One of the most visited Dexalytics blog posts is titled “WHAT SHOULD I TRACK- BMC OR BMD?” In that post, I talked about bone mineral content (BMC), bone mineral density (BMD), the difference between these two measures, and how they were determined. In addition, I also discussed the difference between BMD T-scores and Z-scores. Given the popularity of that post, I thought we would revisit BMD and BMC, but this time examine their association with bone stress injuries.
For starters, BMD is the basis of the World Health Organization’s definition of osteoporosis. A T-score lower than -2.5, indicates an individual’s BMD is 25% lower than the average 30-year-old (WHO, 1994). A number of studies have demonstrated that a low BMD is predictive of osteoporotic fractures (Marshall et al., 1996; Curtis et al., 2016). However, a majority of these studies have examined older adults or have only studied women. So the question of whether BMD can be predictive of bone stress injuries in males or athletes remains relatively unclear.
BMD is highly influenced by physical activity (Herbert et al., 2019). In fact, it has been estimated that physical activity accounts for 30% of the variability in BMD (Valdimarsson et al., 1999). The fact that athletes tend to have higher BMD levels than their non-athlete counterparts is further proof of the importance of physical activity on BMD (Herbert et al., 2019). Unfortunately, there are some exceptions to athletes having greater BMD. It has been reported that endurance athletes often have low BMD possibly resulting in lower-extremity stress fractures (Polluck et al., 2010; Loucks, 2007). One reason for this is excessive physical activity (i.e., overtraining) can actually have a negative effect on bone health (Kuiper & Keizer 1988). Low BMD and the higher risk of fracture observed in endurance runners can be attributed to site-specific loading that happens with running as well as the low energy availability often seen in this population (Loucks, 2007).
It should be noted that there is a large genetic component to BMD as well. It has been suggested that the heritability of BMD is approximately 50–85% depending upon anatomical location (Ralston and Uitterlinden, 2010). In addition, a woman’s menstrual status also plays a significant role on BMD and bone stress injuries. Ackerman and colleagues (Ackerman et al., 2015) reported that amenorrheic athletes have a lower BMD and a higher stress fracture risk than eumenorrheic athletes. Although a lot of focus on low BMD and bone stress injuries have focused on women, recent studies in men have shown similar findings regarding low BMD and bone stress injuries. Tenforde et al. (Tenforde et al., 2018) studied 28 athletes who had a history of lower-extremity bone stress injuries and reported that 43% of these athletes that sustained body stress injuries in trabecular-rich regions of the body (i.e., sacrum, pelvis, femoral neck and calcaneus) met the criteria for low BMD (BMD Z-score <-1.0). , Kettunen et al. (Kettunen et al., 2009) reported that former male athletes who maintained their physical activity had substantially higher BMD than a population-based control group, and osteoporotic hip fractures among the former athletes were postponed on average by 6 years.
What about BMC and bone stress injuries? Most of the research to date regarding predicting bone stress injuries and fractures has been done examining only BMD. However, a recent study by Curtis et al. (Curtis et al., 2016) examined the predictive value of both BMD and BMC for incident fractures. Curtis et al. (Curtis et al., 2016) reported that both BMC and BMD were able to predict incident fracture with similar precision. It should be pointed out that this study was done in a female-only population so whether these findings can be applied to males or athletes is still unknown.
It appears that both low BMD and BMC are predictive of body stress injuries. Therefore, clinicians, coaches and athletes may want to consider obtaining DXA on both male and female runners as well as other athletes who sustain body stress injuries. In addition, low BMD and BMC levels may point to the need to optimize nutritional factors such as appropriate energy intake and adequate calcium and vitamin D (Tenforde & Fredericson, 2011; Tenforde et al., 2016).
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Curtis EM, Harvey NC, D’Angelo S, Cooper CS, Ward KA, Taylor P, Pearson G, Cooper C. (2016) Bone mineral content and areal density, but not bone area, predict incident fracture risk: a comparative study in UK prospective cohort. [published online December, 2016]. Arch Osteoporosis 11(1):39, December 2016.
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About the Author
Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.