Exploring the association between BMI and liver fat in children: a study using ultrasound-derived fat fraction (UDFF)

Childhood obesity is rising worldwide, leading to an increased prevalence of metabolic dysfunction-associated steatotic liver disease, the most common pediatric liver disease. Ultrasound-derived fat fraction (UDFF) is a recently developed technique for quantifying hepatic fat that has been validated in children, providing a rapid, reliable, and non-invasive alternative to conventional imaging.

To evaluate the association between weight status, expressed as body mass index (BMI) z-score, and UDFF in a pediatric population without underlying liver disease.

Abdominal ultrasounds including a UDFF measurement in patients without liver disease were retrospectively evaluated. We calculated the BMI z-score for all patients and classified them as normal weight (zBMI) or overweight (zBMI≥1). UDFF values were compared across zBMI categories, sex and ethnicity (Hispanic/Latino or non-Hispanic/non-Latino), using chi-square tests (P<0.05). Pearson's correlation was used to assess the relationship between continuous UDFF values and zBMI. Logistic regression analyses were performed to assess associations between elevated UDFF (>6%) and adiposity markers (zBMI and ultrasound-measured abdominal wall thickness (AWT)), as well as ethnicity.

Of 223 subjects, 93 (41 males/52 females, mean age of 8.2 years) met the inclusion criteria. In the normal-weight population (n=46), 41 (89.1%) had normal UDFF, and 5 (10.9%) had elevated UDFF. In the overweight group (n =47), 29 (61.7%) had normal UDFF and 18 (38.3%) had elevated UDFF. UDFF values showed a positive correlation with zBMI, and higher zBMI increased the odds of elevated UDFF. In the multivariable model including zBMI, AWT, and ethnicity, ultrasound-measured abdominal wall thickness was the strongest predictor of elevated UDFF.

BMI z-score was positively associated with hepatic fat content and with higher odds of elevated UDFF. When multiple factors were considered together, ultrasound-measured abdominal wall thickness showed the strongest independent association with elevated UDFF supporting the central role of adiposity in pediatric hepatic fat accumulation. UDFF may serve as a valuable complement to routine clinical markers, using zBMI, for early identification of children with hepatic steatosis. Larger prospective studies are needed to define its role in clinical practice.