Background: At least 30% of Americans have fatty liver disease. This means that they store too much fat in the liver. At the moment lifestyle changes are the only way to treat this problem.

Objective: To test how (1) a low-carbohydrate diet and (2) a supplement called nicotinamide riboside (NR) affect how a person s body uses dietary fat.

Eligibility: Men aged 18 to 65 and women aged 18 to 50 who are healthy volunteers with a body mass index (height to weight ratio) of 25 or more. Adults with maturity-onset diabetes of the young type 2 (MODY2) are also needed.

Design: Participants with diabetes will have 1 screening visit and a 9-day clinic stay. Healthy volunteers will have 1 screening visit and 2 clinic stays of 1 to 2 weeks each. During screening, all participants will have a physical exam with blood and urine tests. They will have their heart rhythm measured while riding a gym bike. NR is a pill taken by mouth. Only healthy volunteers will take the NR, for 14 days at a time, during 1 clinic stay. During their other stay, they will take placebos; these are pills that look like the study supplement but contain no NR. During each clinic stay, all participants will eat a normal American diet. Then they will eat a ketogenic low-carb diet for 5 days. Participants will have many tests, including: Sleeping every night and having two 24-hour stays in a special room that measures the gases their body uses and produces. Drinking a high-fat shake, then remaining seated for 5 hours while their blood and breath are monitored. Having a substance injected into the arm and remaining seated for 3 hours while their blood is measured. Wearing monitors to measure their activity levels. Another monitor will measure their blood glucose levels. Having imaging scans. ...