Influencing variables on total and free 25(OH)D levels in healthy population

Vitamin D plays a fundamental role in calcium homeostasis and bone metabolism. It mainly comes from cutaneous synthesis through the action of sunlight. Therefore, variations in exposure to this radiation modify serum levels. We studied two different analytes of vitamin D in the healthy Spanish population and the influence of seasonality, climate, and latitude on its levels.

This work is a cross-sectional, descriptive study. A total of 206 blood donors from Burgos and Valencia between 18-60 years of age were recruited during March-April and October-November 2018. Total and free serum 25-hydroxycholecalciferol (25(OH)D) were analyzed.

After summer, total and free serum 25(OH)D medium levels were 24.31 ± 5.25 ng/mL and 5.01 ± 1.25 pg/mL in Burgos and 25.99 ± 6.87 ng/mL and 8.97 ± 2.82 pg/mL in Valencia. After winter, they were 17.66 ± 5.04 ng/mL and 4.08 ± 0.66 ng/mL in Burgos and 21.38 ± 3.77 ng/mL and 7.23 ± 2.44 ng/mL in Valencia. The seasonal changes were statistically significant for both components studied both in the sample as a whole and in the separate populations. The differences found between the two populations in total and free 25(OH)D levels were statistically significant except for total 25(OH)D after summer (24.07 ng/mL vs. 26.03 ng/mL; p = .408). Latitude was also shown to be a factor that influences concentrations of both analytes in summer and winter.

Our study shows lower vitamin D levels than expected for a healthy Spanish population. Seasonality, climate, and latitude were demonstrated to influence total and free 25(OH)D levels.