OBJECTIVE: In healthy, nonobese, adolescent males and females to: (1) Determine the relationship between fitness and energy intake; (2) assess the effect of five-weeks endurance training on energy intake and food choice and (3) compare food record assessments of energy intake with doubly-labeled water (DLW) measurement of total energy expenditure (TEE). DESIGN: (1) Cross sectional analysis of fitness and food intake and (2) Prospective, randomized, controlled interventional study of endurance-type exercise training in 44 females and 44 males (age range, 15-17 y). MEASUREMENTS: Pre and end interventional three day food records were successfully collected from 32 females (15 controls, 17 trained) and 39 males (19 controls, 20 trained). Fitness was assessed from cycle ergometry as peak oxygen uptake normalized both to thigh muscle mass and body weight. Thigh muscle mass was measured by magnetic resonance imaging. TEE using the DLW technique was measured in 12 females (6 controls, 6 trained) and 20 males (10 controls, 10 trained) during weeks 4-5 of the exercise training program (simultaneously with the second assessment of food records). Food record data were analyzed using the Minnesota Nutrition Data System. RESULTS: Fitness was correlated with self reported total caloric intake in males but not females. In females, there was a significant increase in fat intake (19.8+/-9%, P < 0.05) and a significant decrease in carbohydrate intake (-9.8+/-4%) in the trained subjects. No changes were observed in the control subjects. Energy expenditure (2072+/-52 kcal/d) was significantly greater than the estimated energy intake (1520+/-112 kcal/d, P < 0.007) during the intervention in the trained, but not control, subjects. However, there was no weight change in either control or trained subjects. In males, no changes were observed in food choice in either control or trained subjects. Similar to the females, energy expenditure (2425+/-22 kcal/d) was significantly greater than the estimated energy intake (2168+/-117 kcal/d, P < 0.05) during the intervention in the trained, but not control, subjects. No weight changes were observed in either group. CONCLUSIONS: Fitness is associated with increased self-reported energy intake in males but not females, while exercise training led to alterations in food selection (greater fat and reduced carbohydrate) only in females. These observations could reflect specific gender differences, or, alternatively, the generally lower levels of fitness in the females. The apparent negative energy balance without evidence for weight loss in both the trained males and females suggests a systematic under reporting of food intake during exercise programs in adolescents, and indicates the possibility that errors in self reported food intake might be greater during transitions from one level of energy expenditure to another.