ORIGINAL ARTICLE Childhood body mass index in relation to subsequent risk of type 1 diabetes—A Danish cohort study Julie C Antvorskov 1 | Lise Aunsholt 2 | Karsten Buschard 1 | Michael Gamborg 3 | Kurt Kristensen 4 | Jesper Johannesen 5 | Thorkild I A Sørensen 3,6 | Jannet Svensson 5 1 The Bartholin Institute, Rigshospitalet, Copenhagen, Denmark 2 Department of Pediatrics, Hans Christian Andersen Children´s hospital, Odense, Denmark 3 Department of Clinical Epidemiology, Bispebjerg and Frederiksberg Hospitals, Copenhagen, Denmark 4 Department of Paediatrics, Skejby Hospital, Aarhus, Denmark 5 Department of Paediatrics, Herlev University Hospital, Herlev, Denmark 6 The Novo Nordisk Foundation Center for Basic Metabolic Research and Department of Public Health, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark Correspondence Julie Christine Antvorskov, The Bartholin Institute, Rigshospitalet, Ole Maaløes vej 5, 2200 Copenhagen N, Denmark. Email: julie.antvorskov@gmail.com Funding information The Health Research Fund The incidence of type 1 diabetes (T1D) is increasing, and obesity may be a contributing factor by increasing the risk and accelerating the onset. We investigated the relation between child- hood body mass index z-scores (BMIz) and the later risk of T1D, including association with age at onset of T1D. The study included 238 cases and 10 147 controls selected from the Copen- hagen School Health Record Register (CSHRR). Cases of T1D were identified in the Danish Registry of Childhood and Adolescent Diabetes and 2 regional studies and linked to CSHRR. Using conditional logistic regression models, the association of childhood prediagnostic BMIz at 7 and 13 years of age and changes between these ages with subsequent risk (odds ratio, OR) of T1D was estimated. A greater BMIz at 7 and 13 years of age was associated with increased risk of T1D with OR of 1.23 (confidence interval, CI 1.09-1.37; P = .0001) and 1.20 (CI 1.04- 1.40; P = .016), respectively. The risk was increased by upward changes in z-scores from birth to 7 years (OR=1.21, P = .003) and from 7 to 13 years of age (OR=1.95, P = .023), but in the latter age interval also by a decline in BMIz (OR = 1.91, P = .034). There were no associations between BMIz at 7 and 13 years of age and the age of onset (P = .34 and P = .42, respec- tively). Increased BMIz is associated with a moderate increase in risk of T1D, but with no rela- tion to age at onset within the analyzed age range. Increased BMIz over time is unlikely to explain the rising incidence of T1D. KEYWORDS accelerator hypothesis, BMI, children, incidence, type 1 diabetes 1 | INTRODUCTION The incidence of type 1 diabetes (T1D) has increased worldwide over the last 2 decades, especially among children under the age of 5 years. 1,2 In Denmark, a relative annual increase of more than 3% over the past 10 years has been observed, corresponding to 40% over 10 years. 3 This increase in incidence occurs more rapidly than can be accounted for by genetic changes, thus emphasizing the impact of changes in environmental factors. At the same time, the prevalence of overweight in children has increased, and is found to be as high as 30% in some parts of Europe. 4 Obesity, is an estab- lished risk factor for type 2 diabetes (T2D). 5,6 However, several stu- dies, have indicated that weight gain and obesity also increase the risk of T1D. 7,8 It has been proposed that increased body weight is a common basis for T1D and T2D by accelerating the debut of the disease. 9 This hypothesis has been tested in different settings, but most stu- dies are based on weight and height measures at the time of diag- nosis, which may induce bias due to the effect of the prediagnostic phase of the disease on body weight. 10–12 One study included pre- diagnostic body size parameters from 62 of 168 children measured at the age of 3 or 5 years and at birth. They showed an inverse relation between prediagnostic body mass index (BMI) and age at onset of T1D. 13 Julie C Antvorskov and Lise Aunsholt shared first authorship. © 2017 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Pediatric Diabetes. 2017;1–6. wileyonlinelibrary.com/journal/pedi 1 Received: 16 August 2016 Revised: 4 July 2017 Accepted: 17 July 2017 DOI: 10.1111/pedi.12568