This study aims to characterize the neurobiology of obesity in American Indians (AI) using functional MRIs to examine the correlation between brain response to food stimuli in AI women. A functional MRI (fMRI) is used to visualize brain activity when obese and lean AI women look at images of fattening food, non-fattening food and non-food objects. Additionally examined is the effect of the drug naltrexone to suppress brain response to visual food cues and calorie intake in the women.

The study looks at both obese and lean women. Each woman comes in for two visits. Each woman takes both the Naltrexone or placebo (one during each visit). Research staff and participants are blinded. Drugs are not used as treatment, but rather to provide information for possible future treatments.

Hypotheses are:

1. Compared with their lean counterparts, obese women will demonstrate more fMRI activation in the brain in response to visual food cues.
2. Naltrexone will reduce the activation in reward-relevant brain sites in response to viewing photographs of fattening food as compared in placebo in both groups.
3. Naltrexone will suppress spontaneous intake of food in obese and lean AI women.

Inclusion Criteria:

• Female
• Self-identified AI
• BMI over 30 OR between 20 and 24.9
• Must see clearly with or without glasses
• Capable of giving informed consent

Exclusion Criteria:

• Smokes more than one cigarette a day
• Drinks more than two alcoholic drinks a day
• Uses recreational drugs
• Is pregnant
• Has had weight loss surgery
• Other major medical problems (e.g. diabetes)
• Taking medications that alter appetite or body weight
• Significant food allergies

Females Only

Self-identified Native American women between ages of 18 and 45.