Association between 25-hydroxyvitamin D and intact parathyroid hormone levels across latitude among adults with african ancestry

Objective: To compare levels of 25-hydroxyvitamin D (25OHD) associated with a plateauing of intact parathyroid (iPTH) across latitudes among adults with African ancestry. Methods: This study included approximately 500 adults of African ancestry ages 25 to 45 years living in 4 sites: Chicago, Illinois (41°N), Jamaica (17°N), Ghana (6°N), and South Africa (34°S). Multivariate linear regression models, a nonlinear logistic growth curve model, and piecewise linear models with a single knot were fitted to estimate the 25OHD level associated with a plateauing of iPTH with adjustment for covariates. Goodness of fit was compared using computer intensive permutation tests. Results: Mean age was 34.7 (SD 6.2) years, and 46.5% were male. Within each site, the percentage of participants with an iPTH level ≥65 pg/mL was higher among females versus males and was most frequent among South African females (17.1%) and lowest among Jamaican males (0.6%). Goodness of fit tests supported linear regression as the preferred model for the association between iPTH and 25OHD in the 4 sites with no 25OHD level associated with iPTH plateauing in any site. The slope of the association between 25OHD and iPTH differed by latitude; it was strongest in the U.S. (β =-0.81; 95% confidence interval CI =-1.03,-0.59), and weakest in Jamaica (β =-0.45; 95% CI-0.71,-0.18) with covariate adjustment, but differences in slopes were small. Conclusion: The association between 25OHD and iPTH appears linear among adults with African ancestry regardless of latitude within a range of 25OHD levels between 10 and 60 ng/mL.