In this pilot study we found that although maximal oxygen uptake and body composition of fitter older adults were higher compared to unfit peers, most of the basal gene expression related to ageing, performance, and associated metabolism as well as fiber type composition did not significantly differ between HPF and LPF. On the other hand, slight differences were observed regarding protein concentrations before and after an acute exercise test.
Performance and body/muscle composition
Several studies have assessed sedentary behavior and physical inactivity in older adults taking into account differing nationalities, social/educational level and sex [4, 25,26,27], and uniformly observed a tendency for increasing inactive physical behavior among the elderly population. Although the popular allusion that “sitting is the new smoking” is scientifically not sound [28, 29], an inactive lifestyle, especially in older people, increases the risk for cardiovascular diseases, loss of skeletal muscle mass and malfunction as well as all-cause mortality [7, 13, 28].
The combination of increasing age and loss of skeletal muscle mass and/or functionality is associated with decreased aerobic capacity, shown by decreased VO2max . The examination of our fitter HPF and unfit LPF showed distinct differences in performance and body composition. Especially the workload at higher intensities could be better maintained by HPF, seen by higher VO2peak/kg, whereas there seemed no difference between groups during moderate intensities at VO2VT1. Maintaining a higher VO2peak during life can reduce the risk of multiple comorbidities, all-cause mortality, and loss of independence during aging .
The aging-associated capillarization and aerobic enzyme activity decline is equally slowed down by lifelong exercise, which can have a direct beneficial influence on cardiovascular health in women and men . In our mostly inactive LPF participants, the body fat percentage was significantly higher. It is well known that a high body fat percentage contributes to increased risk of developing cardiovascular diseases, diabetes and immobility . On the other hand, several studies have shown that weight loss intervention with a combination of diet and exercise improves muscle strength and muscle quality in addition to fat loss among older obese people and consequently attenuates sarcopenic symptoms and pro-inflammatory responses . On the muscular level, increasing age and immobility lead to alterations in the MyHC isoform distribution, fiber type composition and fiber activation [2, 12, 34]. Interestingly, we did not detect any correlation between skeletal muscle mass, VO2peak and age in both groups and, what is more, skeletal muscle mass as well as MyHC composition were not different between HPF and LPF.
Fiber size was not assessed, which may have given further insights into muscle adaptations and decline as muscle strength is a strong predictor of severe mobility limitation, slow gait speed, increased fall risk, risk of hospitalization, and high mortality rate [2, 34,35,36]. For example, Miljkovic et al. showed that elderly adults with low muscle strength have a 2.6-fold greater risk of severe mobility limitation, 4.3-fold greater risk for slow gait speed, and 2.1-fold greater risk of mortality compared to elderly adults with high muscle strength .
Skeletal muscle gene expression
Beside adaptations in fiber type morphology, regular exercise can slow down age-related alterations on the molecular skeletal muscle level with beneficial changes in oxidative enzyme activity, DNA repair and skeletal neurotrophic factors like BDNF [11, 12, 37]. Skeletal muscular expression analysis of genes involved in gene stability, energy metabolism and mitochondrial activity (PGC1a, G6PD, UCP3, VEGFa, NR4A1, NR4A2, SCO2, SOD2, IL16, and TP53) revealed no difference in HPF and LPF regarding energy metabolism or mitochondrial activity. Significant differences could only be observed in BDNF and BRCA1 levels with higher basal expression in LPF. The latter contrasts with previous studies, which showed increased expression in BRCA1, responsible for genomic stability, with regular exercise [38, 39]. We observed a high intra-group variability in the expression profiles which may contribute to this result. A further possible explanation may be the known blunted response on the protein level in older people .
Serum protein adaptations
Although a higher basal BDNF skeletal muscle mRNA expression was seen for LPF, serum protein concentrations showed non-significant elevated levels in HPF pre and post exercise compared to LPF. Until now, skeletal muscle expression of BDNF in older people after exercise is incompletely understood and has been rarely examined: accumulating evidence on circulating protein levels seem to hint to transiently up-regulation of BDNF concentration after acute exercise, but with no long-term changes with regular training [40,41,42]. Therefore, BDNF may not be differently regulated depending on physical fitness status in older individuals and similar levels might be expected.
Beside BDNF, Küster et al. evaluated Irisin and the kynurenine pathway as novel blood-based biomarkers for physical and cognitive performance . Ten weeks of cognitive and physical exercise training in older people were associated with potentially beneficial decreased kynurenine metabolites, whereas no significant changes in Irisin levels could be observed, which is in contrast to other studies . The main function of the myokine Irisin is thermogenesis regulation by hormonal inducement of increasing energy expenditure, promoting weight loss, and decreasing insulin resistance . Irisin is also a potential circulating mediator of exercise which benefits brain health and neuroprotection, as it was shown that it is secreted upon exercise in mice and humans, where it apparently promotes BDNF release . Furthermore, the PGC-1α-FNDC5/Irisin-BDNF-signaling pathway has been proposed for resistance exercise that increases the expression of FNDC5/Irisin and in turn induces BNDF with subsequently beneficial adaptations in the transcription and transport of mRNA along dendrites, growth, differentiation, and survival of neurons .
In our study, significantly higher serum baseline values of circulating Irisin in HPF compared to LPF were found. Circulating Irisin levels were shown to be negatively correlated with cardiometabolic risk in sedentary participants, with an active lifestyle positively increasing Irisin levels . This is in line with higher VO2peak data of HPF, which implies a higher general fitness of this sub-group. For serum kynurenine, only a trend for decreased concentrations in HPF and increased concentration in LPF was observed post. This might suggest better coping capacity with the physical exercise test, as elevated kynurenine levels were associated during and after stressful cognitive and physical periods .
Serum HSP70 concentration did not significantly differ between HPF and LPF. The versatile heat shock protein HSP70 serves as a molecular chaperone involved in many cellular processes . It has a profound impact on protein turnover, energy metabolism, muscle function/regeneration, hypertrophy and adaptation, while sarcopenia leads to decreased HSP70 content due to muscle loss . Furthermore, a blunted expression of HSP70 in skeletal muscle in response to chronic exercise during aging was observed [9, 46], which may support our observation of unchanged HSP70 response in both HPF and LPF.
The proinflammatory cytokine Il-6 is induced after stressful events like intensive exercise, but has the additional function as an anti-inflammatory myokine . It is one of the key molecules which controls the catabolic effect in myoblasts through inhibition of myogenesis and protein synthesis . Higher plasma levels of Il-6 are predominately found in older people and were associated with lower muscle mass and muscle strength , which may be driven by the chronic low-grade inflammatory status and contributes to muscular functional impairment . Il-6 serum basal levels of HPF and LPF were not significantly different and both groups showed increased values after the exercise test. This observation indicates a similar inflammation process regardless of physical fitness status, which may modulate the so called “inflammageing” in older people . A third sample time point after the exercise test, e.g. 1 or 2 h later, may have given further insights in the long-term process of the Il-6 regulation. Short-term increases of inflammatory cytokines are common responses to exercise, whereas prolonged increases have harmful consequences.
In this pilot-study, physically fit and unfit older men and women performed an acute exercise test with pre and post blood sampling, but a muscle biopsy sample was only taken at rest and so restricted to one time point. The relatively small participant number and muscle sample amount limits to a certain extent the genetic profiling, muscle protein analysis possibility and thus definitive conclusions thereof due to high interindividual expression. Nevertheless, the examined rare muscle biopsy samples in this susceptible older population give valuable insights into structural and adaptive processes in the ageing musculature.