A Cochrane review included 18 studies. For weight, in the RCTs and CBAs (controlled before and after studies) from lower income countries, experimental group children gained an average of 0.39 kg (95% CI 0.11 to 0.67) over an average of 19 months and 0.71 kg (95% CI 0.48 to 0.95) over 11.3 months respectively. Results for weight were mixed in higher income countries. For height, results were mixed; height gain was greater for younger children. Attendance in lower income countries was higher in experimental groups than in controls; our results show an average increase of 4 to 6 days a year. Math gains were consistently higher for experimental groups in lower income countries; in CBAs, the standardized mean difference was 0.66 (95% CI 0.13 to 1.18). In short-term studies, small improvements in some cognitive tasks were found.
Comment: The quality of evidence is downgraded by inconsistency (heterogeneity in interventions and outcomes). and by indirectness (differences in reported outcomes).