BackgroundHuman vitamin D status primarily depends on skin exposure to the ultraviolet B (UVB) spectrum of the sunlight.Despite the many days of sunshine in Mongolia, the northern latitute means that much of the UVB is filteredout as it passes through the atmosphere. Studies of Mongolian infants, schoolchildren, and pregnant womenreveal prevalent and profound vitamin D deficiency in the winter months in Mongolia. To date, there has notbeen a single study of the vitamin D levels of Mongolian men, and no studies of working age women outside ofUlaanbaatar. The goal of this study is to determine Vitamin D levels among Mongolian working age populationin different geographical areas, in different seasons, and in different work settings.MethodsThis cross-sectional study was conducted among 120 healthy adults, recruited by a multistage clustersampling method in Ulaanbaatar, South Gobi, and Bulgan. Each participant was tested for serum 25(OH)Dconcentrations, twice in winter and summer. Samples were measured by ELISA. The paired sampling (120summer samples/120 winter samples total 240 samples) frame allowed us to compare an individual’s winter25(OH)D levels to their own summer 25(OH)D levels, avoiding any confounding by differences betweenindividuals. A paired T-test (two sided) with unequal variances was used to test for differences in 25(OH)Dlevels among study groups.Results95% of all participants were Vitamin D deficient (<20 ng/ml) in winter, 24% deficient in summer (p < 0.001).The mean winter serum 25(OH)D levels were (±SD) 10.7±5.3 ng/ml, which were doubled in the summer to(±SD) 26.1±8.1 ng/ml. In all three regions, men and women had similar mean 25(OH)D levels. In Ulaanbaatar,office workers had higher winter 25(OH)D levels than urban outdoor workers. Surprisingly, office workersin the Gobi had higher 25(OH)D levels than nomads in both winter and summer. In Bulgan, there were nodifferences between office workers and nomads in any season.ConclusionWe observe that low vitamin D levels are more prevalent in our winter samples of healthy working age adults.The prevalence of vitamin D deficiency is very high amongst the adult population. These data suggest a needto increase vitamin D intake either through improved fortification and/or supplementation.