Timing of Cow's Milk Protein Introduction and Childhood Adverse Reactions to Cow's Milk.

Background: Current guidelines emphasize early introduction to potentially allergenic foods, but the optimal timing, amount, and exposure routes for foods other than peanut are not well-established. Cow's milk is often the first allergenic food introduced through infant formulas.

Objective: To examine timing of cow's milk protein introduction (CMPI), including interaction with formula supplementation after delivery, in relation to reported cow's milk adverse reactions throughout childhood.

Methods: Among children in the Boston, Mass-area Project Viva cohort, we assessed timing of CMPI (<2 weeks, 2 weeks-<6 months, ≥6 months) with questionnaires administered in infancy. Outcomes were (1) parent-reported cow's milk adverse reactions via annual questionnaires from age 2 to 13 years (n = 1298) and (2) milk sensitization (n = 505) and IgE-mediated milk allergy (n = 491) at age approximately 8 years. We used multivariable logistic regression models adjusted for parental atopy history, delivery mode, gestational age at delivery, child race, and census tract median household income, and investigated effect modification by ever/never breast-fed status and formula supplementation in the delivery hospital.

Results: A total of 32% were introduced to cow's milk protein at age less than 2 weeks, 38% at age 2 weeks to less than 6 months, and 30% at age 6 months or more. Compared with children with CMPI at age less than 2 weeks, children with CMPI at age 6 months or more had a higher risk of cow's milk adverse reaction at age 2 to 5 years (odds ratio, 2.4 [1.2-4.7]). Children given formula after delivery + CMPI at age less than 2 weeks had the lowest risk overall. Among children not given formula after delivery, CMPI at age 2 weeks to 6 months appeared most protective. Results were similar among ever-breast-fed children. Timing of CMPI was not associated with milk sensitization or IgE-mediated milk allergy in mid-childhood.

Conclusions: Early, consistent cow's milk exposure appears protective against adverse reactions to cow's milk.