Table 2

Grading assessment of the certainty of evidence of each component

Certainty assessmentSummary of findings
No of participants (studies) follow-upRisk of biasInconsistencyIndirectnessImprecisionPublication biasOverall certainty of evidenceRelative effect (95% CI)
Aspirin user compared with non-aspirin user in prevention of colorectal cancer
 6 Meta-analysesNot serious aSerious bNot serious cNot seriousPublication bias strongly suspected d⨁⨁⨁◯Moderate 0.71 to 0.86
Low dose aspirin user compared with control in prevention of colorectal cancer
 3 Meta-analysesNot seriousSerious eNot seriousNot seriousPublication bias strongly suspected⨁⨁◯◯Low 0.71 to 1.02
NSAIDs compared with no NSAIDs for prevention of colorectal cancer
 3 Meta-analysesNot seriousSerious fNot seriousNot seriousPublication bias strongly suspected d⨁⨁◯◯Low 0.57 to 0.74
Magnesium compared with control for prevention of colorectal cancer
 3 Meta-analysesNot seriousNot seriousNot serious gNot seriousNone⨁⨁◯◯Low 0.78 to 0.89
Folic acid compared with control for prevention of colorectal cancer
 3 Meta-analysesNot seriousSerious hserious iNot seriousPublication bias strongly suspected j⨁◯◯◯Very low 0.85 to 0.88
Folic acid in combination with other antioxidants for the chemoprevention of colorectal cancer
 3 Meta-analysesNot seriousNot seriousvery serious kNot seriousNone⨁◯◯◯Very low 0.96 to 1.15
Dairy products compared with control for prevention of colorectal cancer
 3 Meta-analysesNot seriousSerious lvery serious mNot seriousNone⨁◯◯◯Very low 0.81 to 0.87
Fibre compared with control for prevention of colorectal cancer
5 Meta-analysesNot seriousSerious nvery serious mNot seriousNone⨁◯◯◯Very low 0.57 to 0.94
Fruits and vegetables
 9 Meta-analysesNot seriousSerious overy serious mNot seriousNone⨁◯◯◯Very low 0.48 to 1.00
Soy products
 3 Meta-analysesNot seriousSerious pvery serious mNot seriousNone⨁◯◯◯Very low 0.85 to 1.01
Vitamin E
 7 Meta-analysesNot seriousNot seriousserious mNot seriousNone⨁⨁◯◯Low 0.82 to 1.05
Vitamin C
 3 Meta-analysesNot seriousSerious qVery serious mNot seriousNone⨁◯◯◯Very low 0.84 to 0.92
Combinations of vitamins A, C, E, D or antioxidants
 5 Meta-analysesNot seriousNot seriousVery serious mNot seriousNone⨁◯◯◯Very low 0.92 to 1.00
β-Carotene
 3 Meta-analysesNot seriousSerious rNot seriousNot seriousNone⨁⨁⨁◯Moderate 0.99 to 1.09
β-Carotene in combination with other products
 5 Meta-analysesNot seriousNot seriousVery serious sNot seriousNone⨁◯◯◯Very low 0.96 to 1.15
Selenium
 3 Meta-analysesNot seriousNot seriousNot seriousNot seriousNone⨁⨁⨁◯Moderate 0.48 to 0.77
Tea
 5 Meta-analysesNot seriousSerious tVery serious mNot seriousNone⨁◯◯◯Very low 0.82 to 0.99
Garlic
 4 Meta-analysesNot seriousSerious uVery serious mNot seriousNone⨁◯◯◯Very low 1.03 to 1.24
Vitamin D
 4 Meta-analysesNot seriousSerious vVery serious mNot seriousNone⨁◯◯◯Very low 0.87 to 0.92
 2 Meta-analysesNot seriousNot seriousVery serious mNot seriousNone⨁◯◯◯Very low 0.62 to 1.08
Coffee or caffeine
 7 Meta-analysesNot seriousSerious wVery serious mNot seriousNone⨁◯◯◯Very low 0.76 to 1.00
Fish and omega-3
 7 Meta-analysesNot seriousSerious xVery serious mNot seriousNone⨁◯◯◯Very low 0.88 to 1.03
Vitamin A
 2 Meta-analysesNot seriousNot seriousVery serious mNot seriousNone⨁⨁◯◯Low 0.77 to 0.87
Vitamin B
 2 Meta-analysesNot seriousSerious yVery serious mNot seriousNone⨁◯◯◯Very low 0.86 to 1.18
Calcium
 5 Meta-analysesNot seriousSerious zSerious aaSerious bbNone⨁◯◯◯Very low 0.34 to 1.38
Statin
 8 Meta-analysesNot seriousSerious ccSerious ddNot seriousNone⨁⨁◯◯Low 0.74 to 1.02
Meat
 7 Meta-analysesNot seriousSerious eeVery serious mNot seriousNone⨁◯◯◯Very low 1.04 to 1.25
Alcohol compared with control for prevention of colorectal cancer
 9 Meta-analysesNot seriousSerious ffSerious ggNot seriousPublication bias strongly suspected⨁◯◯◯Very low 1.03 to 1.52
  • a. Source of bias was difficult to assess as included meta-analyses included observational studies and did not perform standard risk of bias tools nor any other biases.

