Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: “The randomisation was done at the class section level for 60 class sections.” Comment: method of sequence generation was not described
Allocation concealment (selection bias)	Low risk	Comment: since randomisation was performed at class section level it is unlikely a selection bias is present at individual level
Similar baseline characteristics	Unclear risk
Similar baseline outcome measurement	Unclear risk
Blinding of participants and personnel (performance bias)	High risk	Comment: all groups received the supplement and no placebo was used
Blinding of outcome assessment (detection bias)	Low risk	Comment: outcomes were objective therefore the blinding of outcome assessors would not impact the results
Incomplete outcome data (attrition bias)	Low risk	Quote: “Seven girls in the daily administered group during the second week of intervention complained of gastric side effects and requested not to continue in the study and were excluded” Comment: incomplete 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	Quote: “Girls with haemoglobin <70 g/L (0.3%)were eliminated from the analysis.” “Plasma ferritin and C‐reactive proteins (CRP) were estimated in every tenth girl of the study groups” Comment: it is unclear how each girl was selected for these analyses. Data not available for the second measurement. However no protocol could be found and therefore there is insufficient evidence to make a judgement.
Other bias	Low risk	Comment: the study appears to be free of other bias