BMC Public Health 2009, 9:22 http://www.biomedcentral.com/1471-2458/9/22 common cancers, diabetes, multiple sclerosis and athero- sclerosis [1-4]. Our study supports the evidence that PTH achieves a pla- teau at 25(OH)D concentrations between 75 and 90 nmol/L demonstrated recently [21]. In winter no plateau was achieved, since the majority of observations (97%) were below 75 nmol/L. Our study demonstrated an unexpected finding that men have a higher amplitude of 25(OH)D variance through the seasons than women. Clothing habits have been shown to influence 25(OH)D levels [11,37]. There might be gender-specific clothing differences (although not of a culture-religious nature in the studied region) which are responsible for a higher 25(OH)D in men after the sunny season. There is also a possibility that men are more likely to work outside during the summer than women, result- ing in longer exposure to UV light. Secondly, fat tissue is the physiological depot for vitamin D suggesting that the obese have an increased storage capacity of 25(OH)D [38,39]. The fact that with equal BMI and age, men have a lower body fat percentage than women [40] might explain the faster decline and also the faster accrual in men. Stud- ies have also shown that vitamin D has a role in neu- romuscular function [41,42]. This might mean that men, having higher muscle mass and lower fat tissue [40], require more vitamin D to sustain muscle cell function and in the absence of UVB radiation, deplete their vitamin D stores faster. The study showed that BMI, age and menopause deter- mined 25(OH)D status in the end of summer, but not in winter. The fact that all these associations lost significance when adjusted for sunbathing habits suggests that most probably it is not the weight and age per se influencing 25(OH)D, but their effect on sunbathing, which impacts the UVB doses they are subjected to in summer, resulting in lower 25(OH)D levels. This is plausible, as we found no correlation between 25(OH)D and BMI, age or meno- pausal status (an indirect indicator of age for women) in winter, when these factors cannot exert their effect on sun exposure. As a study limiting factor we might be reporting 25(OH)D values, which are slightly higher than the average year to year measurements at this latitude, as in summer 2006 the sun was comparatively plentiful compared to previous years. In our study people with a higher 25(OH)D value in winter were more likely to attend the follow-up test in summer, which is another minor aspect of bias. Therefore it is possible that we might be overestimating vitamin D levels in summer. However, since the drop-out rate was not remarkable, this bias should not influence the results significantly. At the time of the study our laboratory was not part of the international vitamin D assays quality assessment scheme (DEQAS) which, as already men- tioned, would have enhanced the comparability of the results with those performed by other laboratories and assay methods. Conclusion We demonstrated that 25(OH)D levels are low all year round in Estonia, a country situated at a high northern lat- itude. Dairy fortification with vitamin D (which is already practiced in neighbouring Finland, Norway, Sweden and also the US) might address these shortcomings in addi- tion to propagating healthy sunbathing habits, since sun had a major effect on vitamin D status and explained much of the age and BMI caused variance in 25(OH)D levels. Competing interests Authors Mart Kull, Riina Kallikorm, Anu Tamm and Mar- gus Lember have no conflicts of interest. This work was possible thanks to the support of the Estonian Science Foundation (Grant 6452) and an unrestricted grant from Merck Sharp and Dohme used exclusively to cover labora- tory analysis costs. Authors' contributions MK is the drafter of the current paper. He was actively involved in the designing and coordination of the study, material acquisition, analysis and interpretation of the material. RK and ML were actively involved in the design- ing of the study, material collection, and interpretation of results, drafting and revising the manuscript as well as acquisition of the funding for the study. In addition ML was the general supervisor of the research group. AT was involved and responsible for laboratory handling and analysis of blood samples, participated in the critical review of the paper. All the authors named have read and approved the final version of the manuscript before sub- mitting. Acknowledgements We wish to thank Dr. Tiina Vilimaa and Dr. Mall Lepiksoo for their help in organising the study; Ms. Inge Ringmets for her help in statistical analysis and Mr. Stewart Johnson for language review. We are also grateful to Dr. Peeter Saadla, Dr. Tarvo Rajasalu, Dr. Triin Kull, Dr. Madis Parksepp, Dr. Mari Parksepp and Dr. Eve Härma for their help in study material collection. References 1. EURODIAB ACE Study Group: Variation and trends in incidence of childhood diabetes in Europe. Lancet 2000, 355:873-876. 2. Hernan MA, Olek MJ, Ascherio A: Geographic variation of MS incidence in two prospective studies of US women. Neurology 1999, 53:1711-1718. 3. Cantorna MT: Vitamin D and autoimmunity: is vitamin D sta- tus an environmental factor affecting autoimmune disease prevalence? Proc Soc Exp Biol Med 2000, 223:230-233. 4. Garland CF, Garland FC, Gorham ED: Calcium and vitamin D. Their potential roles in colon and breast cancer prevention. Ann N Y Acad Sci 1999, 889:107-119. 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