The association between 25-hydroxyvitamin D and parathyroid hormone in adolescents living with HIV in southern Africa: a cross sectional study.

Low vitamin D associated with high parathyroid hormone (PTH) is commonly reported in the context of HIV infection. We determined the association between total 25-hydroxyvitamin-D [25(OH)D] and PTH in adolescents living with HIV, in Zambia and Zimbabwe. Adolescents (11-19 years) perinatally-infected with HIV and established on antiretroviral therapy (ART) for ≥6 months were recruited into a cross-sectional study. Socio-demographic and clinical characteristics were recorded, anthropometry measured, and fasted serum concentrations of 1,25-dihydroxyvitamin-D (1,25(OH)D), total 25(OH)D and intact PTH measured. The association between total 25(OH)D and PTH was examined using natural cubic spline regression modelling. 842 participants (female: 53.2%) with median age 15.5 (IQR:13.2-17.9) years were enrolled. Median ART duration was 9.8 [IQR:6.3-12.3] years and 165/841 (19.6%) had an HIV viral load >60copies/ml. Stunting (HAZ score<-2) and underweight (WAZ score<-2) were observed in 29.9% and 30.0% respectively. Three quarters (n=639) reported daily calcium intakes <150mg/day. The mean (SD) concentrations of total 25(OH)D and 1,25(OH)D were 66.1(16.5) nmol/L and 210.6(70.4) pmol/L respectively, and median PTH level was 4.3 (IQR:3.3-5.5) pmol/L. There was an inverse non-linear relationship between total 25(OH)D and PTH, 25(OH)D levelling-off at 74.6nmol/L (95%CI: 74.5-75.2). Results were consistent in those taking tenofovir disoproxil fumarate (81.7%) and in those who were virally unsuppressed. In this population with extremely low habitual calcium intakes, the lack of association between 25(OH)D and PTH when 25(OH)D exceeded 75nmol/L, potentially suggests levels of 25(OH)D >75nmol/l may need to be achieved to improve bone health; investigation is needed in future research studies.