Vitamin D Supplements Show Only Modest Benefit Against Respiratory Infections in Young Children
A Cochrane review of 107 RCTs finds vitamin D may slightly cut ARI-related doctor visits in under-5s, but evidence remains low-certainty.
Summary
This Cochrane systematic review pooled data from 107 randomized controlled trials involving 31,521 participants to assess whether vitamin D supplementation during pregnancy or early childhood reduces acute respiratory infection (ARI) healthcare visits in children under five. Results showed a slight reduction in the proportion of children needing ARI-related visits when given vitamin D versus placebo, but no meaningful reduction in how often each child visited. Higher doses showed no advantage over lower doses. Hypercalcemia was rare across all groups. Overall evidence certainty was low to moderate, meaning larger, well-designed trials are still needed before firm recommendations can be made.
Detailed Summary
Acute respiratory infections are the leading cause of illness and death in children under five worldwide, driving enormous healthcare utilization globally. Vitamin D deficiency is widespread among pregnant women and young children, and vitamin D plays a known role in immune system development. Given its safety profile, low cost, and ease of administration, vitamin D supplementation has long been proposed as a practical public health tool to reduce ARI burden in this vulnerable age group. This Cochrane review, updated to March 2025, represents the most comprehensive synthesis of evidence to date on this question.
The review included 107 randomized controlled trials enrolling 31,521 participants. Studies compared vitamin D supplementation (D2, D3, or unspecified forms) against placebo, or higher doses (≥1000 IU) against lower doses (≤1000 IU). Supplementation was administered during pregnancy, early childhood, or both, and in some cases to both infants and lactating mothers. Dosing schedules ranged from daily to quarterly boluses, with study durations spanning a single dose to 18 months. Settings included hospitals, day-care centers, communities, and homes across multiple countries. Risk of bias was assessed using the Cochrane RoB 1 tool, and evidence certainty was graded using GRADE.
For the primary outcome — proportion of children making ARI-related healthcare visits — vitamin D versus placebo showed a statistically significant but clinically modest effect: RR 0.95 (95% CI 0.91 to 1.00; P=0.03; 10 studies, 2,447 participants; low-certainty evidence). This translates to a 5% relative reduction in the proportion of children needing a visit. However, for the mean number of ARI-related visits per child, no significant difference was found: MD 0.07 (95% CI −0.06 to 0.20; P=0.32; 3 studies, 561 participants; moderate-certainty evidence). These two findings together suggest vitamin D may marginally reduce who gets sick enough to seek care, but does not meaningfully reduce how frequently any given child seeks care.
Comparisons of higher versus lower vitamin D doses yielded no benefit on either outcome. The proportion of children making ARI visits was not significantly different: RR 0.94 (95% CI 0.81 to 1.10; P=0.46; 2 studies, 1,382 participants; moderate-certainty evidence). Mean visits per child also showed no difference: MD −0.10 (95% CI −0.59 to 0.39; P=0.69; 1 study, 579 participants; low-certainty evidence). This finding is clinically important — it suggests that simply increasing vitamin D dose beyond standard levels does not confer additional respiratory protection.
Regarding safety, hypercalcemia was rare across all comparisons. No cases were reported in pregnant women receiving vitamin D versus placebo. In children, vitamin D versus placebo showed RR 0.81 (95% CI 0.53 to 1.24; 7 studies, 1,542 participants; very low-certainty evidence), and higher versus lower dose comparisons showed RR 1.23 (95% CI 0.82 to 1.85; 7 studies, 1,287 participants; low-certainty evidence). The overall safety signal is reassuring, though the very low certainty for some comparisons limits definitive conclusions.
The authors conclude that while vitamin D supplementation appears safe and may offer a slight reduction in ARI-related healthcare visits, the low certainty of evidence and heterogeneity across trials prevent strong policy recommendations. The lack of benefit from higher doses is notable. Large, well-designed, placebo-controlled trials with standardized ARI definitions and dosing protocols are urgently needed to resolve remaining uncertainty and guide supplementation policy for pregnant women and young children.
Key Findings
- Vitamin D vs. placebo reduced the proportion of children with ARI-related healthcare visits by 5% (RR 0.95, 95% CI 0.91–1.00; P=0.03; 10 studies, 2,447 participants; low-certainty evidence)
- Vitamin D did not reduce the mean number of ARI-related visits per child (MD 0.07, 95% CI −0.06 to 0.20; P=0.32; 3 studies, 561 participants; moderate-certainty evidence)
- Higher-dose vitamin D (≥1000 IU) vs. lower-dose (≤1000 IU) showed no reduction in proportion of children with ARI visits (RR 0.94, 95% CI 0.81–1.10; P=0.46; 2 studies, 1,382 participants; moderate-certainty evidence)
- Higher vs. lower dose also showed no reduction in mean ARI visits per child (MD −0.10, 95% CI −0.59 to 0.39; P=0.69; 1 study, 579 participants; low-certainty evidence)
- No cases of hypercalcemia were reported in pregnant women receiving vitamin D vs. placebo across included trials
- Hypercalcemia risk in children was not significantly altered by vitamin D vs. placebo (RR 0.81, 95% CI 0.53–1.24; 7 studies, 1,542 participants; very low-certainty evidence)
- The review synthesized 107 RCTs with 31,521 total participants — the largest meta-analysis on this topic to date
Methodology
This Cochrane systematic review searched CENTRAL, MEDLINE, Embase, CINAHL, Web of Science, WHO Global Index Medicus, and four clinical trial registries through March 18, 2025, identifying 107 eligible RCTs with 31,521 participants. Eligible studies compared vitamin D (any form, any dose) to placebo or higher versus lower doses (threshold ≥1000 IU vs. ≤1000 IU) in children up to age five or their pregnant/lactating mothers. Meta-analyses calculated risk ratios for dichotomous outcomes and mean differences for continuous outcomes with 95% CIs; GRADE was used to assess evidence certainty. Studies involving children with chronic respiratory or systemic conditions were excluded.
Study Limitations
The primary limitation is low-to-very-low certainty evidence for the key benefit outcomes, driven by heterogeneity in dosing regimens, ARI definitions, and outcome measurement across trials. Many included studies were small, limiting statistical power and generalizability. The review had no dedicated funding, and the authors note that differences in supplementation timing, form (D2 vs. D3), and delivery setting made cross-study comparisons difficult.
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