Does the age at complementary feeding introduction have an impact of health outcomes later in life?

The timing of introduction of complementary feeding is a pivotal issue in paediatrics. In most Low-Medium Income Countries (LMICs), national guidelines recommend beginning complementary feeding at 6 months of age, and the same stands for the American Academy of Paediatrics. According to the Committee of Nutrition of European Society for Paediatric Gastroenterology Hepatology and Nutrition, complementary foods (i.e., solid foods and liquids other than breast milk or infant formula) should not be introduced before 4 months but should not be delayed beyond 6 months. This indication is provided because an early introduction of complementary foods (CFs) before 4 months, results harming for both renal and gastrointestinal function and inadequate with respect to neurodevelopmental skills. The concern about late introduction after 6 months mainly arises from inadequacy of breastmilk in providing critical nutrients, particularly iron. Moreover, accumulating evidence suggests a potential higher risk of food allergies (i.e. peanuts) when delaying the exposition to allergens, rather than preventing from their occurrence. Therefore, the window of introduction of CFs between 4 to 6 months, has been generally regarded as safe, particularly in high-income countries where paediatric societies often support this recommendation. Surprisingly, the World Health Organization has recently raised some concern whether the introduction of CFs before 6 months of age (180 days) might pose potential health risks, with a particular focus targeted for LMICs. In its newly released dietary guidelines for complementary feeding, 4 major topics are addressed: higher morbidity from gastrointestinal disorders (such as diarrhoea) in areas where food and water hygiene are an issue; the provision of complementary feeding with lower nutritional quality compared to breastmilk in low-resource environments; inadequate developmental readiness to have foods; and the programming to later onset and risk of obesity. According to the WHO report, globally, in LMICs the introduction of CFs before 6 months occurs in 29% of infants. The report collected a total of 40 observational studies to assess the association between early introduction of complementary foods, defined as <6 months of age, and later introduction, defined as ≥6 months of age. Regarding growth parameters, no association were found for early introduction and stunting or wasting compared to late introduction (≥6 months). Interestingly, an association indicating more underweight among infants with early compared to later introduction was found across studies (OR = 1.29 [1.08, 1.53]). When evaluating the association between timing of CFs introduction and BMI or BMI z scores, in four studies an earlier introduction was linked to a higher BMI or BMI z scores values, compared to late introduction. Four studies looked at overweight, obesity, and overweight and obese together. Overweight and obesity did not show any association, but when looking at the combined category, early introduction was related with greater levels of overweight/obesity (OR = 1.34 [1.09, 1.65]). While 2 studies did not find an association between anaemia and early introduction of CFs, one study identified an association with iron deficiency anaemia, suggesting that earlier exposure was related with decreased risk (OR = 0.34 [0.18, 0.63]). Other health outcomes, i.e. diarrhoea, food allergy, asthma, lower track respiratory infections, wheeze, eczema or rickets, revealed no statistically significance. It should be noted that the type of observational studies severely limits the possibility of drawing conclusions and, collectively, the evidence for all previous outcomes was rated as low to very low. Overall, there is substantial uncertainty about the harms associated with introduction earlier than 6 months, arising the question whether the window of timing of complementary feeding should be graded according to high and low-resource settings.