Relevance of hydration state of the fat free mass in estimating fat mass by body impedance analysis

Body impedance analysis (BIA) is widely used to assess body composition, however, accurate estimates of fat mass (FM) are sometimes rendered impossible due to over- or dehydration states. In fact, BIA is accurate in total body water (TBW) estimates, and TBW can be converted into fat free mass (FFM) assuming 73% hydration; but, if there is an alteration in the hydration state, such a fixed value does not fit with the real content of fluids in the FFM compartment, thus causing wrong estimates of FFM and in turn of FM. For this reason, phase-sensitive bioelectric analyzers operating at a single frequency or at multiple frequencies have recently been under investigation in order to produce an equation that is also able to estimate the volume of extracellular water (ECW). Some authors, rather than trying to find the best equation or the best instrument approach, have tested the direct use of resistance, reactance and phase angle values derived from 50 kHz impedance measurements, assuming that normal bioelectrical values are associated with normal hydration states. Using a large series of data we have found strong correlations between BIA-derived FM and BMI, and our data show that screening the subjects with normal hydration states using the direct bioelectrical parameters (BIAGRAM) improves the predictivity of standard equations, since it avoids trying to estimate FM in overhydrated subjects.