Seasonality of vitamin D concentrations and the incidence of vitamin D deficiency in children and adolescents from central Poland.

Vitamin D3 [25(OH)D] deficiency is a significant problem in Polish children. In many regions of the world, 25(OH)D concentrations show seasonal variation and differences between boys and girls, due to seasonal differences in insolation, as well as different sociological and cultural factors.The aim of the study was to assess the seasonal variations of 25(OH)D concentrations and the incidence of vitamin D deficiency in children from central Poland.The analysis comprised 1275 children, age 3-18 (11.2 ±4.0) years, with disorders of growing and/or puberty, obesity, and other complaints that could be related to endocrine diseases, except for ones with calcium-phosphorus imbalance, impaired parathyroid hormone secretion, and diseases that may influence vitamin D supply.Seasonal variability of 25(OH)D concentrations was observed with maximal levels in August and minimal in January and close relationship between 25(OH)D levels and insolation in the previous two months. In all the quarters, 25(OH)D concentrations were lower in girls than in boys and in older vs. younger children. The median value of 25(OH)D concentrations was below the lower limit of optimal range during the whole year. High incidence of 25(OH)D deficiency was observed (from 10.7% in August to 80.4% in January) together with low proportion of normal 25(OH)D levels (from 3.6% in January to 42.1% in August).Our results are consistent with previous reports on inadequate vitamin D supplementation in Polish children and adolescents, pointing out the necessity to implement current recommendations concerning vitamin D supplementation and the need for further studies on the consequences vitamin D deficiency for health of children and adolescents, with special attention to the pleiotropic effects of vitamin D.