Population Serum 25-Hydroxyvitamin D Status in Kazakhstan: A Systematic Review and Meta-Analysis

Background/Objectives: The aim of this study was to systematically synthesize and quantitatively estimate the mean serum 25-hydroxyvitamin D concentrations across populations in Kazakhstan and to examine variations according to age group, health status, and geographic region. In addition, we specifically evaluated healthy subgroups to establish reference estimates that may be useful for future epidemiological surveillance and public health planning. Methods: A systematic review and meta-analysis was conducted in accordance with the PRISMA 2020 guidelines. PubMed, Scopus, ScienceDirect, and Google Scholar were searched through March 2026 without date restrictions. Studies reporting the mean serum vitamin D concentrations among Kazakhstani populations were included. Random-effects meta-analysis was performed in R. Subgroup analyses were conducted by age group, health status, and geographic region. Meta-regression, influence diagnostics, publication bias assessment, JBI risk-of-bias evaluation, and GRADE certainty-of-evidence assessment were performed. Results: Sixteen studies comprising 28 groups and 5771 participants were included. The pooled mean serum 25(OH)D concentration in the overall cohort was 22.3 ng/mL (95% CI: 19.3-25.3), while the healthy cohort demonstrated a slightly higher pooled mean of 24.4 ng/mL (95% CI: 20.3-28.4). Adolescents had the lowest vitamin D levels among all age groups. Significant regional variability was observed, and meta-regression identified male participant proportion as a significant moderator (p = 0.03). Heterogeneity was extremely high across analyses (I² ≈ 99.9%). Conclusions: Mean serum 25(OH)D concentrations were generally within the insufficient range across the included study groups in Kazakhstan, including healthy subgroups. However, because the certainty of evidence was very low and between-study heterogeneity was extreme, the findings should be interpreted cautiously. These results support the need for standardized national surveillance and locally evaluated public health policy considerations, including targeted supplementation for high-risk groups, screening strategies where clinically indicated, and assessment of food fortification options.