Vitamin D (VitD) is essential for the normal functioning of the body, as it participates in the metabolism of calcium and phosphorus, and the lack of it is associated with risk factors for developing various diseases. Endogenous synthesis of VitD in the skin—which happens upon exposure to ultraviolet B radiation—is a primary source of VitD, while in the absence of this process, the crucial source becomes dietary intake. Only a few foods naturally contain VitD, and even those often contain in smaller quantities. Therefore, fortified foods and dietary supplements can be an important source of VitD. The aim of the doctoral thesis was to assess the VitD status and factors influencing VitD status in Slovenia. A systematic literature review revealed a high prevalence of low VitD status, particularly during the wintertime. The nationally representative Nutrihealth study confirmed alarming results. During an extended winter period, insufficiency was observed in 81.6% of adults and 78.8% of older adults, while deficiency was observed in 40.8% of adults and 34.6% of older adults. Additionally, factors influencing VitD status were investigated. The Nutri-D study has indicated a higher prevalence of deficiency in winter among individuals with higher a body mass index and a lighter constitutive skin color. In contrast, suboptimal levels during summertime were more common in those with a higher body mass index, lighter constitutive skin color, lower dietary VitD intake, and those protecting themselves from the sun with clothing. Furthermore, a systematic review of the dietary VitD intake revealed low intakes across all populations. The SI.Menu research study which was included in the systematic review, reported VitD intake of 2.7 µg in adolescents, 2.9 µg in adults, and 2.5 µg in older adults. In the scope of the doctoral thesis, we identified highly prevalent low VitD status, which was influenced by various factors, especially body mass index, constitutive skin color, and dietary intake. VitD deficiency is a major public health risk, that requires a systematic approach on multiple levels. It is of the utmost importance to design conductive policies and take rigorous public health measures at both individual and national levels to reduce the prevalence of VitD deficiency in Slovenia.
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