Teenage girls taking low-dose oral contraceptives showed abnormally low levels of bone growth, and sometimes even lost density, compared with teens who took birth control pills with a higher dose of estrogen, Czech researchers found.
In a randomized, crossover trial, bone mineral density (BMD) failed to increase in girls 15 to 19 years old who took pills with a low dose (15 micrograms) of ethinyl estradiol for nine months, while bone density increased normally in participants taking pills with a high dose (30 micrograms) of ethinyl estradiol, according to Dr. Jan Stepan of Charles University in Prague.
Ethinyl estradiol is a form of the female hormone estrogen that's commonly used in birth control pills.
In a presentation here at the American Society for Bone and Mineral Research, Stepan said that based on these findings, girls who need oral contraceptives "could be counseled toward preparations with higher estrogen levels."
The study involved 82 girls in their middle to late teens -- a period during which they should be accumulating bone density.
Twenty-eight girls were given no medications and served as controls. The other 54 were randomly assigned to nine months of treatment with oral contraceptives containing either 15 or 30 micrograms of ethinyl estradiol.
After the initial treatment period, those in the treatment group were switched to the other contraceptive dosage for another nine months.
Lumbar spine BMD and whole body bone mineral content were measured at the outset and after each nine-month period. In the control participants, spinal BMD increased by 1 percent during each treatment period, and whole body bone mineral content rose 2 percent in each period.
Those initially assigned to the 30 micrograms ethinyl estradiol dosage also showed a 1 percent increase in spinal BMD, but it returned to baseline levels when they switched to the 15 micrograms dosage.
Participants first receiving the 15 micrograms dose showed virtually no increase in spinal BMD. After switching to the higher dosage, spinal BMD accumulation paralleled that of control participants.
With the 15 micrograms formulation, "physiological BMD increase in lumbar spine was interrupted and, after nine months, its users had 2 percent lower lumbar spine BMD compared with controls," the author summarized.
Whole body bone mineral content did not accumulate as quickly in the girls taking the oral contraceptives as in the control group.
Those starting at the low-dose birth control and switching to the high dose ended up with about half the level of increase seen in the control group (1.7 percent versus 3.7 percent), while girls taking the drugs in the reverse order had an even smaller increase in whole body bone mineral content (0.4 percent)
During the first nine months of the study, serum PINP -- a marker of bone turnover -- declined in all three groups and somewhat faster in those taking oral contraceptives. However, those switching from the high- to low-dose product then showed a 30 percent increase in PINP levels.
The final treatment period saw no change in mean PINP among controls or those switching from the low- to high-dose ethinyl estradiol.
Stepan explained that the lower dose of ethinyl estradiol suppresses the body's normal estrogen release without fully replacing it. Those on the higher dose wound up with a more normal level of estrogens, he said.