RESEARCH Central fatness and risk of all cause mortality: systematic review BMJ: first published as 10.1136/bmj.m3324 on 23 September 2020. Downloaded from and dose-response meta-analysis of 72 prospective cohort studies Ahmad Jayedi,1,2 Sepideh Soltani,3,4 Mahdieh Sadat Zargar,5 Tauseef Ahmad Khan,6,7,8 Sakineh Shab-Bidar2 For numbered affiliations see ABSTRACT follows: waist circumference (10 cm, 3.94 inch end of the article. OBJECTIVE increase): 1.11 (95% confidence interval 1.08 to Correspondence to: S Shab-Bidar To quantify the association of indices of central 1.13, I2=88%, n=50); hip circumference (10 cm, 3.94 [email protected] 2 (ORCID 0000-0002-0167-7174) obesity, including waist circumference, hip inch increase): 0.90 (0.81 to 0.99, I =95%, n=9); Additional material is published circumference, thigh circumference, waist-to-hip thigh circumference (5 cm, 1.97 inch increase): 0.82 2 online only. To view please visit ratio, waist-to-height ratio, waist-to-thigh ratio, body (0.75 to 0.89, I =54%, n=3); waist-to-hip ratio (0.1 the journal online. adiposity index, and A body shape index, with the risk unit increase): 1.20 (1.15 to 1.25, I2=90%, n=31); Cite this as: BMJ 2020;370:m3324 of all cause mortality in the general population, and to waist-to-height ratio (0.1 unit increase): 1.24 (1.12 http://dx.doi.org/10.1136/bmj.m3324 clarify the shape of the dose-response relations. to 1.36, I2=94%, n=11); waist-to-thigh ratio (0.1 unit 2 Accepted: 11 August 2020 DESIGN increase): 1.21 (1.03 to 1.39, I =97%, n=2); body Systematic review and meta-analysis. adiposity index (10% increase): 1.17 (1.00 to 1.33, I2=75%, n=4); and A body shape index (0.005 unit DATA SOURCES increase): 1.15 (1.10 to 1.20, I2=87%, n=9). Positive PubMed and Scopus from inception to July 2019, and associations persisted after accounting for body the reference lists of all related articles and reviews. mass index. A nearly J shaped association was found ELIGIBILITY CRITERIA FOR SELECTING STUDIES between waist circumference and waist-to-height ratio Prospective cohort studies reporting the risk estimates and the risk of all cause mortality in men and women. of all cause mortality across at least three categories A positive monotonic association was observed of indices of central fatness. Studies that reported for waist-to-hip ratio and A body shape index. The continuous estimation of the associations were also association was U shaped for body adiposity index. included. CONCLUSIONS DATA SYNTHESIS Indices of central fatness including waist circumference, A random effects dose-response meta-analysis was waist-to-hip ratio, waist-to-height ratio, waist- http://www.bmj.com/ conducted to assess linear trend estimations. A one to-thigh ratio, body adiposity index, and A body stage linear mixed effects meta-analysis was used for shape index, independent of overall adiposity, were estimating dose-response curves. positively and significantly associated with a higher RESULTS all cause mortality risk. Larger hip circumference and Of 98 745 studies screened, 1950 full texts were fully thigh circumference were associated with a lower reviewed for eligibility. The final analyses consisted risk. The results suggest that measures of central of 72 prospective cohort studies with 2 528 297 adiposity could be used with body mass index as on 2 October 2021 by guest. Protected copyright. participants. The summary hazard ratios were as a supplementary approach to determine the risk of premature death. WHAT IS ALREADY KNOWN ON THIS TOPIC Introduction Existing evidence suggests that central fatness might be more strongly Two recent comprehensive meta-analyses assessed associated with the risk of mortality than overall obesity, however the results the association of general adiposity, represented by have not been quantitatively gathered body mass index, with the risk of all cause mortality in 1 2 Several large scale prospective cohort studies have suggested a positive the general population. The authors included more linear or J shaped association between indices of central fatness with all cause than 200 prospective cohort studies and the results mortality risk indicated that a U shaped and a J shaped association Systematic reviews and meta-analyses assessing the dose-response association existed between body mass index and the risk of all between indices of central fatness and all cause mortality risk are lacking cause mortality in the general population and healthy never smokers, respectively. The lowest risk was WHAT THIS STUDY ADDS observed for a body mass index of 22-23 in healthy Central fatness, reflected by large waist circumference, waist-to-hip ratio, and never smokers. Body mass index is easy to obtain and waist-to-height ratio, independent of overall adiposity, was associated with a so is the most frequent anthropometric measure used higher risk of all cause mortality to investigate obesity-mortality and obesity-morbidity The results suggest a nearly