These novel longitudinal findings support our hypotheses regarding both smoking and depressive symptoms as negatively influencing bone accrual in adolescent girls. We found effects on BMD of the lumbar spine and total hip, which are common sites of osteoporotic fractures later in life, but not for TB BMC, which is an aggregate measure of bone health throughout the skeleton. To our knowledge, our longitudinal findings are the first delineated in the adolescent population. Specifically, higher-frequency smokers entered adolescence with the same total hip and lumbar spine BMD as lower-rate smokers (or non-smokers), yet a higher frequency of smoking was associated with a lower rate of bone accrual across adolescence. For example, at age 19, the clinical relevance of the difference in the lumbar spine BMD of a non-smoker versus a daily smoker approximates the amount an adolescent gains in BMD in 1 year. With continued smoking behavior, one could surmise that lumbar spine BMD could become dramatically lower throughout adulthood. Smoking uptake across time was typical of adolescents in which levels of smoking are generally low and highly variable across 30 day periods, and pack years cannot be ascertained. However, it is concerning that even relatively low levels of smoking have a negative impact on bone accrual. Alternatively, smoking may be a marker for other unmeasured behaviors that negatively impact bone. Further research is needed to confirm findings in girls with higher rates of smoking.
The associations between depressive symptoms and bone were somewhat different. Lower lumbar spine BMD persisted across ages 11–19 for those with higher compared to lower depressive symptoms, and there was no interaction with age. An increase of 1 SD (10 points) in depressive symptoms was associated with lower BMD of the lumbar spine; similar to what a girl might typically gain in BMD in 1 year2
. Importantly, in a nonclinical sample (i.e., not depressed), it is worrisome that effects in bone accrual were evident even at these lower levels of depressive symptoms. The magnitude of effects of diagnosable depression on bone accrual may be even greater.
In addition to its novelty, the attention to a wide range of potential covariates that may have confounded previous adult studies, was another strength of our study. Few studies have accounted for this array of relevant covariates. Importantly, even after accounting for significant covariates, the two key predictors—depressive symptoms and smoking— still played a role in lower bone accrual.
In spite of the strengths of the study, future research is essential and limitations should be addressed. First, further exploration should focus on whether anxiety and alcohol play a role in accrual of bone as reported in animal models and adult human studies. Anxiety is common in pubertal-age girls37
and often is comorbid with and precedes depression; thus, we hypothesized that anxiety might have a similar impact on bone accrual as did depressive symptoms. Our prior cross-sectional findings revealed negative associations with anxiety and bone.22,23
Second, alcohol might not have been significant in this study because of low use, particularly in the younger cohorts, but also because alcohol was not the focus of the study and its measurement was limited. Third, the study may not be generalizable to all adolescent girls; it may only represent the sample at hand. However, characteristics of our sample with respect to BMI, calcium intake, physical activity, serum 25 (OH)D, and bone health are similar to published studies on those measures. Notably, our sample fell below recommended guidelines for calcium intake and physical activity, and findings may not generalize to girls meeting those national standards. Importantly, future studies should expand to other geographical regions and include other races beyond black and white girls.
In sum, adolescence is a crucial period of development that lays the foundation for women’s health across the lifespan. To our knowledge, our study is the first to test and demonstrate that smoking behavior and depressive symptoms in girls have a negative impact on bone accrual in adolescence. For several decades, such studies have been conducted in adults but not in adolescents, in whom nearly 50% of bone is accrued. Importantly, as much bone is accrued in the 2 years surrounding menarche as is lost in the last 4 decades of life.3,38
Thus, adolescence is paramount when considering factors that interfere with optimum accrual. In addition to the reported gender difference in osteoporosis in adults,39
reproductive-age women generally exhibit higher rates of depression than men40
and differing outcomes of substance use. Because depression and substance use often become chronic, their deleterious effects on bone might result in long-term and costly public health problems. Based on our study, it may be premature to advocate DXA screening for BMD in adolescents with depressive symptoms or those who smoke. Our study should be replicated to determine whether greater vigilance in monitoring bone mineral status is necessary in those with smoking behavior and higher depressive symptoms. While adolescent depressive symptoms already signal a red flag for future adult depressive episodes, they also may become a red flag for a future constrained by low bone mass or osteoporosis and higher fracture rates in postmenopausal years.