This study yielded new information about biological outcomes and commuting distance, an understudied and habitual source of sedentary behavior that is prevalent among employed adults and important for individuals with the additional exposure of occupational sitting. The findings suggest that commuting distance is adversely associated with moderate-to-vigorous physical activity, CRF, adiposity, and blood pressure but not blood lipids or fasting glucose. This information provides important evidence about potential mediators in the relationship between time spent driving and cardiovascular mortality observed previously in this study population.15
A plausible mechanism between commuting distance and adiposity could be that longer commutes displace physical activity participation given (1) the independent associations with physical activity and CRF and (2) attenuation in associations with adiposity after adjustment by physical activity. At the same time, when examined as continuous variables, both BMI and waist circumference were associated with commuting distance even after adjustment for physical activity and CRF, suggesting an independent effect of commuting distance on adiposity likely via a reduction in overall energy expenditure.39,40
Another factor that may contribute to the observed associations with adiposity may be that participants with long commutes were more likely to live in suburban neighborhoods, which often possess built environment features that are associated with physical inactivity and sedentary behavior.41,42
Associations of commuting distance with the other metabolic risk indicators were largely weak or nonsignificant, with the exception of blood pressure. This is plausible, given the strong influence of individual and environmental factors on these health indicators43–46
and that commuting long distances via motorized travel represents only a portion of total sedentary time. Yet, associations with blood pressure were as strong in magnitude as those with physical activity and persisted even after adjustment for physical activity and adiposity.
Multiple mechanisms could be contributing to this relationship. First, automobile driving has been identified as a salient source of everyday stress, especially when drivers are faced with traffic congestion.47–50
Because the Dallas–Fort Worth region is ranked among the top five most congested metropolitan areas in the U.S.,51
those with longer commutes may be more likely to be exposed to heavy traffic resulting in higher stress levels and more time sitting. Daily commuting represents a source of chronic stress that has been positively correlated with physiologic consequences including high blood pressure, self-reported tension, fatigue, and other negative mental or physical health effects in some studies.49, 52–55
Another explanation of the observed association between commuting distance and blood pressure, as well as adiposity, may be that commuting distance is related to unmeasured risk factors of hypertension, including worse diet, poor sleep, depression, anxiety, or social isolation.56–58
These unmeasured variables may be related to long commutes as well as neighborhood factors associated with suburban communities that may limit opportunities for physical activity and social cohesion.59–61
This study has several strengths. Commuting distance was calculated based on street networks using a GIS instead of relying on self-report. In addition, the extensive physical examination provided a unique opportunity to assess CRF and measured BMI, as well as elements of the metabolic syndrome.
Limitations include the cross-sectional study design and limited generalizability of the study population, consisting of predominantly white, well-educated and healthier adults of middle-to-upper SES and under-representation of women. Although the homogeneity of the population with respect to education and race/ethnicity may improve internal validity, some residual confounding may be present due to other unmeasured socioeconomic variables (e.g., occupation and income). Other limitations include lack of information about the mode and frequency of commuting; however, it was anticipated that the vast majority of participants commuted by automobile given that more than 95% of the workers >16 years who worked outside the home commuted by private vehicle in 2005–2007 in the Dallas–Fort Worth–Arlington Metropolitan Statistical Area.22
In addition, information about time spent commuting and the validity of the network route to actual distance traveled by the participant were lacking. Differences in shortest versus actual route are expected for a variety of reasons (e.g., traffic, childcare). Categorizing commuting distance may have minimized some of this measurement error. Also, participants whose work addresses were their home addresses were excluded. Future studies are needed to examine how telecommuting and working from home affects health indicators.
Finally, commuting by automobile represents only one of many forms of sedentary behavior, and this study lacked data on other important contributors to sedentary time, such as occupational sitting and TV viewing. At the same time, time spent riding in a car has been shown to be a predictor of cardiovascular mortality in the population.15
In addition, TV viewing is poorly correlated with total sedentary time in working populations,62
and occupational sitting is expected to be common given that an estimated 90% or more of adults in this study population work in sedentary professional or managerial positions based on job title. Commuting distance represents a measured source of sedentary behavior with variability in this study population. Because we cannot rule out all competing explanations with these methodologic limitations, future prospective studies are needed in more-diverse populations with precise assessment of sedentary time across multiple behavioral domains to tease out the independent effects of passive commuting on health.41