Downloaded from oem.bmj.com on October 13, 2014 - Published by group.bmj.com
PostScript
Shift work and diabetes: a meta-analysis In a meta-analysis, Gan et al1 found a stronger association between shift work and diabetes for men than for women. In that meta-analysis, three studies contributed to the pooled estimate by 90% and nine studies by 10% only. Studies conducted among women were large-to-medium sized (3 studies, N=179 891), while studies conducted among men were small-to-medium sized (9 studies, N=24 629). It seems that gender difference in the magnitude of the association between shift work and diabetes is due to small-study effects. Small studies tend to report larger effect sizes than larger studies.2 804
Occup Environ Med November 2014 Vol 71 No 11
Downloaded from oem.bmj.com on October 13, 2014 - Published by group.bmj.com
PostScript REFERENCES 1
2
3
4
5
Gan Y, Yang C, Tong X, et al. Shift work and diabetes mellitus: a meta-analysis of observational studies. Occup Environ Med Published Online First: 16 Jul 2014. doi:10.1136/oemed-2014-102150 Nuesch E, Trelle S, Reichenbach S, et al. Small study effects in meta-analyses of osteoarthritis trials: meta-epidemiological study. BMJ 2010;341:c3515. Mikuni E, Ohoshi T, Hayashi K, et al. Glucose intolerance in an employed population. Tohoku J Exp Med 1983;141(Suppl):251–6. Ika K, Suzuki E, Mitsuhashi T, et al. Shift work and diabetes mellitus among male workers in Japan: does the intensity of shift work matter? Acta Med Okayama 2013;67:25–33. Morikawa Y, Nakagawa H, Miura K, et al. Shift work and the risk of diabetes mellitus among Japanese male factory workers. Scand J Work Environ Health 2005;31:179–83.
Figure 1 A random-effects meta-analysis of 12 studies on the association between shift work and diabetes according to the size of included studies. The size of the grey shaded area indicates the weight of each study. Horizontal lines show the 95% CIs (ES, effect size).
To explain the gender difference, I estimated a prevalence ratio for a cross-sectional study,3 and used a fixed-effect to combine subgroups of a single study and a random-effects meta-analysis to combine the estimates of different studies. Since the power of a study depends on sample size and number of events, I stratified included studies into three equal groups (large, medium, small) using SE. Using sample size yielded similar results. A meta-analysis of 12 studies showed small-study effects (figure 1). Small-sized and medium-sized studies reported larger effect size than large studies ( p=0.001). Gender difference was not due to publication bias. A funnel plot of nine studies conducted among men was symmetrical ( p for Egger’s test=0.78). The pooled estimate attenuated by 1.5% only after adjustment for publication bias. The gender difference can be due to the following reasons:
original sample in the analysis, and another study5 limited the analysis to a subgroup. Third, different exposure for men and women. Rotating shift was studied only among men. The stronger association for men can partly be explained by this type of exposure. Rahman Shiri Correspondence to Dr Rahman Shiri, Centre of Expertise for Health and Work Ability, and Disability Prevention Centre, Finnish Institute of Occupational Health, 00250 Helsinki, Finland; rahman.shiri@ttl.fi Competing interests None. Provenance and peer review Not commissioned; internally peer reviewed.
To cite Shiri R. Occup Environ Med 2014;71:804– 805.
First, confounding. For women, all studies controlled their estimates for body mass index and physical activity, whereas for men, half of the studies did not control their estimates for these two factors. For men, small-to-medium-sized studies exaggerated their risk estimates by more than threefold by not controlling for these two confounding factors.
