The complementary feeding (CF) period is a short transitional period from breastfeeding and formula feeding to family food. While the recommendations state that CF should not start before 6 months of age (World Health Organization) or between 4 and 6 months of age (e.g. ESPHGAN (1) timing varies considerably together with quantities and quality of complementary foods given depending on previous and concurrent feeding (breastfeeding, formula, mixed), ethnicity, country of residence, secular trends, socio economic status and many other factors. Formula-fed children, for instance, start approximately 2 weeks earlier than breastfed children and in almost 40% at or before 4 months of age (2).
During the transitional period fat intake decreases and protein and carbohydrate intake increase. Protein intake often exceed European recommendations from 9 months onwards (3) and early solid introducers have higher energy intakes in the first year of life (4). Depending on the region commercial infant foods and self-cooked products have a different share on infant intakes; both generally show differences in nutrient content and variety. While introduction of solids between 4 and 6 months or after 6 months does not seem to impact growth and later obesity risk, solids before 4 months of age increase the risk (OR for obesity 1.33, 95%CI 1.07-1.64)(5). There is also some indication that too much dairy protein during CF might lead to a higher obesity risk later in life. Only few studies related the mode of feeding and other qualitative aspects of CF to growth. In general, these findings are not conclusive. The impact of CF on short term growth and later obesity risk should be further explored.