Background Frustrated with the onslaught of articles reporting fascination with results that appear paradoxical but are merely due to selection bias, we studied the apparent effect of obesity on diabetes risk in youth who had a test for diabetes. We hypothesised that obese subjects would have lower rates of diabetes than non-obese subjects due to selection bias, and consequently, obesity would appear to lower the risk of diabetes.
Methods Retrospective cohort study of children (4–9 years), pre-teens (10–12 years) and teenagers (13–19 years). Participation was restricted to those who had a test of haemoglobin A1C along with measured height and weight. Body mass index percentile via the Centers for Disease Control and Prevention age and sex standards was calculated and categorised. The main outcome was A1C%, subsequently categorised at the level for diagnosis of diabetes mellitus (≥6.5%).
Results The sample consisted of 134 (2%) underweight, 1718 (30%) healthy weight, 660 (12%) overweight and 3190 (56%) obese individuals. 16% (n=936) had an A1C≥6.5%. Overall, healthy weight children had 8.2 times the risk of A1C≥6.5% (95% CI 5.3 to 12.7) compared with those in the obese category. The relative risk was 13 in pre-teens (95% CI 8.5 to 20.0) and 3.9 in teenagers (95% CI 3.3 to 4.7).
Conclusions Healthy weight was associated with a 4–13 times higher relative risk of diabetes mellitus compared with being obese. While apparently shocking, the study’s fatal flaw (selection bias) explains the ‘paradoxical’ finding. Ignoring selection bias can delay advances in medical science.
- study design
- epidemiological methods
- research design in epidemiology
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Contributors All authors contributed to the concept and design of the study, as well as the analysis and manuscript writing.
Funding HRB received funding from the Canadian Institute of Health Research Banting Postdoctoral Fellowship Award.
Competing interests None declared.
Ethics approval Our study involved deidentified analysis of pre-existing data and was determined by the University of Minnesota’s Institutional Review Board to be exempt for the need for further review.
Provenance and peer review Not commissioned; externally peer reviewed.