functions of vitamin D seem to be higher (30-50 ng/ml; 75-125 nmol/l), 25 but still are considered uncertain. Some authors, however, argue that even for proper bone mineralization, levels higher than 30 ng/ml (75 nmol/l) are necessary. 26 Indisputably, low 25(OH)D levels (below 20 ng/ml) are common and were reported worldwide, 1-7 and this is a drawback because epidemiological data underlined an association between vitamin D deficit and a higher risk for chronic conditions and multimorbidity, including musculoskeletal disorders, cancer, autoimmune diseases, cardiovascular disease, diabetes, and infectious diseases. 1,3,[27][28][29][30] One of the countries with limited data on vitamin D status is Poland (49-54°N; Central Europe). Results of studies carried out in our country indicated that the problem of vitamin D deficiency considerably affects the Polish population. In a study of 448 adult residents of urban areas INTRODUCTION Vitamin D is an important prohormone that can be synthesized by the skin exposed to sunlight (UVB) or ingested with food. However, low outdoor activity, sun protection, and low vitamin D content of staple foods reduce the significance of sun and diet as natural sources for efficacy of vitamin D metabolism and related health effects. In consequence, evidence from various populations highlighted vitamin D deficiency as a public health problem with high prevalence.1-17 The prevalence of vitamin D deficiency depends on diagnostic thresholds defining vitamin D status that is determined by total serum 25(OH)D levels, 18 and the recommended levels of 25(OH)D are still an issue of debate.19-22 Currently, it is accepted that maintaining serum 25(OH) D at a level of 20 ng/ml (50 nmol/l) or above is beneficial at least for bone health and calcium homeostasis.23,24 Levels required for noncalcemic PATIENTS AND METHODSThis cross-sectional study included a total of 5775 adult volunteers (4464 women; 1311 men; mean age, 54.0 ±15.9 years; range, 15.6-89.8 years), who were enrolled and examined through late winter and spring 2014. Serum concentrations of 25(OH)D were determined using the Liaison XL system (DiaSorin; CLIA method). Demographic and anthropometric data were also analyzed. RESULTSThe mean 25(OH)D concentration in the studied population was 18.0 ±9.6 ng/ml; 65.8% of the patients had 25(OH)D levels of less than 20 ng/ml; 24.1% had suboptimal levels of 20 to 30 ng/ml; and only 9.1% demonstrated the optimal levels of 30 to 50 ng/ml. In 89.9% of the studied population, 25(OH)D levels of less than 30 ng/ml were found. Obesity, defined as body mass index (BMI) over 30 kg/m 2 , was associated with lower 25(OH)D levels compared with normal weight (15.8 ±8.5 vs 18.5 ±9.7 ng/ml; P <0.0001). Lower 25(OH)D levels were observed in men, younger individuals, and individuals with excess body weight and higher BMI. CONCLUSIONSThe results of our study, which involved the most representative sample size of Polish adults, support the previously reported data on vitamin D status. The levels of 25(OH)D determined f...