsewhere.14 In all countries, the selection procedure resulted in an equal number of neighborhoods among four types (quadrants) stratified as follows: high-walkable/high-income, high-walkable/low-income, low-walkable/high-income and low-walkable/low-income.
The participant recruitment strategy used in IPEN Adult was a systematic selection of participants living in the predefined neighborhoods. Random samples of adults living in the selected neighborhoods were contacted and invited to wear an accelerometer for objective PA assessment. Three countries recruited and conducted data collection by phone and mail/online surveys; six countries visited participants in person to deliver study materials (Table 1). In Hong Kong, intercept interviews were conducted in residential areas where individual addresses were not available. Study dates ranged from 2002–2011. Recruitment age ranged from 16 to 94. Because only three countries had a wider age range (Table 1), only adults aged 18–66 were included in our analyses. In six countries, participants were recruited across seasons to control for variations in weather that may affect PA. In the other countries, participants were recruited equally across the quadrants by season. Further details on the participant recruitment techniques and response rates can be found elsewhere.14
In this paper, data from 12 sites (total N=9065) in 10 countries were included. Of these 9065 participants, 3100 did not have accelerometer data and 253 had fewer than four valid days of data, yielding a final sample of 5712. In general, when compared with participants who did not wear accelerometers or had fewer than four valid days of accelerometer data, those who had at least four valid days were more likely to be older (P<0.001), married (P<0.001), employed (P=0.014) and overweight (P=0.036). No significant differences were found for gender, educational attainment, BMI (kg m−2), being obese vs non-obese, neighborhood socio-economic status and objectively assessed neighborhood walkability. The socio-demographic characteristics of the sample with valid accelerometer data by study site are presented in Table 2.
All investigators completed the San Diego State University Institutional Review Board training, and met the NIH Fogarty International Center and their own country’s ethics requirements. All participants provided informed consent for participation in their country-level study. Participant confidentiality for pooled data was maintained by de-identification using numeric identification codes. For data transfer, a secure file sharing system was used. Survey data (demographics and BMI) were assessed for completeness by the study sites and double checked by the Coordinating Center in San Diego. Accelerometer data were provided in pre-processed format (that is, DAT or CSV files) to the Coordinating Center where trained researchers screened all data using MeterPlus software version 4.3. (www.meterplussoftware.com). Protocols for screening data to identify valid wearing time were developed for different Actigraph models, methods of deployment, available documentation of wearing time, and cultural differences in activity patterns.18
Participants reported height and weight (six countries) or were measured in person using standard techniques (four countries), and BMI (kg m−2) was calculated. Previous studies showed that self-reported and objectively measured BMI are highly correlated and that BMI can be used as a proxy measure for adiposity in large-scale studies.19 Both BMI (continuous) and weight status (dichotomous) were examined as outcomes. Weight status was defined as being non-overweight (BMI ⩽24.9 kg m−2) versus being overweight and obese (BMI ⩾25.0 kg m−2). Due to multicollinearity with study site, not mode of collection but only study site was entered as a covariate in the statistical analyses.
Objectively assessed PA and sedentary time
Mean minutes per day of moderate-to-vigorous-intensity physical activity (MVPA), mean minutes per day of sedentary behavior and mean counts per minute were assessed objectively using accelerometers. Reliability and validity of accelerometers have been documented extensively.20, 21, 22 In three countries, accelerometers were mailed to participants; in others they were hand-delivered. Participants were asked to wear the accelerometer above the right hip for seven consecutive days during waking hours and to remove it only for water activities (for example, swimming, bathing). Different models of the ActiGraph accelerometer (Pensacola, FL, USA) were used in the study, including the 7164/71256 models, GT1M, ActiTrainer and GT3X models. Because previous studies do not provide univocal results on whether MVPA and sedentary time data of different ActiGraph models can be pooled23, 24, 25, 26 and no definite solution is available yet to take into account the use of different ActiGraph models in statistical analyses, it was decided to control for ‘Actigraph model’ in all analyses.