  • b. Heterogeneity was noted in three of the five meta-analyses9 10 (only for case–control); and11, not reported in one13 and no heterogeneity noted in one.12

  • c. All meta-analyses, any dose aspirin definition.

  • d. Publication bias was detected but no value provided in the meta-analysis.

  • e. Heterogeneity was noted in one of the three meta-analyses,10 not reported in two.12 13

  • f. Heterogeneity was noted in one of the three meta-analyses,16 not reported in two.10 15

  • g. Intervention and control are based on highest category compared with lowest category of magnesium intake as opposed to primary question, similar quartiles.

  • h. Heterogeneity reported in one meta-analysis22 but not in two others.20 21

  • i. Intervention and control are based on different controls as well as high vs low intake.

  • j. Publication bias was detected in two meta-analyses.20 22

  • k. Different intervention and comparison within each meta-analysis included.

  • l. Heterogeneity found in one meta-analysis.27

  • m. Different intervention and comparison within each meta-analysis included.

  • n. Heterogeneity was reported in one meta-analysis30 none in two26 29 and not reported in one.31

  • o. Varying heterogeneity in meta-analyses.

  • p. Heterogeneity was noted in Zhu 2015.38

  • q. Strong heterogeneity was reported in one meta-analysis.21

  • r. Two meta-analyses reported heterogeneity42 ,46 and one did not report any results.45

  • s. Different intervention and comparison within each meta-analysis included.

  • t. Heterogeneity was reported in three meta-analyses26 ,50 ,51 but none in one.48

  • u. Heterogeneity was noted in one meta-analysis54.

  • v. Heterogeneity was reported in two meta-analyses21 ,22 none in one56 and not reported in another.55

  • w. Heterogeneity noted in five meta-analyses.58 ,60 ,26 ,61 ,63

  • x. Heterogeneity was noted in three meta-analyses,65 ,66 ,68 none was noted in four.26 ,33 ,64 ,67

  • y. Heterogeneity was found in one meta-analysis.69

  • z. Heterogeneity was noted in two meta-analyses,21 ,73 none was noted for three.70 ,71 ,72

  • aa. Intervention and control are based on different controls as well as high vs low intake.

  • bb. Imprecision due to wide confidence interval in one meta-analysis.70

  • cc. Heterogeneity was reported in two meta-analyses80 ,81 no heterogeneity in two,74 ,75 varying strength of heterogeneity between publication type in two77 ,78 or not reported in the last two meta-analyses.76 ,79

  • dd. Databases used in meta-analyses are population based therefore include all population, not only average-risk population.

  • ee. Heterogeneity was reported in two meta-analyses,82 ,84 not in five26 ,33 ,39 ,83 ,85.

  • ff. All included meta-analyses for this outcome reported heterogeneity.

  • gg. Different intervention and comparison within each meta-analysis included. Doses standard.