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What do systematic reviews say?

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Evidence found and GRADE certainty

948 questions · 998 findings · 100 reviews · Headline outcomes only

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No studies found 233 · 23.3%GRADE not reported 40 · 4%Very low 203 · 20.3%Low 271 · 27.2%Moderate 176 · 17.6%High 75 · 7.5%

Vitamin A vs placebo

Incidence: all cancers · healthy people and healthy male physicians

Vitamin A showed no clear difference in incidence: all cancers compared with placebo (high certainty).

1 finding from 1 review

Vitamin A vs placebo

Mortality: all cancers · healthy people and healthy male physicians

Vitamin A showed no clear difference in mortality: all cancers compared with placebo (high certainty).

1 finding from 1 review

Vitamin A vs placebo

Mortality: all causes ‐ smokers and asbestos workers · healthy people and healthy male physicians

Vitamin A leads to higher mortality: all causes ‐ smokers and asbestos workers compared with placebo (high certainty).

1 finding from 1 review

Vitamin C vs placebo

Incidence: lung cancer · healthy male physicians, period: 1997 to 2007 and female health professionals, period: 1998 to 2005

Vitamin C showed no clear difference in incidence: lung cancer compared with placebo (moderate certainty).

1 finding from 1 review

Vitamin C vs placebo

Incidence: lung cancer ‐ men · healthy male physicians, period: 1997 to 2007 and female health professionals, period: 1998 to 2005

Vitamin C showed no clear difference in incidence: lung cancer ‐ men compared with placebo (high certainty).

1 finding from 1 review

Vitamin A and fish oils (docosahexaenoic acid) vs placebo, vitamins, or no treatment

Visual acuitychange in logMAR visual acuity, may be reported as ETDRS · retinitis pigmentosa

It is very uncertain whether vitamin A and fish oils (docosahexaenoic acid) changes visual acuitychange in logMAR visual acuity, may be reported as ETDRS compared with placebo, vitamins (including vitamin A), or no therapy (very low certainty).

1 finding from 1 review

Summary of findings table ‐ Vitamin B12 supplementation vs no supplementation or placebo

Maternal anaemia · pregnant women

It is very uncertain whether summary of findings table ‐ Vitamin B12 supplementation changes maternal anaemia compared with placebo/no vitamin B12 supplementation (very low certainty).

1 finding from 1 review

Summary of findings table ‐ Vitamin B12 supplementation vs no supplementation or placebo

Low birthweight · pregnant women

Summary of findings table ‐ Vitamin B12 supplementation showed no clear difference in low birthweight (< 2500 g) compared with placebo/no vitamin B12 supplementation (low certainty).

1 finding from 1 review

Summary of findings table ‐ Vitamin B12 supplementation vs no supplementation or placebo

Preterm birth · pregnant women

Summary of findings table ‐ Vitamin B12 supplementation showed no clear difference in preterm birth (< 37 weeks) compared with placebo/no vitamin B12 supplementation (low certainty).

1 finding from 1 review