Modulation of Mitochondrial Biogenesis Through Manipulations of Endurance Exercise Volume and Intensity
The mitochondria is often referred to as “the powerhouse of the cell” and for good reason; it is responsible for generating most of the energy your body uses. Over the last 20 years we have learned a great deal more about the complex signaling pathways that help improve the functioning of our bodies' mitochondria, and we move ever more towards optimizing those adaptations through manipulations of exercise intensity and duration. This is of particular interest for endurance athletes whose success is rooted in maximizing energy production, however it is also important for health due to mitochondria dysfunction being common in individuals with metabolic syndrome (Kolodziej and O’Halloran, 2021). The creation of new mitochondria is referred to as mitochondrial biogenesis.
While much remains to be determined, research has shown that we can increase the amount of mitochondria (also referred to as mitochondrial content) and improve mitochondrial function/efficiency (the amount of energy produced by each mitochondrial matrix). Current evidence suggests that endurance exercise volume is the primary driver of changes in mitochondrial content, while exercise intensity is the primary driver of changes in mitochondrial functioning. Over the rest of the paper I will elaborate on the signalling pathways responsible for these differing adaptations as well as outline the evidence supporting these conclusions.
One of the difficulties in studying mitochondrial is that there is not a consensus on the best way to measure mitochondrial biogenesis, or even complete consensus on the definition of mitochondrial biogenesis (Bishop et al, 2018). The gold standards for measurement are histological in nature, and those methods require expensive equipment, are time intensive, and frequently invasive. Due to that scientists often measure the activity of particular biomarkers to provide an estimate of mitochondrial biogenesis (Granata et al, 2018). Two of the most common are Citrate Synthase (CS) and mitochondrial protein synthesis (MitoPS).
It is no surprise for such an important organelle that there is not just one signalling pathway. While likely not comprehensive in this essay we are going to focus on three of the most important signalling events: Calcium (Ca2+) release, increase of AMP relative to ATP, and the production of Reactive Oxygen Species (ROS) (Bishop, Granata, and Eynon, 2014). Please refer to Appendix 1 for a graphic outlining some of the events (Lundby and Jacobs, 2016). All of these processes create a cascade that leads to the activation of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) which is considered to be the regulator of mitochondrial biogenesis (Wu et al, 2002), though some there are countering views on the exact role it plays (Islam, Edgett, and Gurd, 2018).
Calcium is a key component of muscular contraction and its release and proliferation in the cytosol occurs to a great degree during endurance exercise. This increase of calcium ions causes activation of calcium/calmodulin-dependent kinase which triggers a cascade ending in PGC-1a activation (Wright et al, 2007). As for the production of ROS, the mitochondria is the main producer of superoxide in the cell, which in turn can go on to form both hydrogen peroxide and hydroxyl radical (which are both ROS). This increase in oxidative stress goes into overdrive during exercise and activated p38 mitogen-activated protein kinase (MAPK) which eventually signals PGC-1a (Kulisz et al, 2002). However, it is important to note that while some ROS exposure is good, too much can have detrimental effects on the body (including mitochondrial production(Bouchez and Devin, 2019)). During exercise our body rapidly consumes adenosine triphosphate (ATP) for energy leading to the formation of more adenosine monophosphate (AMP), the shift in the ATP:AMP ratio towards AMP activated AMP-activated protein kinase (AMPK) which can also signal PGC-1a (Lundby and Jacobs, 2016). Lactate may also play a role in mitochondrial biogenesis, but the exact mechanisms remain to be elucidated (Genders et al, 2019)
Just by looking at the different pathways we can start to piece together methods of using exercise interventions to trigger beneficial adaptations in our mitochondria. From a research perspective the main independent variables that have been examined for exercise prescription are intensity and volume, with in general the two having an inverse relationship. Continuous exercise tends to be designated as either low or moderate intensity, and intermittent exercise is usually classified as either high intensity interval training (HIIT) or sprint interval training (SIT). While these two terms are often used interchangeably, for this review I will be referring to HIIT as any exercise performed above the lactate threshold and up to ~110% of power output at VO2max, and SIT as training done above that.
