Non-Linear Association Between Phase Angle and Body Fat in a Sample of US Adults

Phase angle (PhA) is a widely used bioimpedance index associated with cell mass, membrane integrity, skeletal muscle mass and quality, and body fluid distribution. This study aims to investigate the relationship between PhA and body fat, which is poorly defined in the current literature. The analysis involved 1533 adults of both sexes from the National Health and Nutrition Examination Survey. Anthropometry, bioimpedance analysis, and dual-energy X-ray absorptiometry were used to estimate body mass index (BMI), PhA (at 50 kHz), and body fat, respectively. Non-linear cubic spline regression models were applied to examine the associations between PhA and fat mass (FM), BMI, fat mass index (FMI), and FM%. The relationship between PhA and body fat was non-linear. An initial positive slope was observed with FM, FMI and BMI, followed by a decline, whereas FM% exhibited a flatter initial pattern before decreasing. Men showed more pronounced flection points, at lower fat levels (FM: 23 kg; FMI: 9 kg/m²; BMI: 37 kg/m²; FM%: 25.5%) than women (FM: 34 kg; FMI: 16 kg/m²; BMI: 32 kg/m²; FM%: 42.5%). Although most associations were significant, the explained variance of the relationships was lower than 10%. In conclusion, this study demonstrates a weak and non-linear relationship between PhA and body fat. This suggests that PhA variability is predominantly influenced by factors other than fat, a point that warrants consideration in both clinical and routine practice.