Use of stable calcium isotopes (42Ca & 44Ca) in evaluation of calcium absorption in Beijing adolescents with low vitamin D status.

Female adolescent populations with low calcium intakes and sub-optimal vitamin D status are found in northern China. Whether these individuals are able to adapt by enhancing calcium absorption and reducing calcium excretion for maximizing calcium retention is unknown. This study examined the association between true-fractional-calcium-absorption (TFCA), plasma 25-hydroxy-vitamin-D, and urinary calcium excretion among adolescents in Northern China. Twelve healthy girls 9 to 17 years old were recruited from Beijing during a winter. Calcium intake, anthropometry, pubertal status, plasma 25-hydroxyvitamin-Ds (25-OHD), serum calcium, and urinary calcium were determined. TFCA was evaluated by dual stable calcium isotopes. The mean +/- SD calcium intake, 24-hour urinary calcium excretion, plasma 25-OHD and TFCA were 591 +/- 164 mg/day, 79.9 +/- 49.6 mg/day, 30.5 +/- 9.8 nmol/L, and 60.4 +/- 14.4%, respectively. TFCA was inversely correlated with 25-OHD (r = -0.73, p = .008). Urinary calcium was correlated with the onset of menarche (r = 0.63, p = .027). Post-menarcheal girls had a higher urinary calcium output than premenarcheal girls (p = .03). Adolescents from north China with sub-optimal vitamin-D status are able to adapt by enhancing TFCA and reducing urinary calcium excretion to retain calcium for bone development. TFCA was inversely correlated with plasma 25-OHD. Whether a higher efficiency of calcium absorption is sustainable if the shortfall of vitamin D persisting remains to be studied.