Oval contraceptive use and bone mineral density in premenopausal women: cross-sectional, population-based data from the Canadian Multicentre Osteoporosis Study

Background: Positive and negative effects on bone mineral density (BMD) hav e been described as a result of the premenopausal use of oral contraceptive s (OCs); increased fracture rates have also been reported. This study asses sed the relation between OC use and BMD in a population-based, 9-centre, na tional sample of women aged 25-45 years. Methods: Premenopausal women who had been enrolled in the Canadian MultiCen tre Osteoporosis Study were classified as having ever been OC users (greate r than or equal to3 months) or as having never been OC users (0 to < 3 mont hs). Data were obtained through extensive questionnaires and measuring of p articipants' weight, height and the BMD of lumbar vertebrae and the proxima l femur. Results: Of the sample of 524 women, whose mean age was 36.3 (standard devi ation [SD] 5.9) years, 454 had used OCs; their mean age when they started u sing OCs was 19.8 (SD 3.5) years and the mean duration of use was 6.8 (SD 4 .8) years. Women who had ever and those who had never used OCs showed no di fferences in age, age at menarche, parity, current calcium intake, exercise , body mass index (BMI), education, past irregular cycles or amenorrhea. OC users reported more alcohol and cigarette use and more use of medications to create regular cycles. Mean BMD values (adjusted for age, BMI and height ) were 0.02-0.04 g/cm(2) (that is, 2.3%-3.7%) lower in OC users, and were s ignificantly lower in the spine and trochanter. The BMD of the spine in OC users was 1.03 (SD 0.12) g/cm(2) versus 1.07 (SD 0.12) g/cm(2) (95% confide nce interval [Cl] of difference -0.07 to -0.001) in those who had never use d OCs. BMD was, neither related to the duration of OC use nor to gynecologi cal age at first use. Current and past users had similar BMD values. Interpretation: National, population-based data show lower BMD values for t he trochanter and spine in premenopausal women who have used OCs compared w ith those who have never used OCs.