Assessment by bioimpedance of forearm cell mass: a new approach to calibration

Objective: Changes in skeletal muscle mass are involved in several important clinical disorders including sarcopenia and obesity. Unlike body fat, skeletal muscle is difficult to quantify in vivo, particularly without highly specialized equipment. The present study had a two-fold aim: to develop a regional 4 K counter for non-invasively estimating cell mass in the arm, mainly skeletal muscle cell mass, without radiation exposure; and to test the hypothesis that cell mass in the arm is highly correlated with electrical impedance after adjusting for the arm's length. Methods: Forearm cell mass was estimated using a rectangular lead-shielded 4 K counter with 4-Nal crystals; impedance of the arm was measured at multiple frequencies using a segmental bioimpedance analysis (BIA) system. The system's within- and between-day coefficient of variation (CV) for K-40-derived elemental potassium averaged 1.8 +/- 1.3 and 5.8 +/- 1.2%, respectively. The corresponding BIA system's CVs were 1.0 +/- 0.4 and 2.1 +/- 1.0%, respectively. Subjects and results: Participants in the study were 15 healthy adults (eight females, seven males; age 39 +/- 2.8y, BMI 22.9 +/- 4.5 kg/m(2)). The right arm's K (5.2 +/- 1.7g) was highly correlated with length-adjusted impedance (r(2) = 0.81, 0.82, and 0.83 for 5, 50 and 300 kHz, respectively; all P < 0.001); multiple regression analysis showed no additional improvement by adding age or sex to the prediction models. Conclusion: These results demonstrate the feasibility of calibrating BIA-measured electrical properties of the arm against estimates of arm cell mass, mainly of skeletal muscle, obtained by regional 4 K counting. This simple and practical approach should facilitate the development of BIA-based regional cell mass prediction formulas. Sponsorship: National Institutes of Health grants RR00645 and NIDDK 42618.