Association of Cycling With All-Cause and Cardiovascular Disease Mortality Among Persons With Diabetes

Abstract

To the Editor We read with great interest the Original Investigation recently published in this journal by Dr Ried-Larsen and colleagues. 1 The authors reported decreased mortality from all causes and from cardiovascular diseases associated with cycling in adults with diabetes. The large sample size and the robust analyses are strengths of the study. However, it is surprising that the reported association between cycling and mortality was independent of practicing other types of physical activity. An almost linear relationship between the amount of physical activity and mortality reduction (4% risk reduction per 15 min of additional physical activity per day) has been demonstrated for healthy individuals and for individuals living with cardiovascular disease. 2 While the findings by Ried-Larsen and colleagues may represent a unique feature of persons with diabetes, these findings may also be associated with a bias comparable to the "healthy worker effect." This phenomenon, known from occupational epidemiology studies, 3 was first discovered in 1885 by William Ogle, 4 who recognized that more physically demanding occupations attract healthier individuals. Reduced mortality rates in those workers may be partly attributable to a related selection bias. Cycling is among the more challenging modes of physical activity. Compared with other exercise types, such as (Nordic) walking, cycling may be especially demanding for older individuals, requiring high muscle strength, postural control, and self-perceived confidence. 5 Thus, whereas regular cycling would likely contribute to reduced all-cause mortality, the results of the study by Ried-Larsen and colleagues may be partly explained by healthier individuals being part of the cycling population. Moreover, the less healthy individuals may be less likely to start cycling during late adulthood; they may choose a safer activity.