A cross-sectional study of smoking and bone mineral density in premenopausal parous women: effect of body mass index, breastfeeding, and sports participation.

The objective of this cross-sectional study was to describe the relationship between cigarette smoking, effect modifiers, and bone density in premenopausal parous women. We studied a sample of 276 women (mean age 33 years) from Southern Tasmania. The study factors were cigarette smoking, body mass index (BMI), sports participation, and breastfeeding history. Bone mineral density was measured utilizing an Hologic QDR 2000 densitometer and converted to Z scores using the group mean and variance. There were 118 current smokers and 158 nonsmokers. Smokers had lower bone mass at all sites (femoral neck, -0.32 SD, 95% confidence interval [CI] -0.60 to -0. 04; lumbar spine, -0.49 SD, 95% CI -0.76 to -0.22; total body, -0.40 SD, 95% CI -0.66 to -0.14). Stratifying by BMI revealed that this association was only present, but greater in magnitude, for those with a BMI <25 kg/m2. Smokers who had breastfed at least one child had an additional deficit in bone mass (femoral neck, -0.48 SD, 95% CI -0.89 to -0.07; lumbar spine, -0.39 SD, 95% CI -0.80 to 0.02; total body, -0.37 SD, 95% CI -0.77 to 0.06) while smokers who took part in competitive sport had significant increments in bone mass (femoral neck, 0.74 SD, 95% CI 0.31 to 1.17; lumbar spine, 0.48 SD, 95% CI 0.03 to 0.93; total body, 0.42 SD, 95% CI 0.00 to 0.84). Neither of these two associations were present in nonsmokers. In conclusion, current smoking was associated with substantial deficits in bone mass in our sample of women, particularly those with a BMI <25 kg/m2. In addition, smoking may prevent the usual postweaning recovery phase of bone after breastfeeding while sports participation may offset the negative effect of smoking on bone mass. These observations need to be confirmed in longitudinal studies but they imply that past studies of smoking in this age group may have missed important associations as they did not consider possible effect modifiers.