1. The relation between dietary carb0hydrate:Epid ratio and the fuel mixture oxidized during 24 h was investigated in eleven healthy volunteers (six females, and five males) in a respiration chamber. Values of the fuel mixture oxidized were estimated by continuous indirect calorimetry and urinary nitrogen measurements.2. The subjects were first given a mixed diet for 7 d and spent the last 24 h of the 7 d period in a respiration chamber for continuous gas-exchange measurement. The fuels oxidized during 2.5 h of moderate exercise were also measured in the respiration chamber. After an interval of 2 weeks from the end of the mixed-diet period, the same subjects were given an isoenergetic high-carbohydrate low-fat diet for 7 d, and the same experimental regimen was repeated.3. Dietary composition markedly influenced the fuel mixture oxidized during 24 h and this effect was still present 12 h after the last meal in the postabsorptive state. However, the diets had no influence on the substrates oxidized above resting levels during exercise. With both diets, the 24 h energy balance was slightly negative and the energy deficit was covered by lipid oxidation.4. With the high-carbohydrate low-fat diet, the energy expenditure during sleep was found to be higher than that with the mixed diet.5. It is concluded that: (a) the composition of the diet did not influence the fuel mixture utilized for moderate exercise, (b) the energy deficit calculated for a 24 h period was compensated by lipid oxidation irrespective of the carbohydrate content of the diet, (c) energy expenditure during sleep was found to be higher with the high-carbohydrate low-fat diet than with the mixed diet. Adaptation of energy expenditure to various energy intakes has been often studied in man (Passmore et al. 1955;Durnin & Norgan, 1969;Danforth et al. 1978; Garrow, 1978). Dauncey (1980) has recently shown that over-eating (or under-eating) for only 1 d induces changes in energy expenditure, and that the resting metabolic rate measured 14 h after the last meal is affected by the previous day's energy intake. However, the relation between dietary composition and the fuel mixture in 24 h has received less attention. The aim of the present study was to investigate the fuel mixture oxidized over 24 h when modifying dietary carbohydrate: lipid.A comparison of the nutrients ingested and the fuel mixture oxidized in 24 h gives the 24 h nutrient balance. Alternatively the relation between nutrient intake and the fuels oxidized can be studied by comparing the mean respiratory quotient (RQ) measured over 24 h with the mean food quotient (FQ), i.e. the ratio, volume of CO, produced:volume of 0, consumed for the combustion of the energy intake (Flatt, 1978). The FQ of a usual balanced diet is approximately 0.85, which corresponds to an energy partition of approximately 45, 40 and 15% for carbohydrates (CHO), fat and protein respectively. When the mean FQ of the 24 h energy intake is similar to the subject's mean 24 h RQ, the fuel mixture oxidized corresponds to the C...
