In line with our main hypothesis, we found an impaired MFO, accompanied of a shifting down and leftward of the oxidation rates curves. Both, CHOox and FATox, were low for a reduced power with age, which points a possible metabolic inflexibility with ageing in women despite of being active. To the best of our knowledge this is the first study to analyze women over 60, providing insight into the presence of metabolic abnormalities in this segment of the population, as well as knowledge to understand their responses to physical exercise.
As a second finding, aligned with our second hypothesis, preserving power to reach higher intensities in exercise suggests ensuring a better ability to oxidize fat in the lower loads (or in an upside-down reading, the better capacity to use both fuel sources may help to preserve muscle power). Moreover, although power decreases significantly the older the women, higher neuromuscular capacity, and non the higher cardiorespiratory fitness, seems to be responsible of a lightly better metabolic flexibility, with the ability to produce lactate also pointing to be key in this response to graded exercise.
The metabolic inflexibility with ageing has been previously described (Calçada et al., 2014), confirming the specific weight of ageing in the impaired ability to combine energy substrates (Fethney, 2010). Despite the unknown mechanisms of the origins of these metabolic behaviors (Goodpaster & Sparks, 2017), our data confirm a notably impaired fat oxidation [0.13 (0.09-0.17 g·min-1)] even compared to other longitudinal studies on populations with limited oxidative capacity, i.e., middle age obese [0.36 (0.31-0.40 g·min-1)] or sedentary [0.24 (0.22-0.25 g·min-1)] women (Amaro-Gahete et al., 2018). CHOox rates do not compensate this low fuel provision leading to an early cessation of exercise. Age-related changes in the metabolic status, but also in the skeletal muscle, may account for it.
On the one hand, there is the lower respiratory capacity of skeletal muscle due to a lower capacity of mitochondrial oxidative enzymes (25-40%) in older people (Brunner et al., 2007), as well as the drop in percentage of type I fibres or mitochondrial density (Holloszy et al., 1998). The acute response to exercise is conditioned by the abundance and function of these mitochondria, becoming an indirect method for understanding their functioning and oxidative capacity in different populations (San-Millán & Brooks, 2017b). The skill to alternate between carbohydrate and lipid metabolism in response to graded exercise would be thus affected in over-60 active women. Noteworthy, previous studies already showed limited PGC-1α mRNA expression with age (Holloszy et al., 1998), as well as the ability of increasing PGC-1α in the elderly with exercise (Cobley et al., 2012), what might explain the large heterogeneity in our sample, but also the strong influence of power in our results (as discussed below).
On the other hand, the body composition may also influence metabolic flexibility. Previous studies have observed a superior lipid metabolism, both for a higher intramyocellular lipid content that leads to the increased fatty acids availability in obese fitness population (Ara et al., 2011), or for an increase in the protein cluster of differentiation 36 (CD36) (Bonen et al., 2004; Søgaard et al., 2019). These authors suggest that this marker could be associated with increased free fatty acids uptake in older adults. However, the women in our sample are not only with better body composition, but also elder. Our data show lower body mass index values as well as fat mass respect to the study of Søgaard et al., (2019) (BMI; 25.5 vs 30.06 kg/m2; 35.03 vs 39.10 kg respectively), while regarding the FFM the differences observed were far superior (39.83 vs 51.8 kg).
In this sense, already Amaro-Gahete et al. (2019) highlighted the relevance of normalizing FATox through FFM, as performed in our study. The ageing process is in turn associated to increased sarcopenia (Cruz-Jentoft & Sayer, 2019) and impoverishment of FFM, affecting body composition. Albeit we consider the FFM in FATox estimation, following the scarce recent studies such as Frandsen et al. (2020), FATox rates were still low in our sample: 5.61 (3.59-7.63) mg/min/kgFFM vs. 7.3 (6.2–8.4) mg/min/kgFFM in a group of middle-aged sedentary people and 7.6 (6.4–8.8) mg/min/kgFFM in a group of middle-aged trained people in Frandsen et al. (2020). Therefore, the worsening with age is maintained despite normalizing FFM.
In this scenario, the age-related fall of MFO through the graded test would require the early involvement of carbohydrates. However, the shifting down and leftward of the CHOox rates curve highlights the big difficulty to get additional fuel metabolism over certain intensities (i.e., the metabolic inflexibility), since the glycolytic pathway might be also punished because of the greater loss of muscle mass in type II fibers (Brunner et al., 2007).
