Regulation of FGF21 by Nutritional Challenges

72-hour fast in healthy volunteers (Study Arm A): Inclusion Criteria: * Age 18-50 years * Body Mass Index 21-26 kg/m\^2 * Ability to give informed consent * Ability to follow verbal and written instructions in English Exclusion Criteria: * Type 1 or Type 2 diabetes mellitus diagnosed according to the American Diabetes Association criteria * Coronary heart disease (history of myocardial infarction, unstable angina pectoris, or congestive heart failure) * Tobacco, marijuana, or intravenous drug use within 1 year of screening visit * Recent weight change (\>3 kg within 6 months of screening visit) * Malignancy treated with chemotherapy within past 3 years * History of depression, psychosis, or other psychiatric illness requiring hospitalization * History of hyperthyroidism * Renal insufficiency (creatinine clearance \< 50 ml/min) * Transaminases \> 2X above the normal range * Known liver disease * Pregnancy within 12 months of screening visit * Lactation within 12 months of screening visit * Failure to use medically approved contraceptive methods (oral contraceptive, 2 barrier methods, surgical sterilization) * History of an eating disorder (anorexia, bulimia, or laxative abuse) * History of surgery for the treatment of obesity (gastric banding, gastric bypass, gastric stapling) * New diagnosis of hypothyroidism within 1 year of screening visit or change in dose of thyroid hormone within 3 months of screening visit * History of alcohol abuse within the past 3 years * History of keloid formation * History of allergy to lidocaine or marcaine * Use of plavix, coumadin, or heparin 72-hour fast or hypocaloric diet in subjects with non-alcoholic fatty liver disease (NAFLD) (Study Arms B \& C): Inclusion Criteria: * Age 18-60 * BMI 25-45 kg/m\^2 * Known nonalcoholic fatty liver disease based on liver biopsy * Ability to give informed consent * Ability to follow verbal and written instructions in English Exclusion Criteria: * Type I diabetes mellitus diagnosed according to the American Diabetes Association criteria or Type II diabetes mellitus with A1c \> 7.5% or taking metformin or thiazolidinediones * Coronary Heart Disease (history of myocardial infarction, unstable angina pectoris, or congestive heart failure) * Uncontrolled hypertension (BP \> 150/90 mmHg on or off antihypertensive medication) * Tobacco, marijuana, or intravenous drug use within 1 year of screening visit * Recent weight change (\> 3 kg within 6 months of screening visit) * Malignancy treated with chemotherapy within the past 3 years * History of depression, psychosis, or other psychiatric illness requiring hospitalization * History of hypo or hyperthyroidism * Renal insufficiency (creatinine clearance \< 50 ml/min) * Pregnancy within 12 months of screening visit * Lactation within 12 months of screening visit * Failure to use medically approved contraceptive methods (oral contraceptive, 2 barrier methods, surgical sterilization) * History of an eating disorder (anorexia, bulimia, or laxative abuse) * History of surgery for the treatment of obesity (gastric banding, gastric bypass, gastric stapling) * New diagnosis of hypo or hyperthyroidism within 1 year of screening visit or change in dose of thyroid hormone within 3 months of screening visit * History of alcohol abuse within the past 3 years * Cardiac pacemaker or aneurysm clips * Metal implants in the body (pins, plates, shrapnel, intact bullets, IUD) * Claustrophobia