Why is it not possible to separate out the contribution of p

Solution

lipid oxidation during submaximal exercise was higher and glucose, lactate and insulin concentrations were lower during an exercise bout performed 3?h after a meal at an intensity corresponding to the COPox, compared with 1?h after a meal for the same level of energy expenditure. Its mean value (100?mg?min-1) was quite similar to that obtained after the standardized exercise calorimetry performed at fast (123?mg?min-1). In addition, the heart rate in the 3-h condition was more stable and was closer to that measured at the COPox, whereas a drift was seen in the 1-h condition. Since middle-age is a period where women are prone to overweight, we focused on this period. For inclusion in this study, the women should be older than 50?y and postmenopausal since at least 2?y. These inclusion criteria have also the advantage of ruling out the effect of menstrual cycle on the balance of energetic substrates. To assess this balance of substrates, we made the choice to calculate lipid and CHO oxidation rates rather than to only use the RER. Actually, since we were mostly interested by lipid oxidation, this measurement appeared preferable, keeping in mind that the relationship between lipid oxidation and RER is not linear, as shows the empirical equation given above (lipid (mg?min-1)=1.6946 VO2–1.7012 VCO2). This equation can be simplified as lipid (mg?min-1)=1.7 VO2 (1- RER), that is, lipid oxidation increases proportional to VO2 and decreases when RER increases. Due to this aspect of ‘bell-shaped curve’ is difficult to evaluate lipid oxidation from RER alone, as can be seen, for example in Figure 2. However, the accuracy of indirect calorimetry during exercise has been much discussed, mainly because bicarbonate kinetics and thus CO2 production can be markedly altered above the lactate threshold. VCO2 may therefore overestimate tissue CO2 production and concomitantly overestimate the rate of CHO oxidation.19 In fact, it has been demonstrated that the contribution of bicarbonate-derived CO2 to VCO2 is rather negligible and that VCO2 can be considered to be a reliable reflection of muscle tissue CO2 production during exercise, including at high intensity, as long as RER is lower than 1 Furthermore, as previously shown, this type of exercise testing can be used to characterize alterations in substrate utilization in the obese during exercise6 and has clear beneficial effects. In this study, the values of Wmax, expressed in absolute value, in terms of percentage of predetermined Wmax and in terms of heart rate corresponding to COPox, were very close to those reported previously in overweight females.6 The exercise session may appear relatively short (30?min) but this duration is well adapted to the deconditioned state of obese sedentary subjects. In addition, there is no significant change in substrate availability and oxidation between 30?min and 2?h at low exercise intensity link is http://www.nature.com/ijo/journal/v29/n8/full/0802991a.html