Seasonal variation of serum vitamin D levels in Romania

We measured serum vitamin D in 8024 Romanian subjects and found a marked seasonal variation with highest levels in September and lowest levels in March. The seasonal variation (early autumn vs. early spring) persisted in all age and sex groups. The prevalence of vitamin D deficiency was very high.

Romania is located in Eastern Europe, roughly between 44°N and 48°N latitude. Seasonal variation of serum vitamin D in Romanian subjects is unknown. We assessed the seasonal variation of 25-hydroxy vitamin D [25(OH)D] in Romanian population.

We retrieved from our endocrinology center database all 25(OH)D measurements between 2012 and 2016. We also evaluated age, sex, diagnosis, and date of blood sampling. The 25(OH)D was measured by two different chemiluminescence or electrochemiluminescence assays.

There were 8024 subjects (median age 50 (37, 62); 1429 men (17.8%)) without a diagnosis of low bone mass (osteopenia or osteoporosis). The median serum 25(OH)D was 18.6 (12.7, 25.4) ng/mL. Of the subjects, 0.73, 14.4, 55.6, and 86.1% had a serum 25(OH)D level below 4, 10, 20, and 30 ng/mL, respectively. Serum 25(OH)D showed a marked seasonal variation with highest levels in September (24.1 [18.3, 30.3] ng/mL) and lowest levels in March (13.5 [9.4, 19.6] ng/mL) (p < 0.001). The seasonal variation (early autumn vs. early spring) persisted in all age and sex groups and was maximal for 21-40 years of age (26.5 (20.8, 33.1) vs. 12.9 (9.7, 17.9) ng/mL) and minimal for >65 years of age (18.6 (13.0, 27.2) vs. 12.7 (7.8, 19.7) ng/mL). Men and women showed similar amplitude of serum 25(OH)D variation.

The prevalence of vitamin D deficiency is high, particularly in the elderly. The data show a strong seasonal variation of serum 25(OH)D in all subgroups of our Romanian population with highest levels in September and lowest levels in March.