Normative Reference Values and Cut-Off Points for Nutritional Ultrasound: The VALORES Study.

BACKGROUND: Nutritional ultrasound (NU) enables direct assessment of skeletal muscle and adipose tissue, yet population-based reference values and cut-offs remain scarce. We aimed to establish age- and sex-specific normative values for rectus femoris (RF) and abdominal adipose tissue NU measurements in apparently healthy adults. METHODS: Multicentre cross-sectional study of 1789 healthy blood donors (842 women, 947 men), aged 18-80 years (mean 41.5 ± 14.9). Standardised NU assessed RF cross-sectional area (CSA), circumference, transverse (X) and anteroposterior (Y) axes, and abdominal and thigh adipose tissue. Anthropometry, bioelectrical impedance analysis (BIA) and handgrip strength (HGS) were obtained. Age- and sex-specific percentiles were generated across predefined age groups (18-30, 31-45, 46-60 and > 60 years). T-scores for RF parameters were derived from a young reference subgroup aged 18-45 years (n = 1001). Associations with sex, age, body mass index and physical activity were analysed. RESULTS: All NU parameters differed significantly by sex (all p < 0.001). Median RF-CSA was 4.60 cm2 (IQR 3.83-5.58) in women and 6.77 cm2 (5.55-7.91) in men; median RF-Y axis at rest was 1.56 cm (1.33-1.80) and 1.91 cm (1.65-2.17), and during contraction 1.97 cm (1.71-2.24) and 2.47 cm (2.15-2.78), respectively. Median total abdominal subcutaneous adipose tissue (T-SAT) was 1.77 cm (1.16-2.39) in women and 1.65 cm (1.13-2.22) in men. RF measurements correlated moderately-strongly with BIA-derived muscle mass (r = 0.45-0.65) and HGS (r = 0.49-0.58). T-SAT correlated with BIA fat mass and waist circumference (r = 0.45-0.65), whereas visceral adipose tissue showed weaker associations (r ≈ 0.33). RF-CSA and RF-Y axis increased across physical-activity categories (p < 0.05). Absolute RF values did not differ by BMI; however, when indexed to body weight, participants with BMI > 25 kg/m2 had lower RF-CSA and RF-Y values (all p < 0.001). RF parameters followed a normal distribution, whereas adipose tissue measures did not. In the young reference group (18-45 years), mean ± SD RF-Y axis was 1.68 ± 0.33 cm in women and 2.05 ± 0.37 cm in men. When applying T-score criteria, the -2 standard deviation cut-off identified 1.4%-11.7% of adults aged ≥45 years as having low muscle mass. CONCLUSIONS: This study provides the first age- and sex-specific population-based reference values for nutritional ultrasound measurements in an apparently healthy adult Spanish population. These findings support the validity, clinical applicability, and standardisation of nutritional ultrasound for the assessment of muscle mass, with potential utility in the diagnosis of sarcopenia and malnutrition.