Trials were identified from Cochrane Library, PubMed, Ovid MEDLINE and Ovid EMBASE databases by two independent reviewers (HH, CB). Keywords were a combination of free texts and MeSH subject headings (Online supplement). The search strategy included filters to limit the results by the study type (RCTs only) and subject age range: infant (0–23 months) and preschool (2–5 years). No date limits were applied. No language restriction was applied. The bibliography of eligible trials was searched for relevant papers. The most recent search was conducted in April 2017. The process of study selection is shown in Fig. 1.Fig. 1Flow chart for selection of studies