Breastfeeding Protects Against Asthma Better than Expressed Breast Milk

Meghan Azad, PhD

The first study to distinguish between effects of breastfeeding and bottle-fed expressed breast milk on risk for asthma found breastfeeding was associated with half the odds of developing asthma compared to formula feeding.

Principle investigator Meghan Azad, PhD — Assistant Professor, Pediatrics and Child Health, University of Manitoba, and research scientist, Children's Hospital Research Institute of Manitoba (CHRIM), Winnipeg, Canada — and colleagues examined the prevalence of asthma in children at 3 years of age relative to the mode of feeding at 3 months, while controlling for other risk factors.

"Although several studies have examined the association between infant feeding and asthma, the present study uniquely distinguishes between direct breastfeeding and expressed breast milk," researchers wrote. "Our results suggest that feeding mode differences could help to explain the apparently inconsistent results observed in 'breastfeeding' studies across different population and settings."

The study included 2,534 infants from the Canadian Healthy Infant Longitudinal Development (CHILD) study, a large national birth cohort that recruited pregnant women from 2009 to 2012. The modes of infant feeding at 3 months were distinguished between direct breastfeeding only (27%); breast milk by breastfeeding and by bottle expressed with breast pump (32%); breast milk and formula (26%); or formula only (15%).

Study lead author Annika Klopp, MD, Pediatric Resident, CHRIM and University of Manitoba, told MD Magazine about the distribution of feeding modes.

"Our study found that over half of exclusively breastfed infants in the CHILD study were receiving some expressed breast milk. In the U.S., where the average mother returns to work after just 10 weeks, the proportion is even higher — 85% according to the U.S. Infant Feeding Practices Study," Klopp said.

The status of asthma at 3 years of age was determined to be possible, probable or not present through a semi-structured assessment including focused history and physical examination by 2 or 3 specialist or specifically trained healthcare professionals.

Possible influencing factors controlled for in the analysis included infant sex, gestational age, method of birth, number of older siblings (parity) and daycare attendance. Maternal conditions that were also controlled included age, ethnicity, history of asthma, and tobacco smoking during pregnancy, and socioeconomic indicators including level of education and home ownership.

By 3 years of age, 12.6% of the children were diagnosed with possible or probable asthma. It was also diagnosed in 8.8% of children who had only breastfed, 12.5% of those taking some expressed breast milk in addition to breastfeeding, 14.9% with combination of breast milk and formula, and in 15.8% receiving formula only.

The mode of infant feeding was associated significantly with diagnosis of asthma, as, to a lesser degree, were male sex, gestational age of less than 37 weeks, maternal asthma, caesarean section delivery and daycare attendance by 1 year of age.

Conditions that were not associated with development of asthma included number of siblings, maternal education, or maternal smoking during pregnancy.

"Compared with formula feeding, direct breastfeeding seems to be most protective against asthma development, while expressed breast milk appears to confer intermediate protection," Azad told MD Magazine.

Klopp offered possible explanations for the greater protective effect found with direct breastfeeding than expressed breast milk.

"Refrigeration, freezing, thawing and heating — routine steps between pumping and feeding — may degrade the bioactive components of breast milk which help protect against asthma," she said. "Another possibility is that infants who are fed at the breast develop stronger lungs through the physical act of suckling, and this could explain the difference in asthma risk."

Azad concurred, adding that the teams hope is that the study will guide “future research on the best ways to store and feed expressed milk, and that it will inform societal policies to protect, promote and support breastfeeding.”

The study, "Modes of Infant Feeding and the Risk of Childhood Asthma: A Prospective Birth Cohort Study," was published online in the Journal of Pediatrics in November.