BMC Public Health 2009, 9:22 http://www.biomedcentral.com/1471-2458/9/22 TFhigeudriest1ribution of serum 25(OH)D in winter and in summer The distribution of serum 25(OH)D in winter and in summer. vs. 61.5 nmol/L; p = 0.01) which was missing in winter. Age also correlated negatively with 25(OH)D only in the summer (p = 0.016). Both these associations lost signifi- cance in regression analysis if adjusted for self-reported sunbathing habits. The 25(OH)D level was strongly dependent on sunbathing habits both in summer and in winter (Table 2). The BMI and 25(OH)D correlated inversely: subjects with high BMI (>30) had lower 25(OH)D than subjects with a BMI under 30 (41.0 nmol/L vs. 44.8 nmol/L; p < 0.05). If a BMI cut-off point of 34 was used the difference was even more obvious (38.8 nmol/L vs. 44.5 nmol/L; p < 0.01). In multiple-regression analysis when adjusting for sunbath- ing habits BMI lost significance in winter. In analysis of variance (ANOVA) body mass index was significantly higher in the subjects who avoided sunbathing, and those who bathed the arms and face only when compared with total body exposers (p < 0.0001). Younger individuals were considerably more likely to sunbathe the whole body than older individuals (p < 0.0001). Sunbathing habits (p < 0.0001), smoking (p = 0.008) and vitamin D supplement usage (p = 0.07) were the signifi- cant determinants of winter vitamin D level in multiple regression analysis explaining 12% of its variance (r2 = 0.12). In summer the determinants for vitamin D levels were sunbathing (p < 0.0001), smoking (p = 0.001) and BMI (p = 0.03) explaining 18% of its total variance (r2 = 0.18). Discussion The study is one of the few population-based studies on vitamin D status in northern Europe. The mean 25(OH)D in winter is well below 50 nmol/L, with only a third of the Estonian population showing sufficient vitamin D levels and just 3% of the population above the optimal level. Although the mean 25(OH)D level was around 60 nmol/ Page 4 of 8 (page number not for citation purposes)