Serum 25-Hydroxyvitamin D Is a Predictor of Serum 1,25-Dihydroxyvitamin D in Overweight and Obese Patients

Recent research suggests that 1,25-dihydroxyvitamin D [1,25(OH)₂D], a steroid hormone that regulates calcium homeostasis, may also play a role in the development and progression of cancer, multiple sclerosis, cardiovascular, and other diseases. Decreased serum 1,25(OH)₂D concentrations are often observed in overweight and obese patients. However, little is known about the factors that may influence 1,25(OH)₂D renal synthesis, because it is generally accepted that serum 1,25(OH)₂D concentration is strictly regulated by parathyroid hormone and serum concentrations of calcium and phosphorus. In this study, the associations among serum 1,25(OH)₂D, serum 25-hydroxyvitamin D [25(OH)D], and body composition were analyzed in 1779 patients with excess body weight registered in a Metabolic and Medical Lifestyle Management Clinic in Oslo, Norway. According to our results, serum 25(OH)D, adiposity, age, season of blood sampling, and gender directly influence serum 1,25(OH)₂D (r = 0.33; P < 0.001), with serum 25(OH)D being the strongest predictor for serum 1,25(OH)₂D. The 1,25(OH)₂D concentrations were 25.4 pmol/L (95% Cl: 19.3-31.5; P < 0.001) lower in the lowest 25(OH)D quartile to compared with highest quartile. A seasonal variation was observed for both vitamin D metabolites. Thus, our results suggest that in patients with excess body weight, serum 1,25(OH)₂D concentrations were associated with 25(OH)D and varied during the year. Therefore, it may also be valuable to measure both serum 25(OH)D and 1,25(OH)₂D for the evaluation of vitamin D status in overweight and obese persons.