title: Breast-feeding and overweight in adolescence: within-family analysis [corrected].
abstract: BACKGROUND
  Previous reports have found associations between having been breast-fed and a reduced risk of being overweight. These associations may be confounded by sociocultural determinants of both breast-feeding and obesity. We addressed this possibility by assessing the association of breast-feeding duration with adolescent obesity within sibling sets.
  
  
  METHODS
  We surveyed 5,614 siblings age 9 to 14 years and their mothers. These children were a subset of participants in the Growing Up Today Study, in which we had previously reported an inverse association of breast-feeding duration with overweight. We compared the prevalence of overweight (body mass index exceeding the age-sex-specific 85th percentile) in siblings who were breast-fed longer than the mean duration of their sibship with those who were breast-fed for a shorter period. Then we compared odds ratios from this within-family analysis with odds ratios from an overall (ie, not within-family) analysis.
  
  
  RESULTS
  Mean +/- standard deviation breast-feeding duration was 6.4 +/- 4.0 months, and crude prevalence of overweight was 19%. On average, siblings who were breast-fed longer than their family mean had breast-feeding duration 3.7 months longer than their shorter-duration siblings. The adjusted odds ratio (OR) for overweight among siblings with longer breast-feeding duration, compared with shorter duration, was 0.92 (95% confidence interval = 0.76-1.11). In overall analyses, the adjusted OR was 0.94 (0.88-1.00) for each 3.7-month increment in breast-feeding duration.
  
  
  CONCLUSIONS
  The estimated OR for the within-family analysis was close to the overall estimate, suggesting that the apparent protective effect of breast-feeding on later obesity was not highly confounded by unmeasured sociocultural factors. A larger study of siblings, however, would be needed to confirm this conclusion.
authors.affiliation: Department of Ambulatory Care and Prevention, Harvard Medical School and Harvard Pilgrim Health Care, 133 Brookline Ave., 6th Floor, Boston, Massachusetts 02215, USA. matthew_gillman@hms.harvard.edu
authors.forename: Matthew W
authors.initials: MW
authors.lastname: Gillman
authors.forename: Sheryl L
authors.initials: SL
authors.lastname: Rifas-Shiman
authors.forename: Catherine S
authors.initials: CS
authors.lastname: Berkey
authors.forename: A Lindsay
authors.initials: AL
authors.lastname: Frazier
authors.forename: Helaine R H
authors.initials: HR
authors.lastname: Rockett
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Camargo
authors.forename: Alison E
authors.initials: AE
authors.lastname: Field
authors.forename: Graham A
authors.initials: GA
authors.lastname: Colditz
mesh_terms: D000293:Adolescent; D001900:Boston; D001942:Breast Feeding; D002648:Child; D005260:Female; D006801:Humans; D008297:Male; D050177:Overweight; D012307:Risk Factors; D035781:Siblings