Accelerometer data were collected in (or aggregated to) one-minute epochs. Non-wear time was defined as 60 min or more of consecutive zero counts. Only data of participants with at least 10 wearing hours for at least four days were included in the analyses. Of these participants, 84.8% had at least one weekend day of wearing time because they had six or more valid days of accelerometer wearing. Mail days and participants with data indicating device malfunction were excluded. Counts per minute were converted into minutes of sedentary time (⩽100 counts per min), moderate- (1952–5724 counts per min) and vigorous-intensity (5725+ counts per min) PA.21,27,28 Because total counts per min are more appropriate measures of energy expenditure than sedentary time and MVPA (as these are categorized based on cut points), accelerometer counts per min were also used as an outcome measure in the present paper. Across countries, the number of adults wearing accelerometers ranged from almost 200 to over 2000.18
Age, gender, educational level, work status and marital status of the participants were assessed. Although types of education varied by country, all country data could be categorized into ‘university degree’, ‘high school diploma’ and ‘less than high school diploma’. Marital status was dichotomized into married or living with a partner versus not. These socio-demographic variables were included as covariates in all statistical models.
Descriptive statistics were computed for the whole sample with at least four valid days of accelerometer data and by study site. Associations of accelerometer-based PA and sedentary time with BMI and weight status were estimated using generalized additive mixed models (GAMMs; 29). GAMMs can model data following various distributional assumptions, account for dependency in error terms due to clustering, and estimate complex, dose–response relationships of unknown form.29 Preliminary analyses based on residuals and Akaike’s information criterion (AIC, a measure of model fit) indicated that for the continuous measure of BMI, GAMMs with Gamma variance and logarithmic link functions would be most appropriate. The reported antilogarithms of the regression coefficient estimates of these GAMMs represent the proportional increase in BMI (kg m−2) associated with a unit increase in the correlates. For dichotomous weight status indicators (non-overweight vs overweight/obese), GAMMs with binomial variance and logit link functions were used. The reported antilogarithms of the regression coefficients of these models represent odds ratios of being overweight or obese.
Main-effect GAMMs estimated the dose–response relationships of objectively measured PA and sedentary time with BMI and weight status, adjusting for study site, socio-demographic covariates, accelerometer wear time and administrative-unit-level socio-economic status. Separate models were estimated for (1) MVPA and sedentary time and (2) average counts per min. Curvilinear relationships of PA and sedentary time with BMI and weight status were estimated using non-parametric smooth terms in GAMMs, which were modeled using thin-plate splines.29 Smooth terms failing to provide sufficient evidence of a curvilinear relationship (based on AIC) were replaced by simpler linear terms. Separate GAMMs were run to estimate PA/sedentary time by study site and by gender interaction effects (two-way and three-way interactions). The significance of interaction effects was evaluated by comparing AIC values of models with and without a specific interaction term. An interaction effect was deemed significant if it yielded a >2-unit smaller AIC than the main effect model.30 Significant interaction effects were probed by computing the site- and/or gender-specific association.
As only 2.6% of cases (n=146) had missing data, the data analyses were only performed on complete cases.31 Participants with complete data were more likely to be older (P=0.004), hold a tertiary degree (P=0.034) and have more valid hours (P=0.013) and days of accelerometer wear time (P<0.001), hence all regression models were adjusted for these variables. All analyses were conducted in R (R Development Core Team, 2013) using the packages ‘car’,32 ‘mgcv’,29 ‘gmodels’ 33 and ‘Epi’.34
Table 2 shows the overall and site-specific descriptive statistics for socio-demographic characteristics, BMI, weight status and accelerometer-based measures of PA and sedentary time. The total sample consisted of 5712 participants; 53% were women, 52% had a college or university degree, 77% were working and 64% were living with a partner. Mean age of the total sample was 43 years (s.d.=12.4), overall mean BMI was 25.8 (s.d.=4.9).
