The association between long working hours and metabolic syndrome remains elusive

In the SUN longitudinal study with 6845 university graduates followed-up for 8.3 years, we found that long working hours did not exhibit any association with an increased risk of MetS. 1 Recently, a cross-sectional study including 466 female employees from two hospitals in Ontario, Canada, reported that full-time work status, extended shift length, and working ≥ 35 paid overtime hours/year were associated with higher risk of MetS. 2 Another cross-sectional study with 4,456 employees in Korea reported no significant differences in the prevalence of MetS according to weekly working hours. 3 These studies had a cross-sectional design, which does not guarantee the temporal sequence for this association. We appreciate the comments on our article by Dr. Kawada. However, it should be consider that (1) Though MetS increases the risk for cardiovascular disease (CVD), not all subjects with MetS will eventually develop cardiovascular events; they are two different entities; (2) No study evaluating the association between long working hours and MetS was quoted by Kawada. Indeed, an important meta-analysis concluded that employees who work for long hours (≥ 55 h/week) have a higher risk of stroke than those working standard hours (< 35 h/week), but the association with coronary heart disease (CHD) was weaker. 4 Moreover, when the analysis was stratified by the occupational socioeconomic status, long working hours increased the risk of CHD only in participants in the low socioeconomic status group; (3) Other labour exposure factors were used to justify the association between long working hours and CVD such as job strain, shift work and sleep restriction. We believe that, beyond the quantification of working hours, further characteristics of the work should also be considered because some of these conditions might be strongly associated with MetS or CVD. As previously cited, Kivimäki et al.4 reported that workers in low socioeconomic status had a higher risk of CHD. Canuto et al.5 reported that higher educational level was protective against MetS in 902 fixed-shift Brazilian workers. It has been proposed that ‘blue collar workers’ have a greater risk of negative health events than ‘white collar workers’. Occupations which require more challenging and mentally active work may have a protective effect against negative health events, because the professional is less affected by job strain. Thus, the lack of association between long working hours and MetS in our study could be explained because the SUN cohort included only highly educated individuals that, generally, are white collar workers; (4) It was suggested that our results could be explained by lack of the adjustment for sleep parameters and other potential confounders. Additionally, we adjusted our results for sleep duration. Long working hours were not significantly associated with MetS; (5) Finally, we used the same cut-off points proposed by Kiwimäki et al.4 in their study to categorize working hours. Once again, long working hours were not related to MetS after multivariate adjustment. Thus, we reinforce our conclusion that long working hours did not increase the risk of MetS development in highly educated subjects with white collar jobs. Spanish Government (Grants PI01/0619, PI030678, PI040233, PI042241, PI050976, PI070240, PI070312, PI081943, PI080819, PI1002658, PI1002293, PND2010/87, RD06/0045and G03/140), the Navarra Regional Government (36/2001, 43/2002, 41/2005, 36/2008 and 45/2011) and the University of Navarra. Conflicts of interest : None declared. Key point Long working hours did not increase the risk of MetS development in highly educated subjects with white collar jobs.

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