The Effect of Obesity on the Relationship Between Serum Parathyroid Hormone and 25-Hydroxyvitamin D in Women

Abstract

Obesity is associated with lower serum concentrations of 25-hydroxyvitamin D (25OHD) and higher intact PTH. The threshold of 25OHD needed to maximally suppress intact PTH has been suggested as a marker of optimal vitamin D status. In this study, we hypothesized that whereas the obese have a higher serum PTH and lower 25OHD, suppression of serum PTH by 25OHD would be independent of body weight. We performed a retrospective analysis on 383 women (ages 24-75 y) with a wide range of body weights (43-185 kg) who were stabilized to 1-1.2 g calcium/d for 1 month before blood draw. Body composition, serum PTH, 25OHD, calcium, and creatinine were measured. Locally weighted regression and smoothing scatterplots were used to depict the association between serum PTH and 25OHD. A nonlinear exponential model determined the point for near maximal suppression of PTH by 25OHD. The point for near maximal suppression of PTH by 25OHD for all women (body mass index, 31.4 ± 7.7 kg/m²) occurred at a 25OHD concentration of 21.7 ng/mL (95% confidence interval, 28-48 ng/mL). No point of maximal suppression was found for nonobese women, yet in the obese women (n = 207; body mass index, >30 kg/m²) suppression of PTH occurred at a 25OHD concentration of 11.1 ng/mL (95% confidence interval, 4.7-17.5 ng/mL). These results suggest that if PTH is suppressed at a lower serum 25OHD in the obese compared to the entire population, the lower average 25OHD concentrations in the obese may not have the same physiological significance as in the general population.