J shaped association between waist circumference associations. and risk of all cause mortality The validity of body mass index as an appropriate indicator of obesity has been questioned.3 Research Larger hip circumference and thigh circumference were associated with a lower suggests that body mass index does not differentiate risk of all cause mortality the bmj | BMJ 2020;370:m3324 | doi: 10.1136/bmj.m3324 1 RESEARCH between lean body mass and fat mass; therefore, Search strategy BMJ: first published as 10.1136/bmj.m3324 on 23 September 2020. Downloaded from when using body mass index as a measure, inaccurate We combined keywords relevant to general and central assessment of adiposity could occur.4 Additionally, obesity, mortality, and study design to find potentially the most important limitation of body mass index is relevant studies (table S1 in appendix 1). We searched that it does not reflect regional body fat distribution. PubMed and Scopus databases from inception to July Existing evidence suggests that central obesity 2019, followed by a hand search of the reference and abdominal deposition of fat is more strongly lists of related articles and reviews. We restricted our associated with cardiometabolic risk factors5 6 systematic search to articles published in English. and chronic disease risk7-10 than overall obesity. Furthermore, most studies, especially large scale Eligibility and study selection cohort studies, that have assessed the association of Two authors (AJ and MSZ) reviewed the title and body mass index with the risk of mortality have used abstract of all articles obtained. They selected self-report height and weight to calculate body mass potentially eligible studies that were conducted in index.1 Participants tend to under report their weight the general population (aged >18 years and older); and over report their height, which could result in had a prospective observational design; considered inaccurate assessment of adiposity.11 In contrast, self-reported or measured anthropometric indices as most studies assessing the association of central exposure and in at least three quantitative categories; fatness with the risk of chronic disease used measured considered all cause mortality as the outcome of anthropometry. interest; reported adjusted effect sizes across categories Taking this evidence into account, indices of central of exposures; and provided the numbers of participants obesity might be more accurate than body mass index or person years and events across categories of when estimating adiposity, and therefore could be exposures. All cause mortality was defined as death by more closely and strongly associated with the risk of any cause, including non-communicable diseases such mortality. Despite the evidence, systematic reviews as cardiovascular disease, site specific cancers, type 2 and meta-analyses are lacking that comprehensively diabetes, and respiratory disease, communicable and assess the association of indices of central fatness infectious diseases, and any other cause of death. with the risk of all cause mortality in the general We also included studies that reported continuous population. Only a few meta-analyses of prospective estimation of the associations. For duplicate publica- cohort studies have assessed the association of tions, those with the largest number of participants indices of central adiposity with all cause mortality were included. However, when different publications risk12-14; however, they have important limitations. from the same study reported effect sizes as categorical For example, they were performed in older popula- and continuous, the study with the categorical model http://www.bmj.com/ tions,13 they did not perform non-linear dose- was selected for inclusion in both linear and non-linear response analyses,14 or they did not include most dose-response analyses. Some study publications of the evidence.12 Another pooled analysis of 11 reported effect sizes as categorical but the exact amount prospective cohort studies assessed the association of the standard deviation was not reported in the text, of waist circumference with the risk of all cause while other publications from the same study reported mortality,15 but they only included studies from the direct effect size for a one standard deviation Western countries. increment in anthropometric measures. For these on 2 October 2021 by guest. Protected copyright. We aimed to perform a systematic review and studies, the publication with the categorical model dose-response meta-analysis of prospective cohort was included in dose-response analyses and another studies to investigate the association of indices of publication that reported the direct effect size for a central fatness with the risk of all cause mortality one standard deviation increment was included in the in the general population, in never smokers, and in standard deviation analysis. We excluded retrospective healthy never smokers.