Associations of accelerometer-derived measures of PA and sedentary time with BMI and weight status
After adjusting for sedentary time, significant curvilinear associations of average daily minutes of MVPA with BMI (F3.63, 3.63=33.76; P<0.001) and weight status (F2.45, 2.45=28.85; P<0.001) were observed. These are shown in the two left panels of Figure 1. BMI and the probability of being overweight/obese decreased relatively linearly with an increase of average daily minutes of MVPA from 0 to 40–50 min per day. The estimated effects of MVPA leveled off at higher levels of PA and were nil at >150 min per day of MVPA. However, we need to note that the latter estimates had a high level of uncertainty (large confidence intervals) due to the small number of participants achieving such high levels of activity (Figure 1). No significant associations of accelerometer-derived sedentary time with BMI (eb=3.20 × 10−5; 95% confidence interval: 0.99, 1.00; P=0.271) and weight status (eb=1.0004; 95% confidence interval: 0.9997, 1.0012; P=0.243) were found.
The associations of average accelerometer counts per min with BMI (F3.27, 3.27=40.94; P<0.001) and weight status (F2.02, 2.02=44.18; P<0.001) were also significant and curvilinear (see right panels of Figure 1), but more uniformly negative across the whole range of values than those observed for daily minutes of MVPA (left panels of Figure 1).
Moderating effects of study site and gender
Study site and gender significantly moderated the associations of PA and sedentary time with BMI (see Table 3) but not with weight status. Stronger negative associations of MVPA and accelerometer counts per min with BMI were observed in men than women in Belgium, Brazil, Colombia, Denmark and Mexico (Table 3 and Figure 2). The opposite was true for the Czech Republic (one site: Olomouc) and the two USA sites (Table 3 and Figure 2): stronger negative associations of MVPA and accelerometer counts per min with BMI were observed in women than in men. No significant associations in men or women were found in Hong Kong, Hradec Kralove (site in Czech Republic), Spain and the United Kingdom. The latter findings cannot be attributed to differences in sample size as the point estimates of the regression coefficients are indicative of smaller (almost nil) effects compared with other sites. The two USA sites were the only study sites to show significant positive associations of sedentary time with BMI. Notably, they were only significant in women (Table 3).
Our first aim was to examine the dose–response associations of accelerometer-assessed MVPA, sedentary time and counts per minute with BMI and weight status in adults living in 10 environmentally and culturally diverse countries. After controlling for sedentary time and socio-demographic covariates, a curvilinear relationship between MVPA and both BMI and the probability of being overweight/obese was identified. This relationship was almost linearly negative when MVPA levels ranged between 0 and 50 min per day and weakened at higher levels of MVPA. A similar curvilinear association of average accelerometer-based counts per minute with BMI and overweight/obesity was identified, but the relationship was more uniformly negative, with less leveling off at higher levels of average counts/minute. No associations were found between sedentary time and the weight outcomes, after controlling for MVPA.
The curvilinear association identified between MVPA and the weight outcomes is similar to the dose–response model proposed by Pate et al.35 and updated by Haskell et al.36 That model represents a curvilinear relationship between PA and overall health, showing that the strength of the health benefits of PA depends on the baseline activity levels: an initial increase from an inactive to a somewhat active lifestyle provides stronger health benefits than a change from a somewhat active to a very active lifestyle. Furthermore, a comparable curvilinear relationship has been previously identified in relation to risk of coronary heart disease in adults.37 Adults who achieved activity levels consistent with the public-health health guideline of 150 min per week of MVPA had a 14% lower coronary heart disease risk compared with those who did not reach the guidelines; engaging in 300 min per week of MVPA led to a 20% lower risk, but higher levels of PA did not provide additional benefits. A previous study that examined the dose–response relationship of PA with body weight,38 showed an inverse dose–response association between leisure-time PA and obesity in US adults, but only in women. Although not statistically tested, the curve showed evidence of curvilinearity, with the greatest decline in the prevalence of obesity between women who engaged in insufficient levels of PA and those who met the health guideline; a floor effect was observed at higher levels of PA.38
Within our own findings, the steepest negative association between MVPA and BMI was found when minutes per day of MVPA ranged between 0 and 50 min per day. When comparing this amount with the health guideline of 150 min of MVPA per week, it seems that more PA (350 min per week in this case) is even more beneficial, specifically in the context of weight gain. This is consistent with the guidelines that have been formulated for the prevention of unhealthy weight gain: according to the Institute of Medicine, normal-weight adults should accumulate 60 min of MVPA per day to prevent weight gain.39,40 Higher levels of PA may have important additional beneficial effects on fitness or other health outcomes.41 However, one needs to keep in mind that the present results are cross-sectional—therefore, no true dose–response relationships can be assumed.
