External validation of a digital anthropometry application for estimating girths and body composition in physically active individuals: The Cybermetron Project – Phase 1

Introduction: Body composition assessment is essential for evaluating nutritional status, which influences physical performance and overall health. In recent years, digital anthropometry (DA) has emerged as a user-friendly tool that provides rapid and non-invasive estimates of anthropometric variables and body composition without the need for expensive equipment. However, external validation is still needed. Methods: This cross-sectional study, conducted as part of phase 1 of the CyberMetron Project, aimed to externally validate a DA smartphone application by comparing its girth estimates against ISAK-standardized manual techniques and its body composition estimates against dual-energy X-ray absorptiometry (DXA) in physically active individuals. Robust statistics, correlation and agreement analyses were performed. Results: A total of 141 adults (69F, 72M; 28.1 [5.9] years; 67.3 [11.4] kg; 166 [8.7] cm) participated in this study. The DA application showed low-to-moderate correlation (r <0.5) and low concordance (Linn’s coefficient, CCC<0.3) against girths measured under ISAK standards as well as low concordance and significant biases in body composition assessed by DXA. In women, the application consistently overestimated central girths but underestimated thigh girths and body fat percentage (26.4% vs. DXA: 31.1%). In men, it underestimated arm and thorax girths while showing low bias for waist and thighs, with body fat overestimation (27.1% vs. DXA: 22.8%). Conclusions: Based on our findings, the methodological rigor of ISAK-standardized manual anthropometry is particularly relevant for initial diagnostics and precise adjustments in sports nutrition, while future research could explore a hybrid human-machine approach, leveraging the accessibility and reliability of DA for longitudinal monitoring of nutritional status.