Seasonal variance of 25-(OH) vitamin D in the general population of Estonia, a Northern European country

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.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.Vitamin D insufficiency is highly prevalent throughout the year in a population without vitamin D dairy fortification living at the latitude of 59°N.Vitamin D plays an important role in calcium and bone metabolism. Low levels of vitamin D lead to compensatory elevation of parathyroid hormone (PTH), which can cause lowering of bone mineral density (BMD) and eventually osteoporosis with fragility fractures or osteomalacia. Epidemiological studies have demonstrated a predisposition of the individuals with low levels of vitamin D to certain extra-skeletal diseases [1-4].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 people of all age groups [7-9]. Vitamin D serum concentrations are influenced by several modifiable and non-modifiable factors such as diet, latitude, season, time outdoors, skin pigmentation, clothing and tanning habits [10,11]. At high northern latitudes (abov