The curvilinear relationship of accelerometer-based MVPA with BMI and weight status was confirmed by the comparable associations found for accelerometer-based counts per minute. This is encouraging, as accelerometer counts are a cumulative measure of PA that is not susceptible to cut point categorizations based on limited consensus. Although the shape of the curves was similar, less attenuation was visible at higher levels of counts per minute than at higher levels of MVPA. This might be due to the fact that counts per minute is a more general measure, capturing every accelerometer movement that exceeds zero. Hence, light-intensity activities and counts associated with sedentary time (categorized as ⩽100 counts per minute; 28) were included in the total-counts measure. The counts per minute measure likely has a lower level of error, so the associations consequently will be stronger. It is reassuring that similar conclusions can be drawn from the graphs representing the associations of MVPA and counts per minute with BMI and the probability of being overweight.
Except in US women, no associations between sedentary time and the outcomes were found. This is in contrast with the previous longitudinal and cross-sectional studies revealing that more time spent in sedentary behavior (predominantly assessed as TV viewing) was consistently associated with higher risk of obesity, even after accounting for PA and other covariates.5,42,43 However, in their review of prospective studies examining associations between sedentary time and health outcomes, Thorp et al.7 concluded that findings on the relationship of sedentary time with BMI and weight gain in adults are inconsistent, with small effect sizes and effects being largely dependent on baseline BMI—suggesting potential reverse causation. Similarly, Proper et al.6 concluded that insufficient evidence was available to draw conclusions on the relationship between sedentary time and weight-related measures. More convincing evidence is available to support the relationship between sedentary time and other health outcomes like premature mortality, all-cause and cardiovascular disease related mortality, cancer and diabetes.6,7
Associations of sedentary time with BMI may be weak and inconsistent because BMI is largely dependent on other factors such as energy intake, PA and heredity.44 Furthermore, previous studies that found a significant association of sedentary time with BMI or weight status mainly used self-reported TV viewing time as a proxy of sedentary time, and generally have not reported findings for overall sedentary time. This may have led to biased results because TV viewing is known to be strongly associated with increased energy intake, particularly snacking.7 In addition, the insignificant findings reported here could be due to the fact that accelerometers were used to assess sedentary time. Accelerometry provides an objective measure of sedentary time, which is not susceptible to biases (for example, social desirability, recall bias) that are inherent in the use of questionnaires, but subjective decisions still need to be made when processing accelerometer data. For instance, counts per minute need to be converted to minutes of sedentary time by using cut points. Although the cut point of ⩽100 counts per minute to define sedentary time28 is widely used, it was not empirically derived and might miss some sedentary activity. Some studies have shown that a higher cut point might be more sensitive to detect sedentary time.45,46 In future studies, it will be informative to use inclinometers—objective, posture-based measures of true sitting or reclining time. Previous studies have used activPAL monitors (Physical Activity Technologies, Glasgow, Scotland) to directly assess posture and thus more accurately capture sedentary time, compared with accelerometers (which primarily capture movement). Inclinometers also provide more accurate measurement of posture due to the placement of inclinometer devices on the thigh rather than on the hip.46 Future (prospective) studies should also focus on the broad range of light-intensity physical activities, in order to find out how these activities relate to weight status and other health parameters. It would be interesting to discriminate between low-light-intensity activities and high-light-intensity activities, because previous cross-sectional research showed that associations with physical health are stronger for high-light-intensity PA than for low-light-intensity PA.47,48 Nonetheless, some discussion still exists about which accelerometer-based cut points should be used to define the different types of light-intensity PA.47,48 Finally, further research should take into account the possible importance of breaks in sedentary time, in addition to total sedentary time. Preliminary evidence from cross-sectional studies revealed that breaks in sedentary time are beneficially associated with BMI and waist circumference in adults.49,50
