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Longer duration of exclusive breastfeeding was associated with a reduced risk of childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL), according to results from a Danish population-based cohort study.

Compared with an exclusive breastfeeding duration of less than 3 months, exclusive breastfeeding for 3 months or longer was associated with a decreased risk of hematologic cancers (adjusted HR 0.66, 95% CI 0.46-0.95), which was largely attributable to a decreased risk of BCP-ALL (aHR 0.62, 95% CI 0.39-0.99), reported Signe Holst Søegaard, PhD, of the Danish Cancer Institute and Danish Cancer Society in Copenhagen, and colleagues.

There was no association between exclusive breastfeeding duration and risk of central nervous system tumors (aHR 0.96, 95% CI 0.51-1.88) or solid tumors (aHR 0.87, 95% CI 0.55-1.41), they noted in .

"Our analyses thereby corroborate previous observations from case-control studies suggesting that breastfeeding is associated with risk of childhood ALL," Søegaard and team wrote. "Notably, our results for BCP-ALL align with the approximately 30% reduced ALL risk in children breastfed exclusively for at least 4 months versus never breastfed in of international case-control studies including more than 10,000 children with ALL."

The authors also suggested that the finding of a reduced risk of BCP-ALL "is consistent with implicating early gut microbiome maturation in childhood BCP-ALL pathogenesis."

In a , Helen D. Bailey, PhD, of Curtin University in Perth, Australia, noted that this is the first population-based cohort study to report that prolonged exclusive breastfeeding may be associated with decreased risk of childhood ALL.

However, she also pointed out that while BCP-ALL is one of the most common childhood cancers, it is still rare, and the reduced relative risk of developing BCP-ALL seen in this study translates into a minimum change in the level of absolute risk.

"Thus, it is unlikely that these findings alone would influence a woman's breastfeeding practice," Bailey wrote.

Yet, prolonged breastfeeding has also been shown to have many short- and long-term benefits for infants in all economic settings, she added. "The findings add to the existing evidence about the critical importance of exclusive breastfeeding."

For this study, Søegaard and colleagues used administrative data on exclusive breastfeeding duration from the Danish National Child Health Register and included 309,473 children (51.3% male) born in Denmark from January 2005 to December 2018. These children were followed from age 1 year until childhood cancer diagnosis, loss to follow-up or emigration, death, age 15 years, or Dec. 31, 2020.

During follow-up, 332 children (0.1%) were diagnosed with cancer at ages 1 to 14 years (mean age at diagnosis 4.24 years, 58.4% male).

Of these kids, 37.3% were diagnosed with hematologic cancers, with 65.3% being ALL -- almost all of which (91.4%) were BCP-ALL. Seven children were diagnosed with acute myeloid leukemia (AML), and fewer than five were diagnosed with Hodgkin lymphoma.

The other children had central nervous system tumors (13.3%), solid tumors (24.1%), and other and unspecified malignant neoplasms (25.3%).

In the full study cohort, 33.6% of children were exclusively breastfed for less than 3 months. Among those diagnosed with any cancer, the proportion of those breastfed for less than 3 months was 38.3%, and among those diagnosed with ALL, it was 42%.

The numbers of children diagnosed with AML or Hodgkin lymphoma prevented the authors from estimating any association of exclusive breastfeeding with these cancers.

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