Of outermost importance, this phenomenon suggests being counterbalanced by training, as shown by the strong association between P100 and MFO in our active women, as well as the moderate influence of BLa in MFO (P100 r=0.71, p=0.01; BLapeak r=0.52, p=0.04 respectively). Figure 2 confirms that power could be explaining up to the 50% of the variability in MFO; and up to the 36% of the variability in BLa. This latter (BLa) would in turn explained the 23% of the variability in MFO, which is a moderate association. Even more, the negative association between MFO and age disappeared when considering P100 as a covariate, since this latter was the one who was really affected by ageing (r=-0.85, p<0.01, R2=0.72), unlike the VO2peak associations. According to San-Millán & Brooks (2017), exercise lows circulating lactate by increasing lactate clearance, thus increasing lipid oxidation, and reducing CHOox. This behavior could be preserved in the stronger women over-60, at least in the very first stages of the graded test, in addition to the benefits of exercise through the PGC-1α participation, leading to mitochondrial biogenesis (Cobley et al., 2012; San-Millán & Brooks, 2017).
Therefore, our data indicate that muscle power is severely affected by age in older women, even being active, confirming their need of power training, both for neuromuscular and cardiovascular (i.e., metabolic) health. Peak of power falls with age due to the deterioration of neural function and the drop in the number of motor units (Yamauchi et al., 2010), despite no changes in the muscle cross-sectional area (Frontera et al., 2000). However, those women who have preserved the ability to produce larger muscle power in our pilot study, have also maintained larger fat and CHOox rates, adding new reasons to increase power exercise training with age. Muscle power becomes thus an important indicator from a metabolic perspective and confirms its importance in active ageing strategies. In line with Cadore & Izquierdo (2018) statements, this parameter is of paramount importance, since the larger the power, the better the physical cardiorespiratory fitness and the better the metabolic flexibility in our older female population.
Concerning other performance key parameters like the FATmax, unlike MFO, we found significantly higher values compared to the obese and sedentary women in the cited previous studies (Amaro-Gahete et al., 2019) [79.52 (66.40-92.64 %VO2peak) vs. 43.3 (28.28-48.21) and 46.1 (42.17-50.02) %VO2peak respectively]. This phenomenon settles down in a low and anticipated VO2peak because of the test was planned to reach an intensity close to RER≈1 in a large-stages test, and this intensity was achieved early on in this older population. Besides the nature of the test, our results reflect once more a limited behaviour across the intensity spectrum. The higher FATmax must be therefore contextualized and considered just a FATmaxpeak in the test, more than the elderly women' FATmax (VO2max %).
In summary, we found metabolic inflexibility reflected by low FATox and CHOox rates and declined MFO values in women over-60 despite being active. MFO was influenced by power and lactate production (both peripheral factors of women’s motor performance and health) and not by age and VO2peak in the test. According to the scarce literature, these results are conditioned by body composition, the test duration, and the intensity at whither we achieve the respiratory exchange ratio. Up to our knowledge, this is the first study to confirm the influence of the status of training (i.e., muscle power and lactate in the test), variables that might revert all the above.
To conclude, the authors acknowledge the presence of several limitations. Among these, most of the women in the sample were Nordic Walking practitioners, so they were not currently familiarized with the bicycle in the higher intensities. This may have conditioned the end of the test, however, a first familiarization session and the VAS and RPE scales helped to ensure that women felt comfortable and secure enough to increase intensities up to VT2. On the other hand, the sample may be somehow low (due to COVID19-pandemic limitations), fact that limits the analysis and conclusions which can be drawn from this research, as well as the transferability of these results. Noteworthy, this is a pilot study, and the sample is very representative of these active women over-60, since Nordic Walking is a widespread sport modality in these ages. Finally, lack of invasive procedures in the study does prevent us from outlining the mechanism behind these findings. Future studies will therefore need to explore the explanation of these phenomena using biopsies or blood samples. Moreover, a more detailed analysis of efficiency in both pathways to complement these findings, as well as further analysis of the premature and advanced glycolytic RER, could also shed light on exercise responses with ageing. In this aspect, it is worth highlighting the absence of maximum values of VO2, power or lactate in the protocol, as this work focused on intensities close to VT2.