Black and White Female Adolescents Lose Vitamin D Metabolites into Urine

The black American population has a higher prevalence of salt sensitivity compared with the white American population. Dahl salt-sensitive rats, models of salt-induced hypertension, excrete protein-bound vitamin D metabolites into urine, a process that is accelerated during high salt intake. We tested the hypothesis that urinary vitamin D metabolite content and 25-hydroxyvitamin D (25-OHD) binding activity of black female adolescents would be greater than that of white female adolescents. Female adolescents (11–15 years old, 11 black and 10 white) were fed low (1.3g, 56mmol/24hours sodium) and high salt (3.86g, 168mmol/24hours sodium) diets for 3 weeks in a randomized order cross-over study design. White and black adolescents had similar mean urinary vitamin D metabolite content (low salt, black versus white: 50 ± 10 versus 58 ± 17pmol/24hours; high salt, black versus white: 47 ± 7 versus 79 ± 16pmol/24hours). Mean urinary 25-OHD binding activities of the black and white adolescents did not significantly differ. Urinary 25-OHD binding activity of 10/11 black adolescents and 7/10 white adolescents was greater at week 3 of high salt intake than at week 3 of low salt intake (r=0.50, P=0.002, n=17). Plasma 24,25-dihydroxyvitamin D concentrations of the white female adolescents were significantly higher than that of the black female adolescents (P < 0.001). Urinary loss of vitamin D metabolites may be one cause of low vitamin D status, in addition to low dietary intake and reduced skin synthesis.