Effect of Diet on Work Metabolism

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Abstract
The participation of fat and of carbohydrate in the energy expenditure during light exercise (300 kpm/min) on the bicycle ergometer was studied in 6 healthy men 19 to 24 years of age, living for periods of about 70 days under strictly controlled metabolic ward conditions while consuming the following diets: (1) 3,000 Cal., 73 gm of protein, 107 gm of fat, 463 gm of carbohydrate (basal diet); (2) 3,000 Cal., 33 gm of protein, 114 gm of fat, 462 gm of carbohydrate; (3) 3,500 Cal., 89 gm of protein, 231 gm of fat, 247 gm of carbohydrate; and (4) 1,600 Cal., 67 gm of protein, 65 gm of fat, 175 gm of carbohydrate. In connection with the metabolic measurements, 24-hour urine samples were collected for nitrogen and creatinine determinations. The basal metabolic rate was determined 14 to 15 hours postprandial using the Noyons BMR apparatus, which was also used for the analysis of expired air collected between 20 and 30 minutes of work at 300 kpm. In separate experiments, metabolic measurements were made during work, two hours following ingestion of 100 gm of glucose. With the control diet (diet 1), the average respiratory quotient (RQ) during exercise was somewhat higher than during rest. A reduction of the protein intake to 33 gm/day (diet 2), which caused roughly a 50% decrease in the oxidation of proteins, failed to induce any consistent change in the basal metabolic RQ or in the work RQ as compared with diet 1. The high fat and low carbohydrate diet (diet 3) caused a gradual decline in the exercise RQ, reaching values of about 0.74 after 52 days, indicating that the exercise energy was supplied mainly by the oxidation of fat. The 1,600 caloric diet, low in both fat and carbohydrate (diet 4), gave results quite similar to those obtained with the high fat diet. In all cases, ingestion of glucose two hours before exercise caused an increase in the exercise RQ. These experiments show that it is possible at light work loads (300 kpm/min) to obtain respiratory quotients that indicate a major participation of fat oxidation in the work metabolism. These data also suggest that the actual carbohydrate intake rather than the amount of fat in the diet is the decisive factor, determining whether fatty acids or glucose is the preferred fuel.