Underestimating the true impact of obesity – Authors' reply
Authors: Solja T Nyberg, G David Batty, Mika Kivimäki
Journal: The Lancet Public Health (2019)

We thank Jacob Bradshaw and colleagues for their careful reading of our paper1Nyberg ST Batty GD Pentti J et al.Obesity and loss of disease-free years owing to major non-communicable diseases: a multicohort study.Lancet Public Health. 2018; 3: e490-e497Summary Full Text Full Text PDF PubMed Scopus (172) Google Scholar and the useful points that they raise. They suggest that we might have underestimated the true impact of obesity on non-communicable disease because outcome ascertainment for selected disorders might have been imprecise, and we did not consider all obesity-related conditions when we formulated our endpoint. We utilised the six most common non-communicable diseases in developed countries which are also WHO's priorities for prevention.2WHOGlobal action plan for the prevention and control of noncommunicable diseases 2013-2020.http://africahealthforum.afro.who.int/IMG/pdf/global_action_plan_for_the_prevention_and_control_of_ncds_2013-2020.pdfDate: 2013Date accessed: November 7, 2018Google Scholar We agree that this list is not exhaustive, and the full impact of obesity might have been underestimated as a consequence. However, there are several reasons to anticipate that an overestimation could have occurred in our computations. First, ascertainment of disease cases was, as Bradshaw and colleagues highlight, made by linkage to records from hospital and death registries. This is of course imperfect, missing less severe forms of chronic disease. This could be a source of overestimation of obesity-related burden because disease severity is concurrent to body-mass index, which leads to a more likely detection of those diseases that accompany severe obesity. Second, we did an observational study which is inevitably subject to the perennial problem of residual confounding. Because confounding typically inflates associations, it is likely that this also contributed to an overestimation of our obesity-disease estimates. Of the obesity-related conditions featured in our analyses, a causal influence on type 2 diabetes, coronary heart disease, and selected cancers only is supported by evidence from randomised controlled trials and Mendelian Randomisation analyses—theoretically less subject to concerns of confounding—in combination with findings from mechanistically-orientated studies.3Goodarzi MO Genetics of obesity: what genetic association studies have taught us about the biology of obesity and its complications.Lancet Diabetes Endocrinol. 2018; 6: 223-236Summary Full Text Full Text PDF PubMed Scopus (217) Google Scholar, 4Van Gaal LF Mertens IL De Block CE Mechanisms linking obesity with cardiovascular disease.Nature. 2006; 444: 875-880Crossref PubMed Scopus (2033) Google Scholar, 5Kahn SE Hull RL Utzschneider KM Mechanisms linking obesity to insulin resistance and type 2 diabetes.Nature. 2006; 444: 840-846Crossref PubMed Scopus (3457) Google Scholar, 6Bianchini F Kaaks R Vainio H Overweight, obesity, and cancer risk.Lancet Oncol. 2002; 3: 565-574Summary Full Text Full Text PDF PubMed Scopus (725) Google Scholar However, the positive associations of body-mass index with asthma and cancers of all sites, which also comprised our outcome, could be at least partially attributable to confounding, again tending towards an overestimation of burden in the present analyses. These points illustrate the multiple factors that could cause bias in effect estimates. Their net effect needs to be considered before one can be confident that our results were indeed an underestimate of the burden of disease due to obesity. We declare no competing interests. Obesity and loss of disease-free years owing to major non-communicable diseases: a multicohort studyMild obesity was associated with the loss of one in ten, and severe obesity the loss of one in four potential disease-free years during middle and later adulthood. This increasing loss of disease-free years as obesity becomes more severe occurred in both sexes, among smokers and non-smokers, the physically active and inactive, and across the socioeconomic hierarchy. Full-Text PDF Open Access