Background: Obesity is associated with lower serum vitamin D (25(OH)D) levels through
several mechanisms. The aim of the study was to examine the possibility of a negative association
between fat mass and 25(OH)D levels in a cohort of otherwise healthy overweight and obese subjects,
independently of age, sex, blood pressure levels and anthropometric and metabolic parameters.
Materials and Methods: 147 overweight and obese subjects (106 women and 41 men), aged between
18 and 69 years, were enrolled into the study. All of them did not show any clinically evident metabolic
or chronic diseases (i.e. hypertension, diabetes mellitus, renal failure, etc.) and did not use any
kind of drug. Serum fasting levels of 25(OH)D, insulin, glucose, uric acid and lipids (triglycerides,
total, HDL and LDL cholesterol) were measured. The season in which the blood samples were collected
was autumn. Insulin resistance was assessed by using the Homeostasis Model Assessment
(HOMA-IR). Body composition parameters (Fat Mass [FM], Fat Free Mass [FFM], body cell mass
[BCM], Total Body Water [TBW]) were measured by electrical Bioimpedance Analysis (BIA). Lastly,
demographic, anthropometric and clinical parameters (age, Body Mass Index [BMI], Waist Circumference
[WC], Systolic (SBP) and Diastolic (DBP) blood pressure) were also assessed.
Results: 25(OH)D levels were significantly and negatively correlated with BMI (P <0.001), WC (P
<0.01), DBP (P <0.05), insulin (P <0.001), HOMA-IR (P <0.01), triglycerides (P <0.01), and fat mass
(P <0.001). A multivariate regression analysis was performed by considering 25(OH)D levels as the
dependent variable and sex, waist circumference, fat mass, DBP, triglycerides, and insulin (or HOMAIR)
as the independent ones, and 25(OH)D levels maintained a significant and independent relationship
only with fat mass (negative) (P <0.01).
Conclusion: This study clearly shows that 25(OH)D circulating levels are progressively lower with the
increase of fat mass, independently of sex, body fat distribution, blood pressure and insulin and metabolic
parameters. These data strongly show that adipose tissue accumulation per se is absolutely the
main factor responsible factor for lower 25(OH)D levels in obese subjects, possibly through sequestration
of fat soluble 25(OH)D in fat mass.