A Systematic Review and Meta-Analysis of Vitamin D Status of Apparently Healthy Adults in Worldwide Regions

Plasma vitamin D, measured as 25-Hydroxyvitamin D [25(OH)D], is influenced by genetics, lifestyle, age, gender, skin pigmentation, sunlight exposure, latitude, sunscreen use, diet and supplementation. Globally, vitamin D deficiency is frequently reported among healthy populations, yet comprehensive analyses summarizing its prevalence are limited. This systematic review and meta-analysis aimed to assess 25(OH)D status among healthy adults across different regions worldwide. We systematically searched PubMed, Web of Science, ScienceDirect and Google Scholar for observational studies published in English between 2013 and 2024. Heterogeneity among studies was evaluated using Cochran's Q test and the I² statistic, with significance set at p<0.10; I² values were interpreted as low (0-40%), moderate (30-60%), substantial (50-90%) and considerable (75-100%). Due to significant heterogeneity, a random-effects model was employed to estimate the prevalence of vitamin D deficiency. A total of 394,548 participants from 56 studies across 30 countries were included. The pooled prevalence of vitamin D deficiency was 55% (95% CI: 49.0-60.0%), with considerable heterogeneity (Q p<0.001; I² = 100%). Subgroup analysis showed the highest deficiency in the Middle East (62%; 95% CI: 54-70%), followed by Asia (54%; 95% CI: 39-69%), Africa (43%; 95% CI: 24-63%), and Europe (43%; 95% CI: 23-63%). Weighted mean vitamin D levels ranged from 8.4±0.5 ng/mL to 37.7±10.8 ng/mL, with an overall mean of 20.14±6.7 ng/mL. Regionally, the Middle East had the lowest recalculated weighted mean level (17.8 ng/mL), followed by Asia (20.4 ng/mL), Europe (26.8 ng/mL), "Others" (28.0 ng/mL) and Africa (29.2 ng/mL, influenced by one high outlier study). These findings reveal a high prevalence of vitamin D deficiency among healthy adults, particularly in the Middle East, but with the lowest mean levels observed in the Middle East and marked regional disparities, highlighting marked regional disparities.