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August 3, 2005

Underweight, Overweight, Obesity, and Excess Deaths

JAMA. 2005;294(5):551-553. doi:10.1001/jama.294.5.551-a

To the Editor: In their study of deaths associated with underweight, overweight, and obesity, Dr Flegal and colleagues1 conclude that excess mortality due to obesity and overweight is much lower than previously reported. We believe that their analysis is flawed and misleading.

A major challenge in such studies is that low weight is often due to underlying chronic disease, which may exist for many years before death. Thus, lean persons are a mix of smokers, healthy active persons, and those with chronic illness (due to the direct effects of disease on weight and sometimes purposeful weight loss motivated by diagnosis of a serious illness). Their analysis does not successfully disentangle this diverse group. In the main analyses, the study apparently did not exclude persons with known chronic disease at baseline. The authors did conduct some analyses to address reverse causation, such as excluding early follow-up and those persons who had lost weight before baseline (data not shown), and they examined never-smokers separately; however, these approaches should be used simultaneously, not one at a time.2 Unfortunately, the size of the National Health and Nutrition Examination Survey (NHANES) data set is too small to yield precise estimates from these simultaneous analyses. Even just restricting the analysis to never-smokers led to the wide confidence intervals seen in Table 2 in the study by Flegal et al.