Ultrasound-Based Assessment of Subcutaneous Adipose Tissue Changes During a 7-Day Ultramarathon: Association with Anthropometric Indices, Not Body Mass

Accurately tracking body-composition changes in endurance field settings remains methodologically challenging. This study aimed to evaluate whether changes in subcutaneous adipose tissue (SAT) across a 7-day ultramarathon are better reflected by anthropometric indices than by body mass (BM) alone. : Twenty ultrarunners were assessed using both anthropometric indices and ultrasound measurements of SAT thickness, applying a novel method that distinguishes layers including (D) versus excluding (D) embedded fibrous structures. Measurements were obtained before the race and after Stages 4 and 7. Indices included body mass index (BMI), mass index (MI), and waist-to-height ratio (WHtR). : Total SAT thickness decreased significantly for both D ( = 0.001) and D ( < 0.001). BM, BMI, MI, and WHtR also declined significantly post-race ( < 0.001). SAT reduction was most pronounced at the abdominal and thigh sites. Additionally, ultrarunners with lower D values exhibited lower fat at the abdomen and distal triceps. BMI was significantly related to D at the upper and lower abdomen and erector spinae; MI was significantly associated with D at the upper and lower abdomen; and WHtR correlated with both D and D at abdominal and erector spinae sites. BM showed no significant association with any SAT parameter. : Ultrasound-derived SAT thickness, in combination with BMI, MI, and WHtR, offers a field-feasible approach to evaluate body-composition change during multistage ultramarathons. In contrast, BM alone does not reliably reflect SAT distribution or loss.