Phase angle is a predictor of basal metabolic rate in female patients with anorexia nervosa

The aim of the study was to evaluate the relationship between basal metabolic rate (BMR) and bioelectrical impedance analysis (BIA) in undernourished female patients with anorexia nervosa. Participants were 86 female patients with anorexia nervosa (age 20.8 ± 4.7 years; weight 39.3 ± 5.2 kg; body mass index 15.4 ± 1.6 kg m⁻²). BMR was measured by indirect calorimetry and single-frequency BIA was determined at 50 kHz on the whole body. The BIA variables considered were resistance, reactance, phase angle and the bioimpedance index (height²/resistance). Fat-free mass was calculated from subcutaneous skin fold thickness. In the study group BMR was 3782 ± 661 kJ d⁻¹ while bioimpedance index varied between 27.6 and 49.9 cm² Ω⁻¹ and phase angle between 2.54° and 6.49°. BMR was significantly correlated with weight, height, body mass index and fat-free mass, and, among BIA variables, with reactance and phase angle. Multiple regression analysis indicated that phase angle was a predictor of BMR not only when solely BIA variables were considered, but also in combination with either weight and age or fat-free mass. In conclusion, phase angle emerged as a strong predictor of BMR in female patients with anorexia nervosa. Nevertheless, further studies are necessary to confirm this finding in other forms of protein energy malnutrition and justify the inclusion of BIA variables in the equations used to predict BMR in the clinical setting.