Breastfeeding for the first six months of life was associated with a lower prevalence of childhood asthma and allergic rhinoconjunctivitis but a higher prevalence of reported food allergy in a large observational study from Japan.
The findings do not show that breastfeeding causes or prevents allergies. Researchers also lacked detailed information about when specific foods, including common allergens such as egg and peanut, were introduced. That makes it difficult to separate the effects of breastfeeding from the timing of complementary feeding.
The study, published in BMC Pediatrics, followed 88,037 mother-infant pairs enrolled in the Japan Environment and Children’s Study, a nationwide birth cohort. Caregivers completed annual questionnaires about physician-diagnosed asthma, allergic rhinoconjunctivitis, food allergy and atopic dermatitis until the children reached age 6.
Participants were divided into four groups based on feeding during the first six months: exclusively formula-fed, breastfed for less than six months, breastfed for six months while also receiving formula or exclusively breastfed for six months.
Children who were exclusively breastfed for six months had a lower reported prevalence of bronchial asthma and allergic rhinoconjunctivitis than children in some of the other feeding groups. The association with asthma was strongest during the first two years of life and became weaker as children grew older.
At the same time, exclusive breastfeeding was associated with a higher reported prevalence of food allergy, particularly among boys. That association remained evident until about age 3.
“Beneficial associations of breastfeeding were observed for the prevalence of bronchial asthma and allergic rhinoconjunctivitis, while adverse associations were detected for food allergy, particularly in male infants,” said Professor Emeritus Hidekuni Inadera of the University of Toyama.
The results may appear to conflict with recommendations supporting breastfeeding, but infant allergy risk is influenced by more than whether a child receives breast milk or formula.
One possible explanation is the timing of complementary food introduction. Research has found that introducing some common food allergens during infancy, rather than delaying them, may help the immune system develop tolerance.
Current U.S. guidance generally does not recommend delaying potentially allergenic foods once an infant is developmentally ready to begin complementary foods. The appropriate timing and form can vary based on the child’s development and allergy risk, particularly for infants with severe eczema or an existing food allergy.
The Japanese study did not capture enough detail about the timing, quantity or type of foods introduced to determine whether delayed allergen exposure explained the higher reported prevalence of food allergy.
“Consistent with our results and some previous observations, current guidelines recommend that solid food be introduced when infants are between 4 and 6 months while breastfeeding is continued,” Inadera said.
That does not mean families should stop exclusive breastfeeding earlier specifically to prevent food allergies. It means breastfeeding duration and complementary feeding practices need to be considered together rather than treated as unrelated choices.
The study also relied on caregiver reports of diagnoses made by physicians. The researchers did not confirm food allergies through medical records, skin testing, blood testing or oral food challenges, which are often needed to distinguish a true food allergy from other reactions.
The reported prevalence of food allergy was also higher than researchers expected. That raises the possibility that some children were classified as having a food allergy even when their symptoms had another cause or had not been confirmed through standard testing.
Because the study was observational, families were not randomly assigned to different feeding patterns. Parents who noticed early eczema, digestive symptoms or possible allergic reactions may have changed how long they breastfed or when they introduced foods. Other family, health and lifestyle factors may also have influenced both feeding decisions and allergy diagnoses.
The findings on atopic dermatitis were less clear. The study did not consistently find a higher prevalence in its primary analysis, meaning the result should not be interpreted as strong evidence that breastfeeding increases eczema risk.
The study’s size and length are important strengths. Following more than 88,000 children through age 6 allowed researchers to look at how associations changed as children grew older. Still, the results reflect feeding practices, health care and allergy diagnoses in Japan and may not apply in the same way to families in other countries.
The Japan Environment and Children’s Study was funded by Japan’s Ministry of the Environment.