BMC Public Health 2009, 9:22 http://www.biomedcentral.com/1471-2458/9/22 Vitamin D inadequacy is being increasingly recognised worldwide [5,6]. This shortcoming in vitamin D status is most prevalent in the elderly population, but affects peo- ple of all age groups [7-9]. Vitamin D serum concentra- tions are influenced by several modifiable and non- modifiable factors such as diet, latitude, season, time out- doors, skin pigmentation, clothing and tanning habits [10,11]. At high northern latitudes (above 40°N) even with adequate sun exposure the dermal generation of vita- min D is low or missing in winter and thus increases demand for dietary intake [5,12]. As very few foods natu- rally contain vitamin D in amounts satisfying such increased demand in winter, this results in marked sea- sonal variation in the levels of vitamin D [13]. It has been hypothesised that annually recurring cycles of vitamin D can cause mild secondary hyperparathyroidism, which leads to variations in bone turnover rate [14]. Such fluctu- ations could influence the rate of age-related bone loss [15] and might hamper peak bone mass attainment [16]. Different cut-off values for the normal threshold for vita- min D have been used until recently. A level of 50 nmol/ L has been widely used to define 25(OH)D insufficiency, while some studies have used 37.5 nmol/L as the lowest level of sufficiency [17-19]. Recent studies, however, sug- gest that a 25-(OH)D level as high as 75 nmol/L or higher is needed to cover all physiological functions of vitamin D and should therefore be considered optimal [7,20-23]. There are wide regional differences in the 25(OH)D levels with an observed latitude effect, which has already been addressed in several countries by implementing dairy for- tifications policies. Therefore, Norway and Sweden, two of the northernmost countries in Europe, have population 25(OH)D serum levels equal to or higher than those in Spain and Turkey, where sun is plentiful [7,24,25]. In Fin- land low 25(OH)D concentrations were reported in win- ter, although vitamin D dietary intake recommendations were met in the studied population [26]. Since then, dairy fortification has been implemented also in Finland. There are few population-based studies looking at seasonal dif- ferences of vitamin D concentrations and no data is avail- able on vitamin D status of the Estonian population, which is one of the northernmost non-fortifying Euro- pean countries [7,14,18,26-33]. The aim of the current study was to investigate vitamin D status and its seasonal dynamics in a random selection of the adult Estonian population. Methods Subjects The study was conducted in Väike-Maarja municipality in Estonia in 2006. Study subjects were randomly selected from the registers of general practitioners in the region. An initial invitation and a second invitation (if needed) were sent to a total of 402 subjects to participate in the study. Of those invited, 243 (60%) responded. The non- responders were substituted once with the next person of the same age and sex from the patient register, in order to retain the population structure of the first selection. A total of 158 substitutions were made and an invitation (and a repeat if needed) was sent to them. An additional 124 subjects responded (response rate 79%) and were included in the study. A total of 367 subjects (200 women and 167 men, aged 25–70 years) participated in the study with a response rate of 66%. No inclusion/exclusion crite- ria were applied. An informed consent was obtained from all the subjects who participated. The Ethics Committee of Tartu University approved the study. All the 367 respond- ers were subsequently invited to a follow-up blood test in the end of summer. Of the 367 subjects 316 (86%) com- plied and a second blood sample was taken in September. Self-reported summertime sunbathing habits and for women menopausal status were recorded with a self- administered questionnaire. The former was categorised as: usually bathes the whole body; bathes only the arms and face or avoids bathing totally. In addition, the use of vitamin D supplements was recorded. Subject weight and height were measured for body mass index (BMI; kg/m2) calculation. Laboratory measurements Fasting overnight blood samples were obtained twice dur- ing the year, in winter (sampling time from January to March) and in the end of summer (September). All the samples were taken after overnight fasting between 8 AM and noon using pre-cooled tubes. Serum was separated and the samples stored at -20°C until analysed. The serum 25(OH)D level was measured in duplicates by radioim- munosorbent assay by DiaSorin (former Incstar), Stillwa- ter, Minnesota, USA. The intra- and interassay CVs were 4.1% and 5.7%, respectively. For group discrimination, we used 25 nmol/L as the critical value for deficiency and 50 nmol/L as the cut-off value for insufficiency. 75 nmol/ L was considered to be the optimal 25(OH)D level. Serum PTH was measured with an Immulite 2000 analyser (DPC). The intra- and interassay CVs were 10.5% and 8.2%, respectively. Statistics The variables were verified for normality (Shapiro-Wilk test) and skewed variables (vitamin D in summer, PTH in winter and in summer) transformed using a natural loga- rithm for analysis. However geometric means are pro- vided for better interpretation in the text. Student t or Mann-Whitney tests or analysis of variance (ANOVA) with Tukey test procedure were used to compare means. Correlations between variables were tested using the Pear- son product-moment correlation test and presented as Pearson correlation coefficient (rp). A 5% probability for Page 2 of 8 (page number not for citation purposes)