The purpose of this study is to learn more about how diet affects the hormone ghrelin. Ghrelin is made in the stomach and causes appetite to increase. Learning about ghrelin will help the investigators understand more about obesity.

Control subjects will be provided 3 different diets for 2 weeks each. At the end of each diet period participants will be admitted to OHSU Clinical Translation Research Center (CTRC) for blood sampling every 30 minutes for 13.5 hours, to measure levels of hormones. Participants will also have a DXA scan to measure body composition.

PWS subjects will have one admission after eating a standardized diet prepared by caregivers for 2 weeks.

Inclusion Criteria:

• Age 18 or older.
• Weight stable for at least 3 months.
• At lifetime maximal body weight.

Exclusion Criteria:

• Actively losing weight by diet or exercise.
• Smokers.
• Alcohol consumption > 2 drinks per day.
• Exercise > 30 minutes 3 times a week.
• Prescription drug use (except birth control pills, vitamins, or minerals).
• Type 2 diabetes.
• Heart disease, cancer, malabsorptive states, or chronic infections that would affect body weight.
• Weight > 300 lbs (exceeds the weight limit of the DEXA machine).
• Hemoglobin < 12.0 g/dL for women, < 13.5 g/dL for men