Predictors of 25(OH)D half-life and plasma 25(OH)D concentration in The Gambia and the UK

Predictors of 25(OH)D-3 half-life were factors associated with vitamin D metabolism, but were different between people in The Gambia and the UK. Country was the strongest predictor of plasma 25(OH)D concentration, probably as a marker of UVB exposure. 25(OH)D-3 half-life may be applied as a tool to investigate vitamin D expenditure. Introduction The aim of this study was to investigate predictors of 25(OH)D-3 half-life and plasma 25(OH)D concentration. Methods Plasma half-life of an oral tracer dose of deuterated-25(OH)D-3 was measured in healthy men aged 24-39 years, resident in The Gambia, West Africa (n = 18) and in the UK during the winter (n = 18), countries that differ in calcium intake and vitamin D status. Plasma and urinary markers of vitamin D, calcium, phosphate and bone metabolism, nutrient intakes and anthropometry were measured. Results Normally distributed data are presented as mean (SD) and non-normal data as geometric mean (95 % CI). Gambian compared to UK men had higher plasma concentrations of 25(OH)D (69 (13) vs. 29 (11) nmol/L; P < 0.0001); 1,25(OH)(2)D (181 (165, 197) vs. 120 (109, 132) pmol/L; P < 0.01); and parathyroid hormone (PTH) (50 (42, 60) vs. 33 (27, 39); P < 0.0001). There was no difference in 25(OH)D-3 half-life (14.7 (3.5) days vs. 15.6 (2.5) days) between countries (P = 0.2). In multivariate analyses, 25(OH)D, 1,25(OH)(2)D, vitamin D binding protein and albumin-adjusted calcium (Ca-alb) explained 79 % of variance in 25(OH)D-3 half-life in Gambians, but no significant predictors were found in UK participants. For the countries combined, Ca-alb, PTH and plasma phosphate explained 39 % of half-life variability. 1,25(OH)(2)D, weight, PTH and country explained 81 % of variability in 25(OH)D concentration; however, country alone explained 74 %. Conclusion Factors known to affect 25(OH)D metabolism predict 25(OH)D-3 half-life, but these differed between countries. Country predicted 25(OH)D, probably as a proxy measure for UVB exposure and vitamin D supply. This study supports the use of 25(OH)D half-life to investigate vitamin D metabolism.