Energy intake is the most difficult part of the energy balance equation to measure. Using an in patient computerized vending machine system, food intake has been measured over 180 individuals. In individuals undergoing the study more than once the intra-class correlation coefficient is very high (r=0.9) indicating that these studies although performed in an inpatient setting are very reproducible. Eating during nighttime hours is a behavior that may predispose individuals to obesity. In our vending machine studies, we found that nearly 30% of volunteers ate from our vending machines during the night. These individuals consumed overall more calories and gained more weight over a 3 year follow-up period. In addition, these individuals have higher respiratory quotients measured in our respiratory chamber while on a weight maintenance diet indicating an increased drive for food intake. Despite these differences, we have not found differences in anorexigenic or orexigenic hormone levels in these individuals. Higher respiratory quotient, the ratio of carbohydrate oxidation to lipid oxidation, is known to predict weight gain. The mechanism of this increase in weight was thought to be secondary to a lower lipid oxidation in these individuals. Higher respiratory quotient measured during weight maintenance also predicted food intake during the ad-libitum vending machine study. However the component of respiratory quotient which best predicted food intake was higher carbohydrate oxidation indicating a role for carbohydrate (likely glycogen) balance in the regulation of short term food intake. To further investigate whether carbohydrate balance influences food intake, we fed individuals (in random order) a high fat versus high carbohydrate diet followed by ad libitum food intake. In preliminary analysis of 20 individuals, overall we did not find any difference in food consumption after either diet. However, more obese individuals tended to eat more after the high fat compared with the high carbohydrate diet and the opposite was true in leaner individuals, indicating that adiposity is associated with alteration in responses to ingested macronutrients. Predictors of food intake in the past have demonstrated that individuals with lower fasting ghrelin concentrations (a hormone which is associated with increased food intake) actually ate more. However, follow-up studies on a larger group of individuals did not show any association between total ghrelin concentrations and food intake. Furthermore, total ghrelin concentrations did not decrease with overfeeding, and change in ghrelin concentration did not predict subsequent food intake. This data indicates that ghrelin may not have a role in control of day to day food intake, nor is it suppressed by overeating. In order to evaluate this further, we evaluated the effect of short term (3 day) overfeeding compared with weight maintaining diets on both total and active ghrelin concentrations, physical activity and subsequent ad libitum food intake. We found that neither total or active ghrelin or food intake was reduced after overfeeding. However, during the overfeeding period, weight gain was associated with increased sedentary time indicating that physical activity may regulate weight gain during periods of overnutrition.