Background Infectious diseases in infants are considered as major public health issues worldwide. Recently synbiotic-enriched formulas have been proposed to mimic the beneficial effects of human milk on infectious diseases, but their clinical implications are still unknown.

Objectives We performed a 3-month observational study in 851 infants fed follow-on formula to assess the incidence and typology of infectious diseases and explore the influence of synbiotic-enriched formulas (EF) compared with standard formula (SF).

Methods Both parents and paediatricians completed questionnaires about infant characteristics (family characteristics, feeding practices, previous history of health issues) and reported every infectious disease with a detailed description of symptoms, severity and treatments. The SF and EF groups were compared using analysis of variance for continuous variables and with χ² or Fisher’s exact tests for categorical variables. Main outcome measures were compared after adjustments on baseline characteristics.

Results In the intention-to-treat population (798 infants), 35.4% of all infants experienced at least one infectious disease during follow-up, mainly upper respiratory tract infectious diseases (24.1%), otitis (6.6%) and gastrointestinal infectious diseases (5.0%). The EF group experienced fewer infectious diseases (EF 31.0%; SF 40.6%; p = 0.005), with a significant difference for gastrointestinal infectious diseases (EF 3.5%; SF 6.8%; p = 0.03).

Conclusions This observational study shows that the incidence if infectious disease is high in infants fed follow-on formula and suggests that supplementation with synbiotics may have beneficial effects. Further studies are required to determine optimal doses and composition of synbiotics in infant food.

Funding The study was supported by a grant from Nutriben, Les Ulis, France.