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.

Objective: Since specific bioelectrical reference values for Italian adults are lacking, this study aims to define specific values and test their suitability in pathological cases and athletes. Methods: A sample of 1049 Italian individuals (441 men, 608 women) aged 30–65 years was considered. Competitive athletes (bodybuilding, streetlifting and tennis) were identified within the general sample, and an independent group of individuals with obesity or anorexia nervosa was analyzed for comparison. Anthropometric (weight, kg; stature, mid-upper arm, waist and calf circumferences; cm) and bioelectrical (resistance and reactance, at 50 kHz) variables were taken. Resistivity, (Rsp, Ωcm), reactivity (Xcsp, Ωcm), impedivity (Zsp, Ωcm) and phase angle (PhA, °) were calculated. Two-way ANOVA and Hotelling’s T² test were applied to assess group differences. These data were then pooled with existing datasets to create a comprehensive reference for individuals aged 18 to 100 years. Results: The specific bioelectrical variables were: Rsp = 352.3 ± 55.5, Xcsp = 41.8 ± 9.1, PhA = 6.8 ± 1.0, r (Rsp, Xcsp) = 0.67 (men); Rsp = 384.9 ± 71.2, Xcsp = 40.7 ± 9.4, PhA = 6.1 ± 1.0, r (Rsp, Xcsp) = 0.72 (women). Men showed higher PhA values (p < 0.001), reflecting higher muscle mass and quality, and shorter vectors (p < 0.001), indicative of lower relative fat mass (FM%), than women. Advancing age was associated with lower PhA and longer vectors (p < 0.001). Bioelectrical vectors of individuals with obesity or anorexia nervosa were outside the 95% variability, indicating abnormal values of FM%, whereas those of athletes fell within the lower left quadrant. Conclusions: The specific tolerance ellipses for the Italian adult population fill a gap in the existing literature, providing essential new tools for evaluating body composition in clinical and sports settings, and for comparative analyses.

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.

The Bioelectrical Impedance Analysis International Database (BIAID) is an initiative aimed at pooling, standardizing and harmonizing bioelectrical impedance analysis (BIA) data across populations. This report provides a synthesis of the data currently available in the BIAID and an overview of the actions undertaken to date. The repository, which is based on raw bioelectrical values as minimal required data, has expanded over the past two years to include 321,680 individuals across the lifespan, encompassing both healthy and clinical populations and representing all WHO regions. A methodological questionnaire was also implemented to assess data quality and measurement protocols across contributing centers. To date, several research proposals using BIAID data have been approved, reflecting increasing scientific interest and use of the database. Continued data sharing and collaboration are essential to enhance the robustness, representativeness and global applicability of the database, promoting high-quality and reproducible research in nutritional science.