The role of vitamin D metabolism in regulating bone turnover in adolescents with perinatally-acquired HIV in southern Africa: a cross-sectional study in Zimbabwe and Zambia

Vitamin D dysregulation can occur in people living with HIV, disrupting calcium homeostasis and bone turnover. We aimed to investigate the potential mechanisms by which vitamin D regulates bone turnover in adolescents living with perinatally-acquired HIV (ALWH) in southern Africa. A pre-planned secondary analysis was performed of baseline data from the VITALITY trial [PACTR20200989766029] which enrolled ALWH (11-19 years) taking antiretroviral therapy for ≥6 months, and recorded socio-demographic, clinical and dietary data. After over-night fasting, vitamin D metabolites [25(OH)D, 1,25(OH)2D, 24,25(OH)2D], intact parathyroid hormone (PTH) and bone turnover markers (BTMs) [CTX and P1NP] were measured. Tandem Mass Spectrometry measured vitamin D metabolites, whilst intact PTH and BTMs were analysed by electrochemiluminescence immunoassay. Stratified by 25(OH)D [<75 vs ≥75 nmol/L], associations between standardized concentrations (β = standard deviations) of vitamin D metabolites, intact PTH and BTMs were assessed using structural equations modelling (SEM) adjusted for age, sex and country (Zimbabwe/Zambia). Among the 842 ALWH enrolled, the median dietary calcium intake was 100 mg [IQR:55-145]. The SEM showed PTH was positively associated [β 0.21, 95%CI: 0.1,0.32] with 1,25(OH)2D, only when 25(OH)D was <75 nmol/L vs ≥75 nmol/L [β 0.23, 95%CI: -0.13,0.59], with evidence of an interaction [β -0.11, 95%CI: -0.20,-0.02]. A positive relationship between 25(OH)D and 24,25(OH)2D was seen irrespective of 25(OH)D concentration. 24,25(OH)2D was inversely related to BTMs, particularly when 25(OH)D was <75 nmol/L [CTX: β -0.15, 95%CI: -0.24,-0.06, and P1NP: β -0.14, 95%CI: -0.22,-0.06]. There was interaction between dietary calcium and 25(OH)D on PTH [β -0.15, 95% CI: -0.22,-0.07] suggesting an interaction between low 25(OH)D and low dietary calcium which increases PTH. In conclusion, associations between 25(OH)D, PTH, 1,25(OH)2D and BTMs in ALWH appear dependent upon 25(OH)D concentrations <75 nmol/L and calcium intake. A novel, potentially causal pathway between 25(OH)D, 24,25(OH)2D and BTMs was seen. Findings enhance understanding of vitamin D metabolism in people living with HIV.