Longer Breast-feeding May Mean Lower Iron Levels

Norra MacReady

April 17, 2013

Breast may be best, but relying on it too long may set a child up for iron deficiency later, a new study suggests.

In a cross-sectional analysis of 1647 children aged 1 to 6 years, the odds of iron deficiency increased by 4.8% for each month of breast-feeding, lead author Jonathon L. Maguire, MD, and colleagues report in an article published online April 15 in Pediatrics.

Dr. Maguire, from the Department of Pediatrics and the Li Ka Shing Knowledge Institute of St. Michael’s Hospital; the Pediatrics Outcomes Research Team, Division of Paediatric Medicine, Department of Paediatrics, The Hospital for Sick Children; and the Department of Pediatrics, Faculty of Medicine, University of Toronto, Ontario, Canada, and coauthors studied children visiting 7 primary care clinics participating in The Applied Research Group for Kids (TARGet Kids!) study, which focuses on the long-term health and development of Canadian infants and preschoolers, between December 2008 and June 2011.

Iron stores, as measured by serum ferritin, were the primary outcome measure, with deficiency defined as anything below 14 μg/L. Iron deficiency anemia was defined as a hemoglobin level of 110 g/L or less in the presence of iron deficiency. Breast-feeding duration was determined through maternal recall. Excluded from the study were children with any chronic condition other than asthma and those with evidence of inflammation, as indicated by a C-reactive protein level of 10 mg/L or more, because of concerns that an acute illness might falsely elevate serum ferritin levels.

The analysis was adjusted for potentially confounding covariates including age, sex, birth weight, body mass index, ethnicity, household income, day care attendance, age of introduction to cow's milk or solid food, and current intake of cow's milk.

The study participants had a median age of 36 months (range, 12 - 72 months). Ninety-three percent had been breast-fed, for a median duration of 10 months (range, 0 - 48 months). Their median serum ferritin level was 32 μg/L (range, 2 - 172 μg/L).

The prevalence of iron deficiency and iron deficiency anemia was 9% and 1.5%, respectively. On univariate analysis, every additional month of breast-feeding was associated with a decrease in serum ferritin of 0.21 μg/L (P = .0006), an association that remained significant even after adjustment for the covariates. Younger age, higher birth weight, and greater consumption of cow's milk also were independently associated with lower serum ferritin levels. On secondary analysis, there was a statistically significant association between breast-feeding duration and the odds of iron deficiency (odds ratio [OR], 1.026; 95% confidence interval [CI], 1.004 - 1.050). A total breast-feeding duration longer than 12 months was associated with an adjusted OR for iron deficiency of 1.71 (95% CI, 1.05 - 2.79) compared with a duration of less than 12 months.

The adjusted OR for iron deficiency was 1.048 for each additional month of breast-feeding, or an odds increase of 4.8% per month (95% CI, 1.02 - 1.08).

Potential study limitations include the relatively low prevalence of iron deficiency and anemia and the use of parental self-report to estimate duration of breast-feeding.

At least one outside observer believes these data should not be interpreted to mean mothers should curtail their breast-feeding. "Mothers should exclusively breast-feed for the first 6 months," and then introduce iron-fortified foods while continuing to breast-feed for 1 year or more, Diane Spatz, PhD, RN, director of the Lactation Program at Children's Hospital of Philadelphia in Pennsylvania, told Medscape Medical News. To determine further relationships between breast-feeding and iron status, "it would be important to study the infant's other dietary intake after 6 months."

Still, the authors warn, "no previous study has documented a relationship between breastfeeding beyond 12 months of age and reduced iron stores."

This study was supported by a grant from the Canadian Institutes of Health Research, the Sick Kids Foundation, and St. Michael's Foundation. One study author is a consultant to AstraZeneca, Bristol-Myers Squibb, GlaxoSmithKline, Hoffmann LaRoche, Novartis, and Pfizer. The other authors and Dr. Spatz have disclosed no relevant financial relationships.

Pediatrics. Published online April 15, 2013. Abstract