A large Japanese study tracking more than 88,000 mother-child pairs has introduced a quiet but significant complication into one of medicine's most settled recommendations: that exclusive breastfeeding protects infants broadly and uniformly. The research found that exclusively breastfed children faced substantially higher rates of food allergies in their earliest years, even as they were meaningfully shielded from asthma — a paradox that points not to breastfeeding's failure, but to the delicate timing of how and when the developing immune system first encounters the world. The findings invite