Non-Linear Association Between Phase Angle and Body Fat

Phase angle (PhA) is a widely used bioimpedance index associated with cell mass, membrane integrity, skeletal muscle mass and quality, and body fluid distribution. However, its relationship with body fat is unclear, with inconsistent findings in the literature. This study aims to investigate the relationship between PhA and body fat. 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, 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 always 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 inflection points at lower fat levels than women. Although significant, the explained variance of the relationships was lower than 10%. In conclusion, this study demonstrates a non-linear relationship between PhA and body fat. The observed decline in PhA at higher fat levels suggests the onset of clinical complications in people with obesity.