Influence of Soft Tissue (Fat and Fat-Free Mass) on Ultrasound Bone Velocity

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Abstract
The authors determine the relative effect of soft-tissue compartments, body fat (percent[% Bfat] and weight [Bfat kg]) and fat-free mass (FFM kg), on measurements of ultrasound bone velocity (UBV m/second). The authors measured UBV in proximal phalanxes and body fat and fat-free mass by near infrared interactance in 40 healthy premenopausal women (mean age ± standard deviation 28.2 ± 3.8 years). Correlation study (Fisher's r to z) showed that UBV correlated negatively with %Bfat (r = -0.61, P < 0.0001), Bfat kg (r = -0.56, P = 0.0001) and marginally with body weight (r = -0.33, P = 0.0403), but did not correlate with FFM kg or H2O L (both r = -0.08, P not significant). When the correlation test was adjusted for weight and age (partial correlation), the negative correlation between UBV and %Bfat persisted (r = -0.54, P < 0.0005; and r = -0.63, P < 0.0001, respectively) and the correlation with FFM kg, adjusted for weight, became positive and significant(r = 0.55, P < 0.0005). These results, to our knowledge, are the first to be obtained by in vivo evaluation of UBV in relation to body fat and fat-free mass. Body fat, but not fat-free mass, was the main factor affecting UBV. This confirms the deficiency of UBV measurements, considering that obesity is a protective factor for bone mass.