Item	Definition
Bias‐related characteristics*
Concealment of allocation (avoiding selection bias)	The guidance from the Cochrane Handbook for Systematic Reviews of Interventions (Higgins 2011) will be used to judge bias related to sequence generation and concealment of allocation using the two Cochrane 'Risk of bias' items. From these, the statistician will derive a single variable to be used in the stratified analysis: allocation concealment will be judged at low risk of bias if the investigators responsible for patient selection were unable to suspect before allocation which treatment was next. Concealment will downgraded to high risk of bias if there is evidence of inadequate sequence generation.
Blinding of patients and personnel (avoiding performance bias)	Low risk of bias will be judged if: ‐ a credible sham procedure was used; or if a placebo supplement or pill was used that was reported to be identical in appearance to the experimental intervention and the specific outcome or group of outcomes is/are likely to be influenced by lack of blinding ‐ blinding is absent or suboptimal and the specific outcome, such as mortality, is not likely to be influenced by lack of blinding
Blinding of outcome assessment (avoiding detection bias)	For self‐reported/partner‐reported outcomes: Low risk of bias will be judged if: ‐ self‐reported outcomes were assessed AND blinding of patients was considered adequate AND there was no information to suggest that there was an investigator involved during the process of outcome assessment; OR if blinding of investigators performing the outcome assessment was reported AND an attempt to blind patients was reported For other outcomes: Outcome assessment was considered to be blinded if the outcome assessment was reported to be blinded.
Statistical analyses (avoiding attrition bias)	For continuous outcomes Low risk of bias will be judged if: ‐ at least 90% of the patients randomised were analysed AND the difference in percentage of participants not analysed was 5% or lower across trial arms ‐ for trials using imputations to handle missing data: the percentage of participants with missing data did not exceed 20% AND the difference in percentage of participants with imputed data was 10% or lower across trial arms AND applied imputation methods were judged to be appropriate For binary outcomes of rare events Low risk of bias will be judged if: ‐ the event rate is low (e.g. incidence of dementia) AND at least 95% of the patients randomised were analysed AND there is no evidence of differential reasons for missing data that may alter the estimate AND the rate of missing data does not exceed the expected event rates For binary outcomes of non‐rare events Low risk of bias will be judged if: ‐ at least 90% of the patients randomised were analysed AND the difference in percentage of participants not analysed was 5% or lower across trial arms AND there is no evidence of differential reasons for missing data that may alter the estimate AND the rate of missing data does not exceed the expected event rates
Trial Size	A large trial will be defined by a sample size calculation for the primary outcome.
Follow‐up duration	For the cognitive outcomes, we will group studies according to these follow‐up cut‐offs to describe immediate results (up to 12 weeks), short‐term (up to 1 year), medium‐term (1 to 2 years) and longer‐term results (more than 2 years). For the secondary outcome all‐cause dementia, only outcome data at 1 year of follow‐up or longer will be considered and therefore the grouping will include short‐term (1 year), medium‐term (up to 2 years) and longer‐term results (more than 2 years).
Treatment‐related characteristics
Treatment duration	The minimum treatment duration of 3 months is considered short‐term, 3 to 12 months as medium‐term, and 12 months for long‐term.
Dose of treatment	Treatment will be analysed as high dose versus low dose according to previously reported cut‐offs.
Mechanisms of action of the supplements**	Supplements postulated to share a main mechanism of action in preventing development of dementia, including: Antioxidant properties ‐ affecting superoxide dismutase (vitamin A, C, D, E, selenium) Regulation/lowering levels of homocysteine: vitamins B12, folate and B6
Participant‐related characteristics
Cognition‐related criteria	No risk of deficiency versus at risk of deficiency for the type of vitamin and minerals investigated (e.g. presence of malabsorptive diseases, malnutrition, comorbidities or concomitant medications, and ethnicity (Vitamin D))