The study will investigate whether the level of fat stored in the liver is related to the liver's ability to burn fat.

The study has two phases. In Phase I, liver samples from patients undergoing bariatric surgery or a diagnostic liver biopsy will be tested for the ability to burn fatty acids. In Phase II, patients with nonalcoholic fatty liver disease will participate in either a nine-month lifestyle treatment program or undergo standard care. The goal is to determine how weight loss and exercise improve liver health.

Inpatient comprehensive metabolic tests are performed before and after the nine-months.

For Phase I: Any patient scheduled to undergo bariatric surgery or a liver biopsy to diagnose nonalcoholic fatty liver disease.

For Phase II: Only subjects who have had a diagnosis of nonalcoholic fatty liver disease and completed Phase I (and no subjects who have undergone bariatric surgery)

Inclusion Criteria:

1. Men and women (pre and post-menopausal)
2. Overweight/obese with BMI ≥ 25.9 or < 50.0 kg/m2
3. Characteristics of the metabolic syndrome, pre-diabetes (fasting glucose 100-125 mg/dL or 2h glucose 140-200 mg/dL) or diabetes type 2, or undergoing bariatric surgery
4. 22-65 years of age
5. Sedentary, < 60 minutes per week of structured physical activity
6. Alcohol intake< 20 g/d

Exclusion Criteria:

1. Having undergone bariatric surgery.
2. Acute disease or advanced cardiac or renal disease, anticoagulation therapy, or any severe co-morbid condition limiting life expectancy < 1 years
3. Other causes of hepatitis including hepatitis B & C, autoimmune hepatitis, hemochromatosis, celiac disease, Wilson's disease, alpha-1-antitrypsin deficiency, medication-induced hepatitis, any clinical or biochemical evidence of decompensated liver disease
4. Use of steroids or other drugs that cause NAFLD
5. Pregnant or trying to become pregnant
6. Inability to exercise on a bike or treadmill