Relative risk estimate in HEAT for cycling

The strongest evidence at the time of the first project on the health effects of cycling was the relative risk data from two combined Copenhagen cohort studies (1,2,3). This study included about 7.000 20–60-year-old participants, followed up for average of 14½ years. It found a relative risk of all-cause mortality among regular commuter cyclists of 0.72 (95% confidence interval (CI): 0.57–0.91) compared to non-cycling commuters, for 180 minutes of commuter cycling per week.

In 2013, a new systematic review on the reduced relative risk of all-cause mortality from regular cycling was carried out (4).

To be included in this review, a study was required:

• to be a prospective cohort study;
• to report the level of regular walking or cycling (such as duration, distance or MET equivalent);
• report all-cause mortality rates or risk reductions as outcome; and
• report results independent of (that is, adjusted for) other physical activity.

A total of 8901 titles were identified, and 431 full texts were screened. Seven cycling studies met the inclusion criteria. Based on the Newcastle-Ottawa quality assessment scale for cohort studies, the studies were generally of high quality, scoring between 6 to 9 of 9 possible points. A meta-analysis was carried out, combining the results of these seven studies. Since the available studies used a range of different exposures, to conduct the meta-analysis it was necessary to estimate for each study the reduced risk at a common exposure level. For this purpose, the different cycling exposures used in the studies were converted into MET-hours per week (assuming a linear dose-response relationship and an average intensity of 6.8 METs for cycling if not otherwise stated). The common exposure level was set at 11.25 MET-hours per week. This value was derived from the global physical activity recommendations as corresponding to the recommended level of at least 150 minutes of moderate-intensity physical activity per week (5) using 4.5 METs as an average for moderate-intensity physical activity. Using 6.8 METs as an average intensity for cycling, this exposure represents about 100 minutes of cycling per week.

The international advisory group recommended that, for HEAT, a linear dose–response curve based on a relative risk of 0.90 (CI 0.87–0.94) for cycling and applying a constant absolute risk reduction. The sensitivity of the results to various possible shapes of dose–response relationships was tested. The differences between the various curves were small and the difference in the final risk estimate was no more than 6%.