Breast milk is the initial natural food for infants, but already during the second half year complementary feeding is essential. Epidemiological research, first on celiac disease and later on atopic diseases, has driven a paradigm shift with respect to most favorable age to introduce complementary feeding. Simplified, this implies a shift from later to earlier introduction, which is now taken into account in recommendations on infant feeding. Complementary feeding, including all foods, should not be initiated for any infant before 4 months of age, and not later than around 6 months, including infants with elevated disease risk (e.g. for celiac disease or atopic diseases). Motivating reasons could be that ongoing breastfeeding provides an 'immunological umbrella' and/ or a different age interval gives a 'window of opportunity' for developing oral tolerance towards gluten and other food antigens. This will for some infants be in conflict with recent WHO recommendations on exclusive breastfeeding for 6 months. Epidemiology has evolved over time and could, if increasingly used, contribute even more to innovations in pediatric nutrition and other phenomena related to population health.