Vitamin D status and its relation to age and body mass index

While numerous studies have examined 25(OH)-vitamin D(3) (25-D) concentrations and their relation to parathyroid hormone (PTH) levels there is only limited information on the interrelation between 25-D, 1,25(OH)(2)-vitamin D(3) (1,25-D) and PTH. It was the aim of this study to assess the vitamin D endocrine system and its relation to age and body mass index (BMI).

This cross-sectional study comprised a convenience sample of 483 adults which attended the endocrinology outpatient service of a university hospital in the years 2002-2004.

The mean concentrations of 25-D, 1,25-D, calcium and PTH were 21.0 +/- 10.6 ng/ml, 47.9 +/- 21.7 pg/ml, 9.48 +/- 0.48 mg/dl and 51.0 +/- 27.2 pg/ml, respectively. 25-D was related (p < 0.01) to BMI, age, PTH and 1,25-D. After correction for 25-D, we found no relation between BMI and 1,25-D. PTH was related (p < 0.01) to serum calcium, BMI, age and 1,25-D (p = 0015). There was a seasonal variation in both, 25-D and 1,25-D serum concentrations: 25-D levels were lowest in January and increased until July while the nadir and zenith of 1,25-D were found in April and October, respectively.

Since BMI was negatively related to 25-D the prevalence of 25-D deficiency (<8.8 ng/ml) increased from 8.8% in subjects with BMI <30 kg/m(2) to 15.0% in subjects with BMI >30 kg/m(2). BMI, age and season should be taken into account when assessing a patients vitamin D status and more aggressive vitamin D supplementation should be considered for obese subjects.