BMC Public Health Research article Seasonal variance of 25-(OH) vitamin D in the general population of Estonia, a Northern European country Mart Kull Jr*1,2, Riina Kallikorm1,2, Anu Tamm2 and Margus Lember1,2 Address: 1Department of Internal Medicine, University of Tartu, Puusepa Str. 6, Tartu, Estonia and 2Tartu University Hospital, Puusepa Str. 1A, Tartu, Estonia Email: Mart Kull* - martkull@gmail.com; Riina Kallikorm - riina.kallikorm@kliinikum.ee; Anu Tamm - anu.tamm@kliinikum.ee; Margus Lember - margus.lember@kliinikum.ee * Corresponding author BioMed Central Open Access Published: 19 January 2009 BMC Public Health 2009, 9:22 doi:10.1186/1471-2458-9-22 This article is available from: http://www.biomedcentral.com/1471-2458/9/22 Received: 13 August 2008 Accepted: 19 January 2009 © 2009 Kull et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: Vitamin D has a wide variety of physiological functions in the human body. There is increasing evidence that low serum levels of this vitamin have an important role in the pathogenesis of different skeletal and extra-skeletal diseases. Vitamin D deficiency and insufficiency is common at northern latitudes. There are few population-based studies in the northern European region looking at the issue in a wider age group. We aimed to measure Vitamin D level in the general population of Estonia (latitude 59°N), a North-European country where dairy products are not fortified with vitamin D. Methods: The study subjects were a population-based random selection of 367 individuals (200 women and 167 men, mean age 48.9 ± 12.2 years, range 25–70 years) from the registers of general health care providers. 25-(OH) vitamin D (25(OH)D) level and parathyroid hormone (PTH) were measured in summer and in winter. Additionally age, sex, body mass index (BMI) and self-reported sunbathing habits were recorded. Results: The mean serum 25(OH)D concentration in winter was 43.7 ± 15 nmol/L and in summer 59.3 ± 18 nmol/L (p < 0.0001). In winter 73% of the subjects had 25(OH)D insufficiency (25(OH)D concentration below 50 nmol/L) and 8% had deficiency (25(OH)D below 25 nmol/L). The corresponding percentages in summer were 29% for insufficiency and less than 1% for deficiency. PTH reached a plateau at around 80 nmol/L. BMI and age were inversely associated with 25(OH)D, but lost significance when adjusted for sunbathing habits. A difference in the seasonal 25(OH)D amplitude between genders (p = 0.01) was revealed. Conclusion: Vitamin D insufficiency is highly prevalent throughout the year in a population without vitamin D dairy fortification living at the latitude of 59°N. Background Vitamin D plays an important role in calcium and bone metabolism. Low levels of vitamin D lead to compensa- tory elevation of parathyroid hormone (PTH), which can cause lowering of bone mineral density (BMD) and even- tually osteoporosis with fragility fractures or osteomala- cia. Epidemiological studies have demonstrated a predisposition of the individuals with low levels of vita- min D to certain extra-skeletal diseases [1-4]. Page 1 of 8 (page number not for citation purposes)