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/L in summer, with 2/3 of the population reaching sufficiency, the majority of the subjects (around 90%) still stayed below the optimal level of 75 nmol/L. The finding is similar to the results of other studies in the region demonstrating that the summer build-up of 25(OH)D is inadequate in northern countries [26
For overview purposes the currently available vitamin D status data in similar populations performed to date is given in Table . Due to the inter-assay and also inter-laboratory variability of different vitamin D measurement techniques some caution is advocated in interpretation [34
]. At 44 nmol/L the winter mean 25(OH)D concentration in Estonia is comparable with levels in several southerly situated countries but falls short if compared with vitamin D levels in Belgium, France, Switzerland and the US [7
] (Table ). Our summer 25(OH)D is the lowest in this comparison, which corresponds well with the latitude effect. The fact that there is a low proportion of vitamin D supplement users in Estonia and the diet here is also scarce in natural sources of vitamin D (such as fish or fish products) [35
] makes the population largely dependent on the vitamin D stores accumulated during the sunny season. In a country populating an even higher latitude, Norway, a study showed that 40% of the population regularly uses cod-liver supplements, almost 60% consumes fish liver products and 57% other vitamin D supplements, beside cod liver oil [36
]. This might explain why Norway, despite being located at 65–71 N is one of the countries with the highest vitamin D levels in Europe.
25(OH)D status in population studies of adults (15+) in the order of latitude.
The proportion of 25(OH)D deficiency and insufficiency in our study population is comparable with other countries (table ) during different seasons even though it is geographically the highest northern country in the comparison after Finland. However we consider the percentages too high to be satisfactory, as there is still a large proportion of the population who especially during winter are at an increased health risk. Addressing these shortcomings is important as the health risks include diseases like osteoporosis, rickets and with some evidence supporting benefits or risk reductions also for several types of common cancers, diabetes, multiple sclerosis and atherosclerosis [1
Our study supports the evidence that PTH achieves a plateau 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
]. 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, resulting 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
]. 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. Studies have also shown that vitamin D has a role in neuromuscular function [41
]. 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 determined 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 menopausal 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 mentioned, would have enhanced the comparability of the results with those performed by other laboratories and assay methods.