title: Childhood obesity: genetic and environmental overlap with normal-range BMI.
abstract: OBJECTIVE
  To understand the overlap between the etiology of obesity and normal variation in BMI in children.
  
  
  METHODS
  Height and weight data were available from a large UK representative sample of twins: 2,342 same-sex pairs at 7 years and 3,526 same-sex pairs at 10 years. The twin method and model-fitting techniques were used to estimate genetic and environmental contributions to BMI. DeFries-Fulker (DF) extremes analysis was used to investigate genetic and environmental influences on the mean difference between obese and normal-weight children. Obesity was classified using the International Obesity Task Force (IOTF) criteria.
  
  
  RESULTS
  At both ages, BMI and obesity were highly heritable (0.60-0.74) and only modestly influenced by shared environmental factors (0.12-0.22). Extremes analyses indicated that genetic and environmental influences on obesity are quantitatively and qualitatively similar to those operating across the range of BMI.
  
  
  CONCLUSIONS
  Obesity is the extreme of the same genetic and environmental factors responsible for variation throughout the distribution of BMI. This finding implies that genes that influence obesity will also be associated with BMI in the normal range, and similar environmental influences will affect BMI in the clinical and normal range. Knowing that obesity is influenced by the same genetic and environmental factors that affect weight at all levels has implications for investigating the mechanisms for weight gain and developing interventions for weight control.
authors.affiliation: Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, King's College London, London, UK. Claire.Haworth@iop.kcl.ac.uk
authors.forename: Claire M A
authors.initials: CM
authors.lastname: Haworth
authors.forename: Robert
authors.initials: R
authors.lastname: Plomin
authors.forename: Susan
authors.initials: S
authors.lastname: Carnell
authors.forename: Jane
authors.initials: J
authors.lastname: Wardle
mesh_terms: D015992:Body Mass Index; D002648:Child; D004777:Environment; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008957:Models, Genetic; D009765:Obesity / Q000235:genetics*; D012016:Reference Values; D012307:Risk Factors; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics; D006113:United Kingdom; D015430:Weight Gain / Q000235:genetics*