A meta-analysis published in the Journal of Occupational and Environmental Medicine has concluded that shift work is associated with an increased risk of diabetes mellitus, especially in men and in those working rotating shifts.

The authors searched PubMed, Embase, Web of Science and Proquest for relevant studies. Overall, 448 potential studies were identified. Of these 434 were excluded on various grounds, including inappropriate inclusion criteria, lack of observational study design, lack of diabetes outcomes, absence of shift work exposure, subjects already with a diagnosis of diabetes or a study design that did not allow the association between diabetes and shift work to be assessed.

Fourteen studies remained, of which one had been reported in two or more papers. This left 12 eligible studies with 28 independent reports involving 226,652 participants and 14,595 patients with diabetes.

The studies were from a range of countries (Japan, Sweden, United States, Belgium and China), eight studies included men only and two women only. There were seven prospective cohort studies, one retrospective cohort study and four cross-sectional studies. The number of participants ranged from 475 to 107,915.

Four studies were classified as rotating shifts, two irregular or unspecified shifts, three night shifts and three mixed or evening shifts only.

• The results showed a pooled adjusted odds ratio (OR) for the association between any exposure to shift work and diabetes risk of 1.09 (95% CI: 1.05-1.12)

• Sub-group analysis showed a stronger association between shift work and diabetes for men OR 1.37 (95% CI: 1.20-1.56), than for women OR 1.09 (95% CI: 1.04-1.14)

• Further sub-group analysis showed that evening shifts and mixed shifts did not increase the risk of diabetes, but all other shift patterns did

• The rotating shift pattern showed the highest risk OR 1.42 (95% CI: 1.19-1.69)

Several observational studies had previously noted an increased risk of diabetes in shift workers, but none had used quantitative techniques to estimate risk.

The headline finding from the analysis is that exposure to shift work increases the risk of diabetes by 9%, and that men may be more at risk than women.

Shift work can disturb health in two main ways. First, it has an effect on physiology, when normal circadian rhythms are overridden. There is some evidence that insulin resistance is increased and that circadian disruption can increase the development of type 2 diabetes in susceptible individuals.¹

Second, it interferes with normal social interaction, and may be associated with less exercise and unhealthy patterns of eating, as well as continual low-level stress.

This meta-analysis did include a very heterogeneous group of research papers, and although I am not a statistical expert, I would be cautious about the strength of the results. Allowing for this, the findings appeal strongly to the commonsense of those of us who work on the front line with diabetes patients. Achieving good control always seems more of a challenge in patients who work shifts, at every level of diabetes treatment. The complex regimens of insulin-dependent shift workers would suggest that diurnal rhythms might well have a significant effect on this aspect of metabolism.

Dr Matthew Lockyer