This is a longitudinal cohort study of how the responses to a 3 day period of controlled overfeeding relate to subsequent weight gain. We hypothesize that thin individuals are resistant to weight gain because they respond to periods of overfeeding by increasing fat oxidation, reducing food intake, and increasing physical activity relative to those who gain weight over time.

• Half of the subjects will be women, half will be men,
• age 25-35,
• a range of BMI's from 19-27, for the 'thin subjects' there should be no 1st degree relatives with a BMI>30.
• Subjects should not engage in planned physical activity more than 3 hrs per week.
• For the obesity prone group there should be at least 1 1st degree relative with a BMI>30.

Subjects have been recruited through advertisements in local news media and on the internet