Nicolai A. Lund-Blix, Stine Dydensborg Sander, Ketil Størdal, Anne-Marie Nybo Andersen, Kjersti S.Rønningen, Geir Joner, Torild Skrivarhaug, Pål R. Njølstad, Steffen Husby and Lars C. Stene Diabetes Care 2017 May; 
OBJECTIVE Our aim was to study the relation between the duration of full and any breastfeeding and risk of type 1 diabetes.
RESEARCH DESIGN AND METHODS We included two population-based cohorts of children followed from birth (1996–2009) to 2014 (Denmark) or 2015 (Norway).
We analyzed data from a total of 155,392 children participating in the Norwegian Mother and Child Cohort Study and the Danish National Birth Cohort. Parents reported infant dietary practices when their child was 6 and 18 months old. The outcome was clinical type 1 diabetes, ascertained from nationwide childhood diabetes registries. Hazard ratios were estimated using Cox regression.
RESULTS Type 1 diabetes was identified in 504 children during follow-up, and the incidence of type 1 diabetes per 100,000 person-years was 30.5 in the Norwegian cohort and 23.5 in the Danish cohort.
Children who were never breastfed had a twofold increased risk of type 1 diabetes compared with those who were breastfed (hazard ratio 2.29 [95% CI 1.14–4.61] for no breastfeeding vs. any breastfeeding for ≥12 months). Among those who were breastfed, however, the incidence of type 1 diabetes was independent of duration of both full breastfeeding (hazard ratio per month 0.99 [95% CI 0.97–1.01]) and any breastfeeding (0.97 [0.92–1.03]).
CONCLUSIONS Suggestive evidence supports the contention that breastfeeding reduces the risk of type 1 diabetes. Among those who were breastfed, however, no evidence indicated that prolonging full or any breastfeeding was associated with a reduced risk of type 1 diabetes.
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