One limitation that goes beyond the scope of this review is that the training status of the individuals will likely have an impact on their adaptations; sedentary subjects will likely see mitochondrial improvements regardless of interventions while endurance athletes will need further stimulus. Patients already with mitochondrial dysfunction may present different challenges. Additionally, in terms of health endurance training, improved mitochondrial functioning is only one of a host of beneficial adaptations from cardiovascular exercise.
Current research suggests that in terms of increasing mitochondrial content volume is the greater driver of adaptation. A review by Bishop, Granata, and Eynon (2014) pooled data from many different studies to look at the correlation between endurance volume and intensity. They found that there was a strong correlation (R2 = 0.80) between total training volume and CS activity (see Appendix 2). In fact, they found that with increasing volume there was no plateau to the increase of CS activity in type I muscle fibers. When looking at all the studies that have compared different volumes of training, higher training volume always leads to higher CS activity (Bishop, Botella, and Granata, 2019). Because there is an inverse relationship between sustainable intensity and volume, increasing volume will likely come at the cost of intensity. It is important to note that there is likely a minimum intensity needed to trigger adaptations; multiple studies have shown that intensities of ~70-80% vo2max had greater impact on PGC-1a mRNA than work-matched training at ~40-50% (Granata, Jammick, and Bishop, 2018). This indicates that for mitochondrial content there is likely a sweet spot between volume and intensity.
However, in terms of increasing mitochondrial function intensity looks to be much more important. A pooled data set by Granata, Jammick, and Bishop (2018) found that there appeared to be a cut off point for intensity at 90% of vo2max; studies at or above that intensity found mass-specific improvements in mitochondrial respiration while studies below that intensity didn’t. Volume does likely play a small role in that increasing volumes of intense work are likely needed to spur further adaptations to HIIT (Bishop et al, 2019). Research indicates that SIT may be a particularly efficient way to achieve increases in mitochondrial respiration. One specific study showed that training with a series of 30s sprints led to significantly greater increases in mitochondrial respiration than either moderate intensity continuous training or HIIT (Granata et al, 2016). In addition, higher intensity training may be beneficial in activating higher threshold type II motor units (Popov, 2018).
In terms of why volume and intensity have markedly different effects on mitochondria content and function we can look at the different signalling pathways involved. Chen et al (2003) found significantly greater AMPK activation following high intensity exercise than low or moderate intensity. This is not surprising, because the high intensity group had an AMP/ATP ratio that was almost 5 fold higher than moderate (and 26x higher than low). During low/moderate intensity exercise relying primarily on fat oxidation we won’t see a huge amount of AMP created so it makes sense that AMPK activation is also lower. During SIT we also see significant increase of AMPK as well as increase of p38 MAPK (Hargreaves et al, 2005). AMPK leads to a catabolic cascade and for mitochondrial specific adaptations this leads to mitophagy; the remodeling of old mitochondria (Laker et al, 2017). So, we don’t see increases in mitochondrial content because the net amount stays the same, but it now functions more effectively. Lower intensity continuous exercise on the other hand likely stimulates PGC-1a primarily through increases in calcium levels; Rose et al (2007) found that 1-2hrs of training at moderate intensity caused a significant increase in CAMK signaling.
This theme of separating volume and intensity is reflected in the current training paradigms of elite endurance athletes. Over the last 15 years there has been a move towards increasingly polarized training models; a vast majority of the training is performed for high volume at a low/moderate intensity, and the rest is performed at a very high intensity, with a minimal amount occurring in the middle zone. In terms of oxidative adaptations this seems to fit a best of both worlds approach, targeting both mitochondrial volume and respiration. By further understanding the mechanisms underlying these adaptations we can continue to dial in more specific workouts for athletes depending on the needs of their sports and their physiology.
High volume endurance training stimulates mitochondrial biogenesis via CAMK signalling and is the primary driver in mitochondrial content. High intensity endurance training on the other hand stimulates mitochondrial biogenesis primarily by the AMPK and p38 pathways and helps improve mitochondrial function. Based on endurance training methodology it seems that these two modes work synergistically which indicates that future research should look at mixed intensity/volume models, however little to no research has been done on that. Additionally, more research needs to be done over increasing time spans, with more varied subject pools (particularly subjects with metabolic dysfunction), and examining other variables such as frequency, effect of resistance training, and nutritional status. Hopefully by taking a holistic approach we can continue to fine tune our knowledge of mitochondrial adaptations and begin to prescribe exercise interventions focused on superior health outcomes.
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