Palmitate Breath Test to Assess Fatty Acid Oxidation in Non Alcoholic Fatty Liver Disease (NAFLD)

* INCLUSION CRITERIA: A. General Inclusion Criteria for all subjects: 1. Age above 21 years, male or female. 2. Estimated average alcohol consumption \< 30 g/d for men or \< 20 g/d for women in the 6 months prior to enrollment and no binge-drinking behavior. B. Inclusion Criteria for NAFLD: 1. Clinical suspicion of NAFLD, defined by a liver biopsy result consistent with NAFLD or the presence of at least two of the following criteria: 1. Suggestion of liver fat by an imaging study (ultrasound, CT scan, MRI or MR spectroscopy) performed in the 6 months prior to enrollment. 2. Elevated aminotransferase levels (ALT \> 31 U/L for men or \> 19 U/L for women, or AST \> 30 U/L) on at least two occasions in the 6 months preceding enrollment. 3. Presence of the metabolic syndrome, defined according to the modified AHA/NCEP criteria30 as the presence of at least three of: * Abdominal obesity, defined as waist circumference \> 102 cm for men or \> 88 cm for women * Elevated triglycerides (\> 150 mg/dL) or the use of medication to lower triglycerides * Reduced HDL cholesterol (\< 40 mg/DL for men or \< 50 mg/dL for women) * Elevated blood pressure (\> 135/80 mmHg) or use of medication for hypertension * Elevated fasting glucose levels (\> 100 mg/dL) or use of antidiabetic medication For the purpose of inclusion, the presence of overt diabetes mellitus type 2 will be considered equivalent to the presence of the metabolic syndrome, even if the other criteria are absent. For the longitudinal analysis, inclusion criteria: 2. Enrolled in 13-DK-0002 or 3. Non-alcoholic liver disease subjects treated off-protocol with lifestyle intervention or vitamin E treatment. C. Inclusion Criteria for non-NAFLD metabolic syndrome: 1. Evidence of metabolic syndrome as defined in NAFLD criterion 1.c. 2. Normal transaminases (ALT less than or equal to 31 U/L for men or less than or equal to19 U/L for women, or AST less than or equal to 30 U/L) at screening 3. Absence of liver fat by imaging or liver biopsy within 6 months of screening with no (or trace) liver fat D. Inclusion criteria for healthy volunteers 1. No history of known liver disease 2. Not on any regular systemic medications (with the exception of oral contraceptives) 3. BMI less than or equal to 25 kg/m2 4. Non diabetic 5. Normal ALT and fasting glucose EXCLUSION CRITERIA: 1. Disorders leading to CO2 retention or significant respiratory abnormalities such as severe congestive heart failure, severe pulmonary hypertension or severe chronic obstructive lung disease. 2. Disorders interfering with substrate absorption such as gastric bypass surgery, malabsorption disorders, use of orlistat or bile acid sequestrants, or extensive small bowel resection. 3. Pregnancy or lactation 4. Inability to remain seated for the duration of the test. 5. Inability to understand and give informed consent 6. Chronic infection with hepatitis C virus (HCV) or hepatitis B virus (HBV). Patients who were treated successfully for HCV and achieved sustained virological response can be eligible for enrollment \> 18 months after treatment cessation. Patients who are inactive carriers of HBV (HBV DNA \< 1000 copies/mL, HBeAg negative, Anti HDV negative) for at least 12 months prior to enrollment are also eligible. Patients receiving antiviral therapy are ineligible. 7. Concomitant liver disease such as autoimmune hepatitis, primary biliary cirrhosis, primary sclerosing cholangitis, Wilson s disease, alpha-1 antitrypsin deficiency. 8. Presence of definite or probable drug-induced liver injury. In the case of lipid-lowering, anti-hypertensive or anti-diabetic medications that are suspected to cause elevation of aminotransferases, patients will be eligible if treatment is associated with stable enzyme levels for at least 6 months and inclusion criteria 2a. and 2c. are both present. 9. Treatment with medications known to cause fatty liver disease such as atypical neuroleptics, tetracycline, methotrexate or tamoxifen 10. Diabetic patients requiring insulin treatment 11. Milk or soy protein allergy 12. Uncontrolled hypo/hyperthyroidism