Purpose:
To examine the association between sitting time and markers of obesity in healthy men and women and explore the potential modifying role of estimated cardiorespiratory fitness (CRF).
Methods:
A cross-sectional analysis of 4,486 men and 1,845 women enrolled in the Cooper Center Longitudinal Study for the period 2010-2013 was conducted. Key exposures included self-reported sitting time and CRF. Sitting time was self-reported as 1) almost none of the time (about 0%), 2) approximately one quarter of the time (about 25%), 3) approximately half of the time (about 50%), 4) approximately three quarters of the time (about 75%), and 5) almost all of the time (about 100%). CRF was estimated from time spent on a treadmill during a graded exercise test. Obesity outcomes were defined in three ways: 1) waist girth (≥102 cm for men; ≥88 cm for women); 2) BMI (≥30 kg/m2); and 3) percent body fat (≥25% for men; ≥30% for women). Sex-specific multivariable models were constructed to examine the association between sitting time and each obesity outcome, separately, and adjusted for covariates including age, physical activity, smoking status, and alcohol consumption. A cross product term of sitting time and CRF was included in the full model to assess the possibility of effect modification.
Results:
In unadjusted analysis, obesity outcomes significantly varied across categories of sitting time in men and women. In men, significant associations were observed between sitting 75% of the time or more and each measure of adiposity which remained after adjustment for CRF and other covariates. For example, men who sat about 100% of the time were more than two times as likely to be obese whether defined by waist girth (OR: 2.61; 95% CI: 1.25, 5.47), or percent body fat (OR: 3.33; 95% CI: 1.35, 8.20) relative to men who sat about 0% of the time. Similar to the results for men, associations between sitting time and each measure of adiposity were seen in women when adjusted for age. However, when CRF and other covariates were added to the model, these associations were no longer statistically significant. In addition, the sitting time x CRF terms did not reach statistical significance for any obesity outcome.
Conclusion:
In this healthy population, the relation between obesity and sitting time varied by sex. Men who sat more were more likely to be obese by any definition. However, no significant associations were observed between sitting time and obesity among women after adjustment for CRF and other covariates. In addition, CRF did not appear to modify this association in either men or women. These results suggest that women, at least, can offset sedentary behavior by being fit. Further research is needed to understand the disparate effects of sitting time on obesity as seen in these healthy women. Furthermore, prospective research studies are needed to elucidate the causal association between obesity and sitting time.
Association between Exercise-Induced Changes in Cardiorespiratory Fitness and Adiposity among Overweight and Obese Youth: A Meta-Analysis and Meta-Regression Analysis