The respiratory exchange and urinary nitrogen excretion of 6 healthy male subjects (age 21 +/- 1 yr; body weight 70 +/- 2 kg; means +/- SD) were followed for 10 hr after ingestion of a large amount of carbohydrates (CHO) in the form of bread, jam, and fruit juice, equivalent to 479 g of starch. Peak values for blood glucose (6.6 +/- 0.6 mM; mean +/- SEM) and plasma insulin (139 +/- 26 microU/ml) were reached after 90 min at which time the nonprotein respiratory quotient (NP-RQ) had risen to 0.97. During the next 8 hr glucose levels remained near 5.5 mM while insulin declined gradually to 22 +/- 7 microU/ml. The average NP-RQ remained in the range of 0.91 to 0.98, though individual values exceeding 1.0 for very short periods were observed. The increase in energy expenditure above basal rates corresponded to a specific dynamic action (SDA) of 5.9 +/- 0.6%. Assuming the CHO load to be completely absorbed after 5 hr, and allowing for glucose oxidation calculated from the gas exchange data, the glycogen content of the subject's body tissue had then increased by 408 +/- 19 g. During the 10 hr after the meal, 133 g CHO, 17 g fat and 29 g protein were oxidized, providing respectively 66%, 19% and 15% of caloric expenditure, and leaving a gain in glycogen stores estimated at 346 +/- 12 g. The data imply that: (1) The capacity for glycogen storage in man in larger than generally believed, and (2) Fat synthesis from CHO will not exceed fat oxidation after one high-carbohydrate meal, even if it is uncommonly large. When a single high-carbohydrate meal is consumed, dietary CHO merely has the effect of reducing the rate of fat oxidation. These findings challenge the common perception that conversion of CHO to fat is an important pathway for the retention of dietary energy and for the accumulation of body fat.