Infant feeding and growth: Putting the horse before the cart

Michael S. Kramer*, Neil Davies, Emily Oken, Richard M. Martin, Mourad Dahhou, Xun Zhang, Seungmi Yang

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

10 Citations (Scopus)
469 Downloads (Pure)


Background Previous observational studies have consistently shown slower weight and length gains in infants with prolonged breastfeeding than in those who were formula-fed from birth or breastfed for a shorter duration. These studies inferred that prolonged breastfeeding causes slower growth in infancy. Objective We compared infant growth associated with ≥12 mo of breastfeeding with a shorter duration of breastfeeding on the basis of 3 different analytic approaches to the same data from a randomized trial: intention-to-treat (ITT; "as randomized"), observational ("as fed"), and instrumental variable (IV; by using randomization as an "instrument" to achieve ≥12 mo of breastfeeding). Design This was a cluster-randomized trial of a breastfeeding-promotion intervention. Anthropometric measurements were obtained at birth and at 1, 2, 3, 6, 9, and 12 mo. Results The 3 analytic approaches yielded different results. The ITT approach showed more rapid growth in the first 2 mo among infants randomly assigned to the breastfeeding-promotion intervention than among control infants, with a decreasing difference over the ensuing months and nearly identical weight, length, and body mass index by 12 mo. The observational analysis showed a different trend: higher weight and length in infants who were breastfed ≥12 mo than in those who were breastfed <12 mo during the first 3 mo and no difference by 6 mo, while infants who were breastfed <12 mo showed increasingly higher weight and length from 6 to 12 mo. The IV analysis showed a temporal pattern that was similar to that seen in the ITT analysis, but with larger (and less precise) differences between infants breastfed for ≥12 compared with <12 mo. Conclusions We observed major differences in experimental (ITT and IV) compared with observational approaches to analyzing data obtained from the same children. These approaches lead to opposite causal inferences about the relation between infant feeding and growth and underline the importance of ensuring that the postulated cause (feeding) temporally precedes its hypothesized effect (growth). This trial is registered at as ISRCTN37687716.

Original languageEnglish
Pages (from-to)635-639
Number of pages5
JournalAmerican Journal of Clinical Nutrition
Issue number4
Early online date9 Apr 2018
Publication statusPublished - Apr 2018


  • breastfeeding
  • infant growth
  • instrumental variable
  • randomized controlled trials
  • reverse causality


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