Decreased urinary calcium loss and lower bone turnover in young oral contraceptive users

The effect of ethinyl estradiol-containing oral contraceptives (OCs) on bone health is still not completely understood. This study was therefore performed to investigate the effect of OC use on biochemical parameters of calcium (Ca) and bone metabolism in young women. Twelve OC users ([OC+ group] age, 24.8 +/- 0.6 years) and 19 eumenorrheic nonusers ([OC-group] age, 25.5 +/- 0.8 years) were studied. Three individual urine samples (fasting 2-hour and 24-hour specimen) and 3 blood samples collected at intervals of 28 days were pooled for data evaluation. Energy, nutrient intake (7-day food record), the body mass index, and serum 25-hydroxyvitamin D levels of the 2 groups were comparable. Serum levels of estradiol (E2) and sex hormone-binding globulin (SHBG) in the 2 groups mirrored the use and nonuse of OCs. Fasting 2-hour renal Ca excretion was markedly lower in the OC+ group compared with the OC- group (80.2 +/- 14.7 v 185.1 +/- 18.8 micromol/mmol creatinine [Cr], P < .001), indicating a decrease in the urinary loss of endogenous, bone-derived Ca. Moreover, 24-hour renal Ca excretion was reduced in the OC+ group (3.65 +/- 1.46 v5.03 +/- 1.90 mmol/d, respectively, P < .01). In addition, serum Ca levels were lower in the OC+ group versus the OC- group (2.19 +/- 0.07 v 2.29 +/- 0.02 mmol/L; P < .05). The OC+ group had lower serum levels of carboxy-terminal propeptide of type I procollagen ([PICP] a biomarker of bone formation) compared with the OC- group (78.3 +/- 3.6 ng/mL in the OC+ group v 96.9 +/- 5.5 ng/mL in the OC- group, P < .001) and lower renal hydroxyproline (OHPr) excretion (a biomarker of bone resorption, 8.3 +/- 1.0 v 11.3 +/- 1.0 micromol/mmol, Cr, P < .001). In summary, OCs reduce urinary Ca loss and slow bone turnover in young women. The results may help to explain the OC effect on bone mass in young women.