Comparative effects of the restriction method in two large observational studies of body mass index and mortality among adults

BackgroundA method applied in some large studies of weight and mortality is to begin with a well-defined analytic cohort and use successive restrictions to control for methodologic bias and arrive at final analytic results. Materials and methodsTwo observational studies of body mass index and mortality allow a comparative assessment of these restrictions in very large data sets. One was a meta-analysis of individual participant data with a sample size of 8 million. The second was a study of a South Korean cohort with a sample size of 12 million. Both presented results for participants without pre-existing disease before and after restricting the sample to never-smokers and deleting the first 5 years of follow-up. ResultsInitial results from both studies were generally similar, with hazard ratios (HRs) below 1 for overweight and above 1 for underweight and obesity. The meta-analysis showed higher HRs for overweight and obesity after the restrictions, including a change in the direction of the HR for overweight from 099 (95% CI: 098-101) to 111 (95% CI: 110, 111). The South Korean data showed little effect of the restrictions and the HR for overweight changed from 085 (95% CI: 084-086) to 091 (95% CI: 090, 091). The summary effect size for overweight was 090 (95% CI: 089-091) before restrictions and 102 (95% CI: 102, 103) after restrictions. ConclusionsThe effect of the restrictions is not consistent across studies, weakening the argument that analyses without such restrictions lack validity.