title: Sources of heterogeneity in studies of the BMI-mortality association.
abstract: BACKGROUND
  To date, the amount of heterogeneity among studies of the body mass index-mortality association attributable to differences in the age distribution and length of follow-up has not been quantified. Therefore, we wanted to quantify the amount of heterogeneity attributable to age and follow-up in results of studies on the body mass index-mortality relation.
  
  
  METHODS
  We used optima of the body mass index mortality association reported for 30 populations and performed meta-regression to estimate the amount of heterogeneity attributable to sex, ethnicity, mean age at baseline, percentage smokers, and length of follow-up.
  
  
  RESULTS
  Ethnicity as single factor accounted for 36% (95% CI, 11-56%) of heterogeneity. Mean age and length of follow-up had an interactive effect and together accounted for 56% (95% CI, 24-74%) of the remaining heterogeneity. Sex did not significantly contribute to the heterogeneity, after controlling for ethnicity, age, and length of follow-up.
  
  
  CONCLUSIONS
  A considerable amount of heterogeneity in studies of the body mass index-mortality association is attributable to ethnicity, age, and length of follow-up.
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Ulm, Germany. Electronic address: raphael.peter@uni-ulm.de.
authors.forename: Raphael Simon
authors.initials: RS
authors.lastname: Peter
authors.affiliation: Institute of Epidemiology and Medical Biometry, Ulm University, Ulm, Germany; Agency for Preventive and Social Medicine, Bregenz, Austria.
authors.forename: Gabriele
authors.initials: G
authors.lastname: Nagel
mesh_terms: D000328:Adult; D017677:Age Distribution; D000368:Aged; D015992:Body Mass Index; D005006:Ethnicity / Q000706:statistics & numerical data; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008875:Middle Aged; D009026:Mortality / Q000208:ethnology; D013997:Time Factors
keywords: Body mass index; Heterogeneity; Mortality