This PhD work aims to address the rising rates of physical inactivity and obesity-related liver diseases by exploring the relationships between sedentary behavior, physical activity, dietary intake, fitness, liver insulin sensitivity, liver fat content, and liver enzyme levels in 44 middle-aged, sedentary adults with metabolic syndrome. The work is structured around four interconnected studies, all framed within a cohesive research design. It has two primary objectives: to investigate these relationships and to assess the effects of reducing sedentary behavior on liver health over a six-month randomized controlled trial.

Sedentary behavior and physical activity were measured using hip-worn accelerometers, liver insulin sensitivity by positron emission tomography during a hyperinsulinemic-euglycemic clamp, and liver fat content by magnetic resonance spectroscopy. Additional measurements included fasting blood samples for liver enzyme levels, maximal bicycle ergometry for fitness assessment, air displacement plethysmography for body composition analysis, and food diaries for dietary tracking.

The results indicated that sedentary behavior, physical activity, and fitness levels were not significantly associated with liver fat or liver enzyme levels. Instead, overall body adiposity demonstrated a strong correlation, highlighting the importance of maintaining a healthy body composition. Dietary factors also played a significant role; higher protein intake, reduced carbohydrate and sugar consumption, and increased unsaturated fatty acids and fiber were beneficially associated with liver insulin sensitivity.

Although increasing daily standing time enhanced liver insulin sensitivity, the intervention aimed at reducing sedentary behavior by one hour daily did not result in notable changes in liver health markers. Suggesting that a further reduction in sedentary time may be needed and/or more extensive or intense physical activity may be necessary. However, we demonstrated that successfully reducing sedentary behavior reduces liver enzyme levels, particularly alanine aminotransferase. This study offers new insights into how reducing sedentary behavior impacts liver health, contributing to future intervention strategies for metabolic syndrome.