The purpose of this study is to identify how certain parts of the brain that control appetite respond to changes in dietary carbohydrate and fat.

To measure fMRI signal change in hypothalamic and brainstem centers associated with control of food intake and energy expenditure in response to IV glucose, fructose, and saline. The fMRI response to these nutrients will then be compared in obese vs. lean subjects. Frequently timed blood samples will be obtained for measurement of nutrients (i.e., glucose) and blood levels of other factors (i.e., insulin, ghrelin, lactate) during these infusion protocols, and to test associations between these levels and the fMRI response.

Inclusion Criteria:

• Age 18 year to 45 years of age
• BMI 18 to 25 kg/m2 (25 subjects) and 30 to 40 kg/m2 (25 subjects), matched for age and sex.
• At maximal lifetime weight.
• Weight stable for at least 3 months.

Exclusion Criteria:

• Those with a major medical illness or who require prescription medication, including: cancer, coronary artery disease, hypertension, and diabetes.
• Subjects taking psychiatric or centrally-acting (CNS) medications, or who are diagnosed with a psychiatric or neurological illness, including: depression, anorexia, bulimia, and seizure disorder.
• Subjects who have lost weight as a result of dieting, pharmacological treatment, or bariatric surgery.
• Weight > 350 lbs or girth > 60 inches (the limits of the MR machine)
• Exercise > 30 minutes, 3 times a week.
• Alcohol consumption > 2 drinks / day.
• Weight > 300 lbs (150 kg) (weight limit for MR machine).
• Blood glucose > 100 mg/dL, abnormal creatinine or liver function tests.
• Illicit drug use.
• Pregnancy.
• Extreme dietary habits as determined by a GCRC nutritionist: very high or low dietary carbohydrate, protein, or fat intakes.
• Those with a contraindication to exposure to strong magnetic fields: presence of metal in the body such as body piercing, shrapnel, cardiac pacemakers or aneurysm clips.
• Those with claustrophobia.