Beyond “Move More”: Combined Physical Activity and Sedentary Behavior Assessment in Individuals with MASLD from Southern Italy

Background: In Southern Italy, metabolic dysfunction-associated fatty liver disease (MASLD) is rising despite adherence to traditional Mediterranean diets. Accurate assessment of physical activity (PA) and sedentary behavior is critical for effective non-pharmacological management but remains methodologically challenging. Methods: We compared subjective and objective PA measures in 133 adults (mean age 49.0 ± 9.8 years; BMI 35.7 ± 4.9 kg/m2) with moderate-to-severe MASLD. Participants completed the International Physical Activity Questionnaire–Short Form (IPAQ-SF) and wore an ActiGraph GT9X wrist accelerometer for seven days. Results: The IPAQ-SF significantly underestimated moderate PA by 865 min/week (p < 0.001) and reported 33.16 ± 14.78 min/week of vigorous activity not detected by accelerometry. Sedentary time was slightly underestimated (0.45 h/day, p = 0.05), with better overall agreement. Stratified analyses showed significant underestimation of sedentary behavior among women and participants <50 years. Spearman correlations were weak (rho = 0.14 for moderate PA; rho = 0.36 for sedentary behavior). Bland–Altman plots confirmed poor agreement for moderate PA but acceptable limits for sedentary estimates. Conclusions: In high-risk Southern Italian populations with MASLD, reliance on self-reported PA may lead to inaccurate clinical guidance. Integrating objective monitoring with subjective tools is essential to deliver precise, individualized exercise prescriptions beyond generic “move more” recommendations.