Moderately Carbohydrate-restricted Diet to Treat NAFLD in Adolescents

This will be a 6-month randomized clinical trial with two arms: moderately carbohydrate-restricted diet and a fat-restricted control diet. This 6-month study will have 2 phases: a 12-week controlled feeding phase and a 12-week "free living" phase. During the controlled feeding phase, all food will be provided to the families of the participants for the entirety of the 12 weeks. Participants (n=80) will have been diagnosed with NAFLD based on the presence of current evidence of active disease, which will be determined by the ongoing presence of hepatic steatosis estimated by diffusely echogenic liver via ultrasound suggestive of fatty liver and a serum alanine aminotransferase (ALT) level of 45 U/L or greater. All participants will be children and adolescents age 10-17 yrs.; will have an HbA1c \85th percentile). It is anticipated that most participants will be sedentary. The investigators will inquire as to routine physical activity at screening. All participants will be asked to maintain their usual level of physical activity throughout the study. Physical activity will be monitored via a smart watch provided to each participant at the beginning of the study, and participants will be queried weekly by the study dietitian regarding changes in physical activity. Participants who use oral contraceptives will be asked to maintain consistent use of these preparations throughout the study. Hormone use will be examined as a potential covariate in statistical analyses.