Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: “Adolescents from 15 schools (four U‐MTs, seven U‐SMP and four R‐SMP) (n¼2990) were randomly selected to receive weekly supplements, while adolescents in the other nine schools (three U‐MTs, two U‐SMP and four R‐SMP) served as controls (n¼1750).”Comment: method of sequence generation was notmentioned
Allocation concealment (selection bias)	Low risk	Comment: since this is a cluster trial, it is unlikely a selection bias at individual level
Similar baseline characteristics	Unclear risk
Similar baseline outcome measurement	Unclear risk
Blinding of participants and personnel (performance bias)	High risk	Quote: “All pupils were aware of which supplement(s) they were taking”Comment: Participants: were aware of the treatment;Personnel: were aware of the treatment;
Blinding of outcome assessment (detection bias)	Low risk	Comment: while outcome assessors were aware of the treatment allocation the majority of outcomes were subjective
Incomplete outcome data (attrition bias)	Low risk	Quote: “4810 out of 5116 participants study (96%). Subjects who dropped out were slightly older than those who remained in the study, and more subjects in the urban religious schools dropped out compared to the other school types. Otherwise, there were no differences between those who finished the study and those who did not.” “Dropout during the study (5.1%) was mainly caused by absenteeism on several consecutive days during the end‐line data collection. There was no difference in dropout rate between the four groups.” Comment: Missing outcome data were matched across groups
Prevention of knowledge of allocated intervention	Unclear risk
Protection against contamination	Unclear risk
Selective reporting (reporting bias)	Unclear risk	Comment: outcomes mentioned in the methods were explored in the results section but no protocol was found
Other bias	Low risk	Comment: the study appears to be free of other bias.