Update on Vitamin D Status and Seasonal Variation in a Non-Supplemented Population Living in a High Polluted Urban Area—A Cross-Sectional Study

Background: Serum concentrations of 25-hydroxyvitamin D [25(OH)D] are associated with the risk of several chronic and acute diseases. However, updated data on vitamin D status in Mediterranean countries, including Italy, remain limited, hindering effective public health strategies. Objective: To assess serum 25(OH)D levels and their seasonal variation in healthy blood donors aged 18–65 years living in Northern Italy and not taking vitamin D supplements. Given the latitude and the high levels of environmental pollution, cutaneous vitamin D synthesis may be impaired in this population. Recent Italian guidelines on supplementation emphasize the need for updated data on the prevalence of hypovitaminosis D and seasonal variation in endogenous vitamin D synthesis. Methods: In this exploratory retrospective cross-sectional study, 534 blood donors (268 men and 266 women) attending the Transfusion Medicine Unit of the Verona University Hospital were enrolled between April 2016 and May 2018. Serum 25(OH)D concentrations were analyzed by season. Clinical, lifestyle, pharmacological and dietary characteristics were also collected. Results: Among healthy, normal-weight individuals, the prevalence of vitamin D insufficiency (25(OH)D < 50 nmol/L) was low and limited to one-two months per year. Overweight and obesity significantly reduced the likelihood of achieving adequate 25(OH)D levels through cutaneous synthesis for several months. Mean 25(OH)D concentrations were higher than those previously reported in the same area, while seasonal variation remained preserved. Conclusions: In a relatively small non-supplemented population of blood donors living in a high polluted urban area of Northern Italy, seasonal vitamin D synthesis seems to be preserved. These updated data show higher 25(OH)D levels compared to past findings. Although these data certainly warrant further validation through a national survey involving other regions of Italy and in not selected population, they appear to be in line with the SIOMMMS recommendations against indiscriminate serum 25(OH)D testing and against routine supplementation for healthy normal-weight individuals under 70 years.