Body mass index represents a good predictor of vitamin D status in women independently from age.

Background & Aims: Vitamin D is a pleiotropic hormone targeting several tissues and is involved in basic homeostatic processes, including bone mineralization, immune response and muscle strength. Although hypovitaminosis D is common in Europe and North America, representing a risk factor for several chronic diseases, the contribution of factors other than sun exposure is largely underestimated.

Methods: In our study, we retrospectively collected data from medical records of women with age between 19 and 80 screened in Central Italy (42°N) for increased risk of metabolic syndrome. Vitamin D status was evaluated by serum 25-hydroxyvitamin D (25(OH)D) measurement and the association among vitamin D status and anthropometric and clinic variables was tested by multivariate logistic analysis.

Results: More than 80% of women presented serum 25(OH)D concentration lower than 30 ng/mL (75 nM), with the majority of values falling between 10 and 20 ng/mL. 25(OH)D concentration was dependent on season, with the highest 25(OH)D mean value measured in September and the lowest mean value in March. Among different clinical characteristics, body mass index (BMI) demonstrated the highest significant inverse correlation with serum 25(OH)D values, independently from season and age. Serum 25(OH)D values demonstrated a seasonal-directed sinusoidal trend and they raised during spring/summer in a similar manner in both obese and non-obese women. However, the obese group had lower mean values of vitamin D respect to overweight and to normal weight groups in both winter and summer, reaching frequently the status of vitamin D deficiency (<10 ng/mL).

Conclusions: In conclusion, at our latitude, seasonal UV irradiance availability determines an obligate sinusoidal trend in vitamin D status. However, body mass is able to reduce proportionally circulating vitamin D over calendar months determining vitamin D deficiency. These results suggest taking in particular account BMI in clinical management of vitamin D status in overweight and obese women.