Commentary: Metabolic syndrome as a result of shift work exposure?

In this new article from Belgium, ‘Rotating shift work and the metabolic syndrome: a prospective study’ by De Bacquer et al., 2 a successfully designed follow-up study (46 years), with reasonable confounding control, has been conducted. The results show adverse patterns of high-density lipoprotein cholesterol, triglycerides, blood pressure and anthropometrical data in subjects engaged in shift work compared with day workers. The authors show adverse outcomes both for combinations of metabolic data, using standard definitions of the metabolic syndrome, and for several isolated metabolic variables. Additionally, an anticipated dose–response effect is observed and discussed, although admittedly the exposure categories for shift workers are rather crude, or include only a few participants. The limitations of the study that might limit interpretation are well discussed and do not compromise the main findings. However, a difficulty arises with inclusion of the persons already affected by disease in the cohort as the direction of causality, and mechanisms underlying disease cannot be determined. Why has it been, and why is it still, difficult to explore metabolic syndrome data similar to those found in the De Bacquer study among shift workers? First, there are circadian variations in lipid and lipoprotein profiles, not only within but also between individuals, 3 that have to be estimated and considered. Secondly, evidence of elevated blood pressure in shift workers compared with day workers has not been convincingly demonstrated. However, a few cohort studies, notably from Japan, have confirmed a greater impact on hypertension progression in shift workers. 4 Ambulatory blood pressure monitoring provides more complete information on blood pressure itself and its variation over a 24-h period when an individual is engaged in routine activities. Future studies may wish to employ this technique to provide a better assessment of the impact of shift work on blood pressure. Importantly, loss of nocturnal dipping of blood pressure has adverse consequences in terms of cardiovascular risk, and may not be evident from daytime measurements alone, therefore assessment of nocturnal blood pressure may be particularly important in this regard. 5