For obese individuals, losing weight and keeping it off are extremely difficult, whereas some other individuals are thin and report trouble gaining weight. In most weight loss and overfeeding studies there is a large variation in the amount of weight lost or gained, and it is not clear whether an individual s response to an intervention can be predicted. Measurement of the amount of energy an individual uses over 24 hours (24 hour energy expenditure or 24-EE) and the response of 24-EE to overfeeding and fasting may help predict which individuals will have the greatest changes in weight with changes in energy balance. In previous work, 24-EE responses to overfeeding and fasting were related such that individuals with the greatest increase in 24-EE upon overfeeding tended to have the smallest decrease in 24-EE upon fasting ( spendthrift phenotype) and those individuals with less increase in 24-EE during overfeeding had a greater decrease in 24-EE with fasting ( thrifty phenotype). Those with the spendthrift phenotype who had a greater increase in 24-EE in response to overfeeding gained less weight over time. Furthermore, studies have shown that diets with very low protein content may magnify the variability in response to overfeeding.