Longitudinal study of vitamin D status among Thai individuals in a sun-abundant country

Background: Vitamin D deficiency is a major public health problem worldwide, even in countries with abundant sunshine. Understanding the risk factors for vitamin D deficiency is important to inform public health recommendations. We conducted a longitudinal analysis of vitamin D status in Thai individuals to assess changes in vitamin D status over time and identify potential determinants.Study design: This study is a long term prospective cohort study.Methods: Of the 1239 participants who were employees of the Electricity Generating Authority of Thailand, serum 25-hydroxyvitamin D (25(OH)D) levels were measured by liquid chromatography/tandem mass spectrometry from samples collected in 2009 and 2019.Results: There was a significant 14.8% increase in serum total 25(OH)D (P < 0.001) from 2009 to 2019, which resulted from significant increases in both 25(OH)D-3 and 25(OH)D-2. The epimeric form of 25(OH)D-2 also increased significantly, while there was no increase in the epimeric form of 25(OH)D-3. A univariate analysis showed significant associations between increased total 25(OH)D and increasing age, male sex, and lower body mass index. After controlling for baseline vitamin D status, multivariate regression analyses found that the direction of association and significance from univariate analyses persisted for total 25(OH)D and 25(OH)D-3. However, a univariate association found between female sex and an increase in 25(OH)D-2 was not significant in multivariate regression analysis.Conclusions: A long-term trend of improved vitamin D status was found among Thai adult individuals over a 10-year period; however, improvements were less noticeable in women.