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

Background: \- Non-alcoholic fatty liver disease (NAFLD) is a common cause of abnormal liver tests. It is closely linked to health problems like obesity and diabetes. Researchers want to use a breath test to measure the metabolism of fatty acids and learn more about the causes of disease. They will compare the results between people who do and do not have NAFLD. They will also repeat the test in people with NAFLD after treatment to see improvement. Objectives: \- To measure the metabolism of fatty acids and compare them between people who do and do not have NAFLD, and people with NAFLD before and after treatment. Eligibility: * People age 21 and older with NAFLD * Healthy volunteers * non-NAFLD metabolic syndrome Design: * Participants will be screened with medical history, physical exam, and fasting blood tests. * Participants will fast overnight. * Participants will sit in a comfortable chair. They will relax for 15 minutes. Then a narrow plastic tube will be put under their nostrils. It will be connected to a breath test device. * Participants will drink palmitate or acetate in a heated liquid meal (Ensure). Palmitate is a fatty acid and acetate is a small molecule that is a building block for fatty acids. Both palmitate and acetate are natural compounds that are in our body and our food. * Participants will sit for 6 hours. They can read, watch television, work on a computer, or other relaxed activity. The air they breathe out from their nose will be collected and analyzed. They may take a bathroom break after 3 hours. They may drink water after 3 hours. * Participants will get a meal after the test. * Participants will have another visit at least 1 week later. They will repeat the test with the other compound (acetate of palmitate). * Participants may be asked to repeat a test to study the result of a change like weight loss or medicine. Thus, participation can last from 1 week to several years.