Received 21 July 2014 Revised 31 July 2014 Accepted 7 August 2014 Published Online First 10 September 2014
Second, selection bias. For example, a small study4 included only 36% of
Occup Environ Med 2014;71:804–805. doi:10.1136/oemed-2014-102469
Occup Environ Med November 2014 Vol 71 No 11
▸ http://dx.doi.org/10.1136/oemed-2014-102512
805
Downloaded from oem.bmj.com on October 13, 2014 - Published by group.bmj.com
Shift work and diabetes: a meta-analysis Rahman Shiri Occup Environ Med 2014 71: 804-805 originally published online September 10, 2014
doi: 10.1136/oemed-2014-102469
Updated information and services can be found at: http://oem.bmj.com/content/71/11/804.2.full.html
These include:
References
This article cites 4 articles, 1 of which can be accessed free at: http://oem.bmj.com/content/71/11/804.2.full.html#ref-list-1
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To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions
To order reprints go to: http://journals.bmj.com/cgi/reprintform
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PostScript
Shift work and diabetes: a meta-analysis In a meta-analysis, Gan et al1 found a stronger association between shift work and diabetes for men than for women. In that meta-analysis, three studies contributed to the pooled estimate by 90% and nine studies by 10% only. Studies conducted among women were large-to-medium sized (3 studies, N=179 891), while studies conducted among men were small-to-medium sized (9 studies, N=24 629). It seems that gender difference in the magnitude of the association between shift work and diabetes is due to small-study effects. Small studies tend to report larger effect sizes than larger studies.2 804
Occup Environ Med November 2014 Vol 71 No 11
Downloaded from oem.bmj.com on October 13, 2014 - Published by group.bmj.com
PostScript REFERENCES 1
2
3
4
5
Gan Y, Yang C, Tong X, et al. Shift work and diabetes mellitus: a meta-analysis of observational studies. Occup Environ Med Published Online First: 16 Jul 2014. doi:10.1136/oemed-2014-102150 Nuesch E, Trelle S, Reichenbach S, et al. Small study effects in meta-analyses of osteoarthritis trials: meta-epidemiological study. BMJ 2010;341:c3515. Mikuni E, Ohoshi T, Hayashi K, et al. Glucose intolerance in an employed population. Tohoku J Exp Med 1983;141(Suppl):251–6. Ika K, Suzuki E, Mitsuhashi T, et al. Shift work and diabetes mellitus among male workers in Japan: does the intensity of shift work matter? Acta Med Okayama 2013;67:25–33. Morikawa Y, Nakagawa H, Miura K, et al. Shift work and the risk of diabetes mellitus among Japanese male factory workers. Scand J Work Environ Health 2005;31:179–83.
Figure 1 A random-effects meta-analysis of 12 studies on the association between shift work and diabetes according to the size of included studies. The size of the grey shaded area indicates the weight of each study. Horizontal lines show the 95% CIs (ES, effect size).
To explain the gender difference, I estimated a prevalence ratio for a cross-sectional study,3 and used a fixed-effect to combine subgroups of a single study and a random-effects meta-analysis to combine the estimates of different studies. Since the power of a study depends on sample size and number of events, I stratified included studies into three equal groups (large, medium, small) using SE. Using sample size yielded similar results. A meta-analysis of 12 studies showed small-study effects (figure 1). Small-sized and medium-sized studies reported larger effect size than large studies ( p=0.001). Gender difference was not due to publication bias. A funnel plot of nine studies conducted among men was symmetrical ( p for Egger’s test=0.78). The pooled estimate attenuated by 1.5% only after adjustment for publication bias. The gender difference can be due to the following reasons:
original sample in the analysis, and another study5 limited the analysis to a subgroup. Third, different exposure for men and women. Rotating shift was studied only among men. The stronger association for men can partly be explained by this type of exposure. Rahman Shiri Correspondence to Dr Rahman Shiri, Centre of Expertise for Health and Work Ability, and Disability Prevention Centre, Finnish Institute of Occupational Health, 00250 Helsinki, Finland; rahman.shiri@ttl.fi Competing interests None. Provenance and peer review Not commissioned; internally peer reviewed.
To cite Shiri R. Occup Environ Med 2014;71:804– 805.
First, confounding. For women, all studies controlled their estimates for body mass index and physical activity, whereas for men, half of the studies did not control their estimates for these two factors. For men, small-to-medium-sized studies exaggerated their risk estimates by more than threefold by not controlling for these two confounding factors.
Received 21 July 2014 Revised 31 July 2014 Accepted 7 August 2014 Published Online First 10 September 2014
Second, selection bias. For example, a small study4 included only 36% of
Occup Environ Med 2014;71:804–805. doi:10.1136/oemed-2014-102469
Occup Environ Med November 2014 Vol 71 No 11
▸ http://dx.doi.org/10.1136/oemed-2014-102512
805
Downloaded from oem.bmj.com on October 13, 2014 - Published by group.bmj.com
Shift work and diabetes: a meta-analysis Rahman Shiri Occup Environ Med 2014 71: 804-805 originally published online September 10, 2014
doi: 10.1136/oemed-2014-102469
Updated information and services can be found at: http://oem.bmj.com/content/71/11/804.2.full.html
These include:
References
This article cites 4 articles, 1 of which can be accessed free at: http://oem.bmj.com/content/71/11/804.2.full.html#ref-list-1
Email alerting service
Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions
To order reprints go to: http://journals.bmj.com/cgi/reprintform
To subscribe to BMJ go to: http://group.bmj.com/subscribe/
