RELATIONSHIP BETWEEN MAXIMAL FAT OXIDATION AND OXYGEN UPTAKE: COMPARISON BETWEEN PATIENTS WITH TYPE 2 DIABETES AND HEALTHY SEDENTARY SUBJECTS

Abstract

Purpose Carbohydrate and fatty acids are the dominant fuels oxidized for energy production during exercise. Their contribution is influenced by intensity of exercise, with a progressive increase in the relative contribution of carbohydrate oxidation to energy expenditure, whereas the absolute rate of fat oxidation increases from low to moderate intensities of exercise and declines as exercise becomes more intense. In patients with type 2 diabetes (T2D) the altered metabolism and the lower capacity to sustain efforts changed the choice and regulation of energy substrates for oxidation. Purpose of our study was to assess the relationship between the highest value of fat oxidation rate (FATmax) and the oxygen uptake (VO2) in sedentary T2D patients and to compare with healthy sedentary subjects. Methods Fifteen sedentary T2D patients (nine males, six females), mean age 58 (range 39 – 65) and fifteen healthy sedentary subjects (eight males, seven females), mean age 53 (range 37 – 71), were studied. All T2D patients were being treated with diet and antiglycemic agents. Significant cardiovascular disease was excluded in all subjects by an instrumental examination. The exercise testing started in the laboratory at 08.00 hours after an overnight fast. All subjects performed a graded exercise test to exhaustion on treadmill, using modified Bruce protocol. Oxygen uptake (VO2) and carbon dioxide (VCO2) were recorded and maximum fat oxidation rate (FATmax) was determined using indirect calorimetry, with the assumption that urinary nitrogen excretion rate was negligible. Substrate oxidation rates were then plotted as a function of exercise intensity, expressed as percentage of maximal oxygen uptake (VO2max). Experimental data are presented as means ± standard error of the mean, and for the statistical analysis of the data linear regression analysis was used. Results The physical and physiological characteristics of subjetcs are shown in table 1. The average VO2max for T2D patients and healthy subjects was 22.29 ± 1.06 ml/Kg/min and 28.63 ± 2.03 ml/Kg/min respectively (P < 0.01). In T2D patients, absolute fat oxidation rate reached a maximum of 6.71 ± 0.46 mg/Kg/min at VO2 of 15.41 ± 0.68 ml/Kg/min (70 ± 1.27% of VO2max)while in healthy subjects 7.19 ± 0.77 mg/Kg/min at VO2 of 18.23 ± 1.28 ml/Kg/min (64 ± 2.61% of VO2max). When linear regression analysis was performed, a positive linear correlation between FATmax and VO2 (r = 0.85; p = 0.0001) in both groups has been found. Also, positive linear correlation between FATmax and VO2max in both T2D (r = 0.76; p = 0.001) and healthy subjects (r = 0.73; p = 0.002) has been found. Conclusions Our data, comparable with those reported in the literature, indicate that in T2D patients fat oxidation rate is not impaired. VO2max, index of exercise capacity, was significantly lower in T2D patients so that, in T2D compared to healthy subjects, we found a maximal fat rate oxidation at higher exercise intensity. Moreover, the positive linear correlation between FATmax and both VO2 and VO2max suggests that even in T2D patients the muscle oxidative capacity might increase in response to aerobic training.