Levels of the two proteins, a hormone-binding protein called SHBG and a marker of bone loss called NTx, dropped by 8 percent and 5 percent, respectively, in the women who quit or cut back on their cigarette intake. Over the same time period, SHBG and NTx levels rose within a control group of women who maintained their smoking habits.
"This may partly explain how smoking contributes to osteoporosis in postmenopausal women," says Cheryl Oncken, M.D., M.P.H., of the University of Connecticut School of Medicine and colleagues.
The study was published in the December issue of the journal Nicotine & Tobacco Research.
Cigarette smoking increases the long-term risk for weak bone and osteoporosis-type fractures, but whether smoking decreases new bone formation or increases bone loss has been something of a mystery.
To pinpoint smoking's exact effect on bone, Oncken and colleagues examined a variety of hormonal factors and known protein markers of bone loss. The factors included estrogen and testosterone, which help protect against bone loss; SHBG, which binds to these hormones and makes them "available" for the body to use; and several protein markers that indicate bone breakdown or formation.
The researchers compared these factors in two groups of menopausal smokers: a group that received three two-hour sessions of smoking reduction therapy and a group of women who continued their smoking habits as they waited to enter similar therapy at the end of six weeks. Both groups contained some women using estrogen replacement therapy.
SHBG and NTx were the only factors to change significantly among women who successfully quit or cut back on their smoking.
The researchers note that the majority of participants in the therapy group didn't achieve complete abstinence throughout the course of the study. They suggest that "differences between the groups may have been greater than what we observed" if members of the group had managed to completely stop, rather than reduce, smoking.
Future research should focus on whether longer periods of reduction or abstinence might produce more significant declines in these or other bone loss proteins, and whether these changes result in stronger bones and less fractures over time, say Oncken and her co-authors.
This work was supported in part by the Claude Pepper Older Americans Independence Center, the University of Connecticut and the National Institutes of Health and the Catherine Weldon Donaghue Foundation.
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Journal
Nicotine & Tobacco Research