Muscle-to-bone And Soft-tissue-to-bone Ratios By Body Mass Index Category

PURPOSE: Obese individuals have more bone injuries despite usually having a higher bone mineral density than healthy weight individuals. This paradox may be explained by imbalances between the amount of soft tissue and bone. A possible way to examine these imbalances is by determining the muscle-to-bone ratio (MBR) and the soft-tissue-to-bone ratio (SBR). Thus, the purpose of this study was to compare total MBR (tMBR) and SBR (tSBR) and appendicular (arms and legs) MBR (aMBR) and SBR (aSBR) in adults with varying body mass indexes (BMI). METHODS: A total of 170 (65 males, 105 females) adults (Age: 28.2 ± 7.5 yrs; BMI: 29.2 ± 5.9 kg/m2) participated in this study. Participants were categorized by BMI values per the Center of Disease Control and Prevention (CDC) cut points: healthy (HW: 18.5-24.9 kg/m2, n = 51), overweight (OW: 25.0-29.9 kg/m2, n = 44), and obese (OB: >30.0 kg/m2, n = 75). Body composition was measured using dual X-ray absorptiometry which provided values of lean mass, fat mass, and bone mineral content (BMC). MBR was obtained by dividing lean mass by BMC. SBR was determined by dividing soft tissue mass (lean mass + fat mass) by BMC. Differences in total and appendicular MBR and SBR between BMI categories were assessed via ANOVA and Tukey HSD. RESULTS: HW had significantly lower tMBR than OB (17.4 ± 1 vs 18.4 ± 1.9, p < 0.05). However, tMBR was not significantly different between OB and OW (17.7 ± 1.9) or between HW and OW. aMBR was significantly different between HW (15.6 ± 1.4) and OB (17.6 ± 2.0, p < 0.001), and between OB and OW (16.3 ± 1.3, p < 0.001), but not between HW and OW. The tSBR in HW (22.9 ± 2.3) was significantly lower than both OB (33.3 ± 5.0, p < 0.001) and OW (28.4 ± 3.1, p < 0.001). In addition, the tSBR was significantly (p < 0.001) lower in OW than OB. The aSBR in HW (20.9 ± 3.0) was also significantly lower than both OB (31.0 ± 5.2, p < 0.001) and OW (26.0 ± 3.2, p < 0.001). Similarly, the aSBR was significantly (p < 0.001) lower in OW than OB. CONCLUSION: These results indicate that the OB group had a higher total and appendicular muscle mass per unit of bone, as well as a higher soft tissue mass per unit of bone when compared with the HW group. This additional weight of soft tissue may increase the amount of stress on the bone, which may translate to an increased risk for bone injury.