Few participants in this population-based cohort reported any walking or biking to work. In men, active commuting was inversely associated with body mass, obesity, triglycerides, diastolic blood pressure, and fasting insulin and positively associated with walking, HDL, and fitness. In women, walking and treadmill time were positively associated with active commuting. However, statistical associations between active commuting and all CVD risk biomarkers in men disappeared with adjustment for BMI, suggesting that BMI is a potential mediator between active commuting and CVD risk. Results were similar when restricted to those living within a 2-mile distance from their place of work.
Associations were clearer for men, who had relatively higher rates and distance of active commuting, thus suggesting that efforts to increase active commuting in women may be particularly relevant for increasing overall physical activity. While the association of active commuting with walking behavior and fitness are clear for women, associations between active commuting and measures of CVD risk were less clear for women. The lack of associations for women could be because women have lower levels of active commuting, or they may have lower intensity of activity during active commuting.
While the positive association between walking and CVD risk has been well investigated for leisure walking (e.g., reviews by Hamer and Chida29
and Murphy, et al.30
), there is less research on the associations with non-leisure forms of physical activity, such as walking for utilitarian purposes.31
A study of Finnish adults observed an inverse association between daily active commuting and ischemic stroke, with highest risk reduction at greater time in active commuting.32
Interestingly, in that study, associations were evident only in the pooled sample of men and women (with BMI and other risk factor adjustment), but not in each separately, indicating relatively modest magnitude of effect. This same research group similarly found reductions in type 2 diabetes,33
other cardiovascular risk factors,34, 35
including reductions in mortality among diabetic men and women36
and hypertensive women37
patients who used active forms of commuting, such as walking or biking. A study in Danish adults observed positive associations between active commuting and high-density lipoprotein cholesterol and negative associations with low-density lipoprotein cholesterol, triglycerides, waist circumference and body mass index..38
There is limited such work in U.S. samples.
The strengths of this study include: extensive CVD risk biomarker data, objective physical activity measures, detailed active commuting data, and additional measures of leisure physical activity. Further, most studies relating walking to CVD risk factors do not adequately control for adiposity and leisure physical activity29
as we have in the present study. Even with these strengths, there are limitations. The CARDIA data are observational in nature and our results do not imply causality. Further, the present study is cross-sectional. Yet, our results suggest that any portion of the commute made by walking or biking is important for maintaining or improving health, regardless of the direction of causation. Unfortunately, the low rates of active transit preclude analyses of dose-response, and thus reduce power to detect effects. Even using the lowest possible threshold (i.e., “any active commuting”) to define active commuting, there were favorable associations with several CVD risk factors in men. Thus, associations could be underestimated due to low variability, and higher levels of active commuting could produce stronger associations with CVD risk factors.
A major limitation is the potential self-selection of active transportation: individuals who are more inclined to be active may be more likely to use active forms of transportation. Indeed, Williams39
has shown that self-selection bias plays a role in the inverse associations between adiposity and walking (leaner individuals selecting to walk greater distances and at higher intensity). Similarly, there is evidence of higher walking among individuals who prefer and live in walkable neighborhoods40
. However, many of the associations in this study remained after controlling for other forms of physical activity.
We are further limited by self-report commuting data and other lifestyle factors, and cannot completely control for misreporting though non-differential measurement error would tend to bias our results toward the null. While our active commuting measure has face validity and was related to fitness levels, no psychometric evaluation was conducted.
While these data do not fully resolve the role of active commuting in health, they contribute information that adds to current thought that additional active commuting would have several benefits. Walking is a particularly good form of activity to target. Among leisure walkers, walking is the sole source of their leisure physical activity.41
Indeed, adherence to physical activity recommendations is higher when considering both leisure and non-leisure forms of physical activity.31
Further, walking can be integrated into other activities beyond leisure into active transportation or commuting42–45
and overall lifestyle activity.1, 6–8
Public support for policies that encourage active commuting has been shown, particularly for individuals with experience using active commuting and with positive attitudes towards walking and biking.46
Intervention research to promote active commuting, reviewed by Ogilvie,47
indicates that the majority of such interventions consistently report a net increase in proportion of trips by foot and in walking in general. Further, increasing active commuting will have dual benefits of increasing population health and in reduction of greenhouse gas emissions.48
Environmental supports for commuting, such as physical environment49–51
and sociocultural49, 50
factors, have been shown to promote active forms of commuting.
These findings support previous studies of health benefits of leisure-time walking and extend these findings to active commuting. Future investigation into the link between active commuting and health outcomes should address the amount of commuting needed for positive health benefit. There is a major need for development of more precise measures of active commuting. Most importantly, the use of longitudinal designs to address selectivity and reverse causality is strongly encouraged. Similarly, research aimed at unraveling the selectivity in active commuting behaviors and understanding whether those who choose to actively commute are healthier and more active is of major importance.