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) (n = 2990) 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 (n = 1750).”
		Comment: Insufficient information to permit judgement.
Allocation concealment (selection bias)	Unclear risk	Quote: “In each of the schools receiving supplements, each of the three grades was randomly allocated to receive one of the three supplementation regimes.”
		Comment: Insufficient information to permit judgement.
Blinding of participants and personnel (performance bias)	High risk	Quote: “All pupils were aware of which supplement they were taking and they were told that the supplements would improve their health and prevent/cure anaemia”
		Comment: Not done
Blinding of outcome assessment (detection bias)	High risk	Quote: “All pupils were aware of which supplement they were taking and they were told that the supplements would improve their health and prevent/cure anaemia”
		Comment: Not done
Incomplete outcome data (attrition bias)	Low risk	Comment:
		Group 1: 63/1,033
		Group 2: 67/1,045
		Group 3: 88/1,130
		Group 4: 138/1,958 (data presented for control group on a stratified random sample of 626)
Selective reporting (reporting bias)	Unclear risk	Comment: Trial registration not reported. Outcomes specified in the methodology section were reported
Other bias	Low risk	Comment: No other biases detected