Geographical differences in type 1 diabetes (T1D) prevalence in Europe have been well documented, but little is known about the geographical distribution of autoantibodies specific to GAD65 (GAD65Ab) in the general population without diabetes, which is reported to range between 0.4 and 3%. However, these studies used different methods to define GAD65Ab positivity with cutoff values based on the 97–99th centile or at +3 SD above the mean among healthy individuals without T1D or type 2 diabetes (T2D). In doing so, the prevalence of GAD65Ab among the study cohorts was, by definition, 1–3%. The application of different cutoff levels greatly impairs the direct comparison of prevalence data between studies. Our aims were to 1) explore the prevalence of GAD65Ab positivity using a cutoff defined by specific competition of antibody binding to radiolabeled GAD65 with added autoantigen across eight European countries and 2) compare characteristics of age, sex, and BMI in relation to GAD65Ab positivity. A center-stratified random subcohort of 16,835 (4.9%) individuals was selected from the original European Prospective Investigation into Cancer and Nutrition (EPIC)-InterAct study (1). After exclusion of individuals with known diabetes, GAD65Ab were analyzed in 15,802 (men/women 5,927/9,875, mean age 52.4 ± 9.2 years) samples. The cutoff for GAD65Ab positivity was determined through a competition assay at ≥65 WHO units/mL, and all samples were analyzed at a single laboratory using a radioligand binding assay (2).

In all, 316/15,802 (2.0%) samples were GAD65Ab positive. Sweden, Denmark, U.K., and Spain had the highest and France had the lowest prevalence of GAD65Ab positivity (Table 1); however, these differences were not statistically significant (P = 0.80). We did not detect any association between GAD65Ab positivity and age, sex, or BMI.

This lack of geographical differences in GAD65Ab prevalence in healthy adults is in contrast to the established differences in incidence of T1D in children in Europe, with the Scandinavian countries having the highest incidence, while lower incidence rates are found in southern Europe with the exception of Sardinia (3). These differences have been attributed mainly to genetic, but also environmental, differences between the countries. A detailed analysis of the underlying HLA types of the participants in our subcohort will be necessary to determine whether GAD65Ab positivity in healthy individuals is associated with distinct HLA haplotypes, as has been previously established in T1D patients (4). Moreover, in contrast to previous studies (5), we found no association between GAD65Ab positivity and age, sex, or BMI. We conclude that GAD65Ab positivity in healthy adults is not associated with geographical location, BMI, age, or sex. While the practice of defining autoantibody positivity on the basis of a distribution is useful when comparing antibody frequencies between control subjects and patients, it is less informative when analyzing antibody levels in a population cohort or when comparing the prevalence of positivity between populations.

Acknowledgments. The authors thank all EPIC participants and staff for their contribution to this study. The authors thank Nicola Kerrison (MRC Epidemiology Unit, University of Cambridge) for managing the data and Matt Sims (MRC Epidemiology Unit, University of Cambridge) for managing the blood samples for the EPIC-InterAct project.

Funding. Funding for the InterAct project was provided by the EU FP6 program (grant number LSHM_CT_2006_037197). The autoantibody measurement was funded by Västerbotten County Council and Umeå University, Sweden (to O.R.), the National Institutes of Health (DK26190 and DK017047) (to C.S.H.), and by the Medical Research Council (MC_UU_12015/1) (N.W.).

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Duality of Interest. No potential conflicts of interest relevant to this article were reported.

Author Contributions. O.R., C.S.H., C.L., and N.W. were responsible for study conception and design and acquisition of data, contributed to statistical analyses and interpretation, drafted the manuscript, and obtained funding. P.W. contributed to statistical analyses and interpretation of data and critically revised the manuscript. J.R. carried out the sample analyses and reviewed the manuscript critically. All authors gave final approval of the version of the manuscript to be published. O.R. is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

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Supplementary data