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Bulgaria is situated in the temperate climactic zone between 41 ◦ and 44 ◦ north. Between November and March the sun is low at the horizon and the skin production of vitamin D is very low. The aim of the study was to investigate the seasonal changes in 25(OH)D levels and their relationship to gender, age and the severity of vitamin D deficiency. MATERIALS AND METHODS: Of the 433 subjects (tested in the winter) invited to participate in the seasonal 25(OH)D study - 315 (72.7%) were tested in the spring, 285 (65.8%) in the summer and 264 (61.0%) in the autumn. In 230 subjects data were available from all four seasonal measurements. One hundred twenty-five (54.3%) were women and 105 (45.7%) were men. The subjects were divided into three age groups: 1) young 20-44 years, n = 107 (46.5%); 2) middle age 45-59 years, n = 75 (32.6%); 3) elderly ≥60 years, n = 48 (20.9%). Serum 25(OH)D and PTH were measured. RESULTS: The mean 25(OH)D levels in the whole group were: winter - 27.14 nmol/l (95%CI: 25.70-28.58), spring - 43.56 nmol/l (95%CI: 41.96-46.17), summer - 61.74 nmol/l (95%CI: 58.95-64.55), autumn - 52.75 nmol/l (95%CI: 50.63-54.88), p < 0.001. The absolute increase of vitamin D was higher in the males than in the females -  30.38 (95% CI: 28.11-32.39) vs.  25.78 (95% CI: 23.67-27.57), p = 0.002. The increase was higher in the younger individuals than in the elderly -  30.23 (95%CI: 28.19-32.26) vs.  26.40 (95%CI: 23.41-29.39), p = 0.038. The seasonal variations were higher in the subjects with vitamin D deficiency (<25 nmol/l) in winter than in those with vitamin D insufficiency (25-50 nmol/l) -  29.83 (95%CI: 27.58-31.88) vs.  26.20 (95%CI: 24.19-28.07), p = 0.015. CONCLUSION: There are significant seasonal variations in 25(OH)D levels in Bulgaria. The men, the younger individuals and those with deficiency have higher potential for an increase in 25(OH)D levels than the women, the elderly and those with insufficiency.

Dynamic of the seasonal levels of 25(OH)D in Bulgaria according to sex, age and winter status of vitamin D