SUBSTRATE OXIDATION DURING EXERCISE IN THE RAT CANNOT FULLY ACCOUNT FOR TRAINING-INDUCED CHANGES IN MACRONUTRIENTS SELECTION

This study investigated spontaneous dietary adaptation to regular exer cise in relation to substrate oxidation measured during exercise. Male Wistar rats were offered permanent access to the three sources of mac ronutrients supplemented with minerals and vitamins. The rats remained sedentary or were trained daily during 3 weeks at moderate intensity (20 m.min(-1), 2 hours). Body weight, total caloric intake, and macron utrients selection were recorded throughout the experiment. Energy exp enditure and substrate oxidation were measured before, during, and aft er an exercise identical for trained and untrained rats (10 m.min(-1), 1 hour). Training reduced body weight gain (2.27 v 5.57 g.day(-1)), i ncreased protein intake (52.6% v 39.2%), and decreased carbohydrate in take (21.3% v 39.5%). Basal and running energy expenditure, as well as glucose and lipid oxidation, remained essentially comparable in train ed and untrained rats. The relative contribution of glucose oxidation (Gox) to total energy expenditure decreased during exercise (52.2%, av erage of all rats) relative to before exercise (60.8%). Gox during exe rcise was positively correlated with resting Gox before exercise, show ing that preexercise substrate oxidation was a strong determinant of r unning substrate oxidation. However, the slope was smaller for the tra ined than for the untrained rats, showing that exercise increases Gox less in trained rats than in untrained ones. We conclude from this stu dy that, since food selection but not substrate oxidation changed foll owing training, food intake adapted to substrate requirements induced by regular training and not the contrary. However, large differences r emained between the mixture ingested, in which lipids accounted for on ly 26% of the energy, and the mixture oxidized during exercise, in whi ch lipids accounted for 50.7% of the substrate oxidized. Such a differ ence may be related to metabolic requirements during the rest of the d ay and/or to the distribution of macronutrients intake relative to exe rcise. This question deserves further investigation with recording of macronutrients selection, energy expenditure, and substrate oxidation over 24 hours. Copyright (C) 1998 by W.B. Saunders Company.