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.