Table 2 Risk of bias in included studies; each criterion was graded as high, low, or unclear risk

• Hollams [13]

• Uncertain risk

• High risk

• Low risk

• High risk

• High Risk

• Although the risk of bias is low for the original cohort, no further details were provided about selection criteria or process for participants in this current study

• “Vit D levels was measured in thawed serum cryobanked at age 6 years” (number of years since blood draw not mentioned)

• Low for lung function and BHR

• Did not match or adjust for maternal atopy, maternal asthma, maternal age, education or household smoking

• Outcome data were missing for 30% of the enrolled cohort

• The used enzyme immunoassay kit “method appeared to overestimate the vitamin D levels at age 6 years”

• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time

Van Oeffelen [12]

• High risk

• High risk

• Low risk

• Low risk

• Uncertain Risk

• Out of the larger cohort, a small “selected” sample included in this study, with no further details about selection provided

• Serum samples were defrosted to measure concentrations of Vitamin D (number of years since blood draw not mentioned)

for asthma (ISAAC score) Low risk for BHR

• Confounders were added to all models (gender, maternal atopy, paternal atopy, smoking by anyone in the house, and serum magnesium)

• Outcome data were missing for 12% of the enrolled cohort

• Measured using a competitive enzyme immunoassay in microtiter plates

• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time

• Also considered playing outside and overweight as potential confounders

Tolppanen [22]

• Uncertain risk

• High risk

• Uncertain risk for asthma, using spirometry & bronchidilatory responsiveness with non-validated questionnaires to diagnose asthma

• Low risk

• High risk

• Except for the loss of follow up, the cohort was from a single community and followed specific eligibility criteria

• The exposures are standardized for age and sex and 25-hydroxyvitamin D3 is adjusted for season and ethnicity

• Model 1 unadjusted

• Of 5765 participants in the assessment of the wheezing and asthma outcome 3323 where included (42% missing data)

• Measured using high pressure liquid chromatography tandem mass spectrometer in the multiple reaction mode (number of years since blood draw not mentioned)

• High risk for wheezing

• Models 2 and 3 adjusted for respectively 8 and 9 potential confounders

• Recall bias

• And of 4488 participants in the spirometric assessment 2259 where included (50% missing data).

• Measuring Vitamin D levels at one point only may not be a reliable measure of integrated 25(OH) D levels over time

• Proportion of incident wheezing, and asthma was higher among children excluded owing to missing data