Physical activity has unequivocal health benefits related to the prevention and treatment of lifestyle disorders associated with metabolic syndrome, obesity and insulin resistance, including type 2 diabetes, cardiovascular and neurodegenerative diseases and some cancers. In addition, regular physical activity is associated with decreases in all cause mortality. Meaning, physical activity = longevity, but not all types of activity contribute equal to our quality of life and longevity. Let's dive into bioenergetics of physical activity and different parameters of exercise.
Fundamentals of exercise bioenergetics
Athletes, but also in fact all humans, ability to exercise ultimately depends on their ability to transform chemical energy into mechanical energy. Skeletal muscles need to synthesize adenosine triphosphate (ATP) for muscle contractions but it is also consumed for energy in many life processes including ion transport, nerve impulse propagation, substrate phosphorylation, and chemical synthesis. These processes, as well as others, create a high demand for ATP. As a result, cells within the human body depend upon the hydrolysis of 100 to 150 moles of ATP per day to ensure proper functioning.
So, ATP is the nucleotide responsible for energy processes in human cells. It is often referred to as the “molecular energy currency” for cells and needs to be constantly synthesized during all work we are doing, and especially exercise. The majority of ATP synthesis occurs in cellular respiration within the mitochondrial matrix.
ATP generation is achieved through two mechanisms - anaerobic and aerobic metabolism. Fats and carbohydrates (CHOs) are two substrates that are mainly used, with some protein contribution. Fat is stored primarily in adipose tissue, but it is also stored in small amounts in skeletal muscles. CHOs are stored as glycogen in skeletal muscles (about 80%) and in the liver (about 15%). Exercise intensity or metabolic and physiological stress, as well as the pattern of recruitment of muscle fibers, will dictate the use of particular energy system and the substrate that is activated, which will then be correlated with different training or „cardio“ zones.
But, let's first define what energy systems enable us to move.
Like most mammals, we humans generate energy via three systems:
· phosphagen (ATP-PC),
· glycolytic, and
· aerobic - oxidative phosphorylation.
All three energy systems are engaged during all forms of physical activity. However, the extent to which each one is involved varies depending on the duration and intensity of the activity.
Most exercise intensity creates ATP through aerobic metabolism, through a process also called oxidative phosphorylation. Regarding of activity intensity, body will use different sources for fuel, and their ratios, as well as engaging different muscle fibers. Please find short summary in form of table below:
How intensity of physical activity gets higher, fats as dominant energy substrate cannot cope with increased demands to synthesize ATP fast enough, so the use of CHO increases and begins to be the predominant energy substrate because the rate of synthesis of new ATP (energy) derived from CHO is faster than that from fat. CHO becomes the main energy substrate used by skeletal muscles at higher exercise intensity (Zone 3+). Above this intensity, ATP cannot be generated fast enough by aerobic glycolysis, so ATP must be generated by an anaerobic mechanism.
Basically, slow movement allows your body to use fat as dominant fuel and as you increase the pace you increase the demand for CHO. Why is that important for our mitochondrial function and longevity?
Types of skeletal muscle fibers
Skeletal muscles consists of 2 types (and some subtypes) of muscle fibers - Type I, also known as "slow twitch fiber", and Type II, or "fast twitch fiber". Fast twich fibers are also divided into two subgroups called type IIa and IIb. Contraction of muscle fibers follows a sequential pattern of recruitment where type I muscle fibers are the first to be recruited.
As the intensity of exercise increases, contractile muscle requirements increase and type I muscle fibers cannot withstand the required demand. Type IIa muscle fibers are activated and eventually, as intensity steadily increases, type IIb will finally be recruited. Simply put, slow fibers are used at lower speeds, and fast at higher speeds (check table above).
Each muscle fiber has different biochemical properties and thus different behaviors during exercise:
· Type I muscle fibers have the highest mitochondrial density and capacity and are therefore very effective in using fats for energy demands.
· Type IIa fibers have a lower mitochondrial density and a higher capacity to utilize glucose.
· Type IIb muscle fibers have low mitochondrial density and a very high capacity to utilize glucose as well as ATP stored in these fibers for needed anaerobic energy.
Therefore, each exercise intensity involves different metabolic responses and patterns of muscle fiber recruitment, which also corresponds to the different training zones. Zones can defined by heart rate intensity, lactate concentration and % of VO2 max values.
Longevity benefits of exercising in Zone 2
Now when we learned that slow twich fibers have the highest mitochondrial density and capacity, and those are dominantly active in Zone 2 we can confirm - Zone 2 training boosts mitochondrial function which directly impacts longevity. So, let's focus on and define Zone 2.
It is the exercise intensity at the one we are stressing mitochondria and oxidative capacity to the most. And activation of mitochondrial stress responses is associated with longevity. Recent studies have provided evidence to support the hypothesis that up-regulation of mitochondrial stress responses contributes to enhanced longevity.
In addition, Zone 2 is where we recruit mainly type I muscle fibers, this is where we mobilize highest amount of fat, both from lipolysis from adipose tissue as well from fat oxidation inside mitochondria. In addition, this is where we stimulate all bioenergetic processes the most – oxidative phosphorylation – we are burning both fat and glucose inside mitochondria. There is really no big glicolitic flux, which would eventually result in increased lactate production.
So, in this training zone, we stimulate type I muscle fibers, thus stimulating the growth and function of mitochondria, which will in addition improve the ability to use fat as a substrate. This is crucial for both longevity and athletic performance.
Next to fat utilization, type I muscle fibers are also responsible for "cleansing" lactate. Lactate is a by-product (not waste!). Of the glucose utilization process, which is used in large quantities as a fuel in fast-twitch muscle fibers. Thus, lactate is mainly produced in fast-twitch muscle fibers which then, export lactate away from these fibers. However, lactate must be "cleansed", otherwise it will accumulate. Then type I muscle fibers play a key role in removing lactate. Training in zone 2 will keep lactates at levels <2mmol/L.
Training in Zone 2 increases mitochondrial density. Training in Zone 2 will not only improve the utilization of fat as a fuel and preserve glycogen stores, but will also increase the capacity to "cleanse" lactate, which is crucial for athletic performance.
Zone 2 training occurs when we perform a steady activity while maintaining a particular heart rate for longer, three or four times a week. It can be accomplished while brisk walking, hiking, biking, or working out on an elliptical machine or stationary bike. This is the exercise intensity that achieves and stimulates mitochondrial function, fat oxidation and lactate clearance capacity the most.
How to know if you are in Zone 2 without monitoring it with some tool such as lactate measurement or heart rate monitor? At this moderate level of training, you can still talk, watch TV or listen to a podcast. Although probably sweating and moving relatively fast, you still do not lose your breath, but your heart rate is elevated (60-70% of your HRmax).
Higher intensity has its role, which we will cover in another blog, but high intensity "cardio" done too frequently is a big mistake made by many people resulting in overtraining issues, such as fatigue, overuse injury, poor sleep, declining performance, and loss of motivation. Also, people practicing only high intensity cardio having chronic elevated cortisol levels, something no one wants to have. This is often seen when people practice i.e. only rowing or running. Those activities and their metabolic demands easily put us in higher zones than Zone 2, with only exception if we are very "cardio-wise" fit and have big endurance capacity.
We advise you to incorporate regular Zone 2 activity in your daily routine and explore and enjoy both longevity and exercise performance benefits in the future!