Data from an experiment with adult humans in which the energy metabolism was measured by indirect calorimetry were used in a biological model of nutrient oxidation, lipogenesis and retention at the whole body level. Six male and four female adult volunteers (53-90 kg body weight) were asked to estimate the amount of a fixed diet, which would cover their normal degree of satiation, corresponding to their requirement of metabolizable energy (ME) for maintenance. The ME intake in relation to metabolic body weight (kg(0.75)) showed a great variation with 466 kT in Group A (n=2), 406 kJ in Group B (n=5) and 379 kJ in Group C (n=3). The heat production was lower than the ME intake in Group A and B, causing a positive energy retention about 1800 (Group A) and 525 kJ/d (Group B). In Group C heat production exceeded ME intake causing a negative retention of 1200 kT and mobilization of body fat. Oxidation of protein contributed with 14% to the total heat production for all groups. Oxidation of carbohydrate contributed in Group A, B and C with 55, 45 and 41% while oxidation of fat contributed with reverse values of 31, 40 and 46%, respectively. Lipogenesis occurred in all participants with 900 kJ/day in Group B and 600 kJ/day in Group A and C, corresponding to 50 and 34 g carbohydrate entering the fat metabolism via de novo lipogenesis. The present results indicate that the rate of lipogenesis in adult humans depends on carbohydrate and energy supply from a diet.