As a second aim, the possible moderating effects of gender and study site on the associations of MVPA, sedentary time and counts per minute with BMI and weight status were examined. For BMI, there were complex site- and gender-specific findings; for weight status, no such moderating effects were present. In most countries except for Spain, the United Kingdom and Hong Kong, accelerometer-based MVPA and counts per minute were related to BMI, but in some countries stronger associations were found in men, whereas in others, associations were stronger in women. Depending on site and gender, both linear and curvilinear associations were observed. No previous studies have examined the country-specificity of such associations, but in the large-scale USA study by Seo and Li,38 a curvilinear association between leisure-time MVPA and obesity was found only in women. No clear explanations for the moderating effects that we have identified can be given. Participants living in the countries where no, or only gender-specific, associations were found did not have particularly high levels of MVPA (so they were not located at the higher end of the continuum, where associations attenuated), and as noted in the results section, the findings cannot be attributed to differences in sample size between countries. Perhaps, non-assessed country- or gender-specific dietary patterns have a confounding role here. In future research, it will be crucial to further combine data from multiple countries and examine the country- and gender-specificity of the associations, as important culturally-dependent associations may be revealed. If confirmed in future prospective studies, the gender- and country-specific findings identified here may have important implications in the context of formulating PA guidelines to help prevent weight gain.
Although the present study had several strengths, including the large sample size, comparable data collection protocols across 10 countries, use of objective methods to assess MVPA and sedentary time, and application of complex statistical models that allowed for curvilinear associations, some limitations need to be acknowledged. First, IPEN Adult employed a cross-sectional design, precluding inferences about causality. Second, estimates of MVPA and sedentary time that were obtained may not be representative of the total population in the participating countries, as participants were recruited from specific neighborhoods selected on their walkability and income levels. Third, response rates and ActiGraph models used varied across study sites. This may imply sampling biases or other methodological biases across study sites. Fourth, only PA and sedentary time were examined in relation to weight outcomes; a more complete perspective could have been provided if diet-related measures, information on sleep duration and a more precise measure of body fat were included as well. Fifth, a combination of self-report and objective measures were used among countries to determine BMI and weight status: this could have biased the results.
In conclusion, this study provided evidence of a curvilinear association of accelerometer-based MVPA and counts per minute with BMI and weight status in adults living in 10 environmentally and culturally diverse countries. Because the curve attenuated at MVPA levels higher than 50 min per day, the currently Institute of Medicine recommendation of 60 min per day of MVPA to prevent weight gain in normal-weight adults was supported. No relationship between sedentary time and the weight outcomes was present, so if confirmed in future studies, it seems that no specific guidelines for sedentary time can be formulated, at least not for weight-related health promotion. As this was the first study to examine the country-specificity of these associations, no definite conclusions can be drawn. However, if confirmed in future prospective studies, the relationship between MVPA and BMI may be country- and gender-dependent, which could have important implications for country-specific health guidelines.
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This study was supported by IPEN, International Physical Activity and the Environment Network, with funding from NIH Grant R01 CA127296.
The authors declare no conflict of interest.
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Dyck, D., Cerin, E., De Bourdeaudhuij, I. et al. International study of objectively measured physical activity and sedentary time with body mass index and obesity: IPEN adult study. Int J Obes 39, 199–207 (2015). https://doi.org/10.1038/ijo.2014.115
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