Body-mass index and all-cause mortality

The meta-analysis of body-mass index (BMI) and mortality from the Global BMI Mortality Collaboration in The Lancet,1Global BMI Mortality CollaborationBody-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents.Lancet. 2016; 388: 776-786Summary Full Text Full Text PDF PubMed Scopus (1340) Google Scholar emphasised the selection of very large studies, few of which use measured weight and height data. Because of this selection bias, 94% of deaths in their primary analyses of North American data came from 20 studies with self-reported weight and height. The Global BMI Mortality Collaboration found that for North America the hazard ratio (HR) for overweight BMI was 1·00 using measured weight and height, and 1·14 using self-reported weight and height. BMIs calculated from self-reported weight and height are systematically biased compared with BMIs calculated from objectively measured data,2Finucane MM Stevens GA Cowan MJ et al.on behalf of the Global Burden of Metabolic Risk Factors of Chronic Diseases Collaborating Group (Body Mass Index)National, regional, and global trends in body-mass index since 1980: systematic analysis of health examination surveys and epidemiological studies with 960 country-years and 9·1 million participants.Lancet. 2011; 377: 557-567Summary Full Text Full Text PDF PubMed Scopus (3129) Google Scholar despite the high correlation between the two. The errors in self-reported weight and height can lead to substantial misclassification into BMI categories and attendant bias in HRs. Since the errors in BMI classification from self-reported weight and height are known to vary by multiple potential confounding factors such as age, sex, race, socioeconomic status, health conditions, region, and language,2Finucane MM Stevens GA Cowan MJ et al.on behalf of the Global Burden of Metabolic Risk Factors of Chronic Diseases Collaborating Group (Body Mass Index)National, regional, and global trends in body-mass index since 1980: systematic analysis of health examination surveys and epidemiological studies with 960 country-years and 9·1 million participants.Lancet. 2011; 377: 557-567Summary Full Text Full Text PDF PubMed Scopus (3129) Google Scholar, 3Gildner TE Barrett TM Liebert MA Kowal P Snodgrass JJ Does BMI generated by self-reported height and weight measure up in older adults from middle-income countries? Results from the study on global AGEing and adult health (SAGE).BMC Obes. 2015; 2: 44Crossref PubMed Scopus (7) Google Scholar, 4Boström G Diderichsen F Socioeconomic differentials in misclassification of height, weight and body mass index based on questionnaire data.Int J Epidemiol. 1997; 26: 860-866Crossref PubMed Scopus (206) Google Scholar, 5Krul AJ Daanen HA Choi H Self-reported and measured weight, height and body mass index (BMI) in Italy, the Netherlands and North America.Eur J Public Health. 2011; 21: 414-419Crossref PubMed Scopus (119) Google Scholar unmeasured residual confounding is likely. This confounding probably varies unpredictably across studies and thus can bias not only individual studies but also comparisons between subgroups and regional comparisons. Because of the high potential for misclassification and unmeasured confounding associated with self-report, the Global BMI Mortality Collaboration's goal of strict control for bias would perhaps be better achieved by excluding studies with self-reported height and weight and only using studies with measured data for analysis. I declare no competing interests. Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continentsThe associations of both overweight and obesity with higher all-cause mortality were broadly consistent in four continents. This finding supports strategies to combat the entire spectrum of excess adiposity in many populations. Full-Text PDF Open AccessBody-mass index and all-cause mortality – Authors' replyAssociations of measured body-mass index (BMI) with mortality within just a few years of the BMI measurement can be strongly distorted by reverse causality (ie, by life-threatening neoplastic, respiratory, vascular, or other diseases having caused weight loss before the BMI was measured). By contrast, associations of measured BMI with mortality several years after the BMI measurement should be little affected by reverse causality, although they can still be strongly affected by confounding, especially in populations in which low BMI is correlated with smoking. Full-Text PDF