Energy Systems, Mitochondrial Fitness and Zone 2 Exercise

Woman running on a trail surrounded by greenery. The runner is wearing a neon green racerback tank top with cyan exercise shorts and gray sneakers.

For this post, we’ll dig into some details on just how your body makes the energy it needs during exercise. In order to do that, we’re first going to introduce: the small but mighty mitochondria!

Mitochondrial Fitness

Mitochondria are specialized components of your cells. What they specialize in is converting the energy from food into energy your cells can use. They take things like pyruvate (from glucose) and fatty acids (from fats) and convert them into ATP (adenosine triphosphate), which your cells can use for energy. Mitochondrial ‘fitness’ just refers to how efficient they are at doing this job—and that can depend on how big they are and how many of them are in a cell. The bigger they are and the more of them there are, the better the energy output capabilities. Which leads to….

Mitochondrial Biogenesis

This term refers to the growth of new and additional mitochondria in cells to aid energy/ATP production in response to stress and exercise adaptation. It tends to decrease with age and disease (boo entropy). This process is supported by autophagy and the recycling of damaged mitochondria, exercise and cold therapy. Certain phytonutrients and natural compounds have also been shown to support mitochondrial biogenesis. These include Co-enzyme Q10, rosemary, polyphenols and antioxidants like astaxanthin1 and grapeseed extract2. (Check out Gandalf™ Astaxanthin(Sold in Canada) and Flora’s Beyond Grape Seed*.)

Exercise Performance

When it comes to our muscles powering our performance for exercise, we have:

  • Slow twitch muscle fiber: Rich in mitochondria; used for low force, low intensity exercise; slower to fatigue; predominately burn fatty acids for fuel; employed in endurance exercise.
  • Fast twitch muscle fiber: High force, high intensity, quick to fatigue; predominately burn glucose for fuel; employed in sprinting and weightlifting.
  • Our muscles, or to be more accurate, the mitochondria within the cells of our muscle tissue have some options when it comes to producing energy:
  • Aerobic glycolysis: using glucose for fuel in the presence of oxygen; for low to moderate exercise of 1-2 hours maximum.
  • Aerobic lipolysis: Burning fatty acids for fuel in the presence of oxygen; for low to moderate exercise this becomes predominate after about 50 minutes and can last for many hours. It aids our endurance by sparing glycogen (glucose) reserves—generally, as the duration or time spent exercising increases, intensity decreases (and more oxygen is available to cells), and fat becomes the more important fuel source over the long term. Definitely something to consider, diet-wise for endurance runners.
  • Anaerobic metabolism: This is where things get interesting! This is where your body is creating energy (ATP) in the absence of oxygen—generally because the intensity of exercise is such that the lungs cannot provide enough oxygen to cells for aerobic metabolism; the mitochondria alone can’t provide enough ATP at this point, so energy is produced within the cytosol—a series of liquid, membrane compartments in the cell. Fatty acids cannot be used for this process, so glucose is converted to pyruvate and lactic acid as energy sources. That pyruvate can then go into the mitochondria to be used for fuel and the liver can convert some lactic acid back into glucose. Interestingly, the brain can also use lactic acid for fuel and it’s actually excess hydrogen as a waste product from all this ATP production that causes muscle pain and discomfort—not lactic acid itself.

These energy producing systems and pathways work concurrently with aerobic metabolism being dominant for longer periods of exercise and anaerobic for short, intense bursts.

Exercise Zones

This is a way of looking at exercise intensity according to what’s going on in your cells in terms of energy production.

  • Zone 1: Basic movements, walking, stairs
  • Zone 2: About 60-70% of maximal heart rate and exertion; slow twitch muscle fibers are at full exertion; the highest rate of fat oxidation, highest amount of ATP from mitochondria under aerobic conditions. This zone Improves mitochondrial density and function.
  • Zone 3: An increase in glucose usage for fuel and a decrease in fat usage; lactic acid levels increase and total ATP production increases. The body transitions from fat oxidation to mostly using glucose.
  • Zone 4: Zero fat oxidation for energy; lots of lactate produced due to increased use of glucose for fuel. Cell cytosol is also burning glucose for increased ATP needs.

How smoothly you are able to transition between these zones reflects how smoothly the cells in your body are able to transition between the different energy production pathways we covered.

Elite Athletes

In terms of what sets elite athletes apart from us mortals—at least when it comes to cellular energy production—they:

  • Generally burn fuel more efficiently due to ‘mitochondrial fitness’. Less glucose is required for fuel and there is more efficient fat oxidation.
  • Have better clearance and re-use of lactic acid as fuel so it doesn’t enter general circulation.
  • Are able to use slow twitch muscle fiber for longer and at higher intensity before needing to recruit fast twitch muscle fiber. They also have Increased size and number of mitochondria in cells.
  • Conversely, in diabetics, they don’t produce cellular energy very well and even at rest are often needing to burn glucose for fuel—due to poor mitochondrial function and density.
  • Exercise increases non-insulin dependent glucose uptake in muscles—through contractions—so less insulin is required from the pancreas. This is how exercise is beneficial for blood-sugar balance.

Wrapping Up

So, with a better understanding of the importance of mitochondrial fitness when it comes to energy and exercise, you may want to consider making some tweaks to your diet and supplementation. Aside from the grapeseed extract and astaxanthin mentioned earlier, omega-3 fatty acids also play a role3 in efficient mitochondrial functioning and turmeric has been shown to have protective4, antioxidant5 effects on mitochondria and their functioning as well. Omega Sport+ from Flora is a liquid oil blend designed for energy and exercise performance and contains omega-3 fatty acids, turmeric, vitamin D and MCTs*. For more information, read these other articles on our blog.

Use code SPORT15 for 15% Off Omega Sport+. Promo code valid through 5/7/2023 in the U.S. and Canada.

  1. Nishida, Yasuhiro, Allah Nawaz, K. T. Hecht, and Kazuyuki Tobe. “Astaxanthin as a Novel Mitochondrial Regulator: A New Aspect of Carotenoids, beyond Antioxidants.” Nutrients 14, no. 1 (December 27, 2021): 107.
  2. Cerbaro, Aline Fagundes, Victoria Rodrigues, Marina Rigotti, Cátia Dos Santos Branco, Giovana Rech, Diogo Losch De Oliveira, and Mirian Salvador. “Grape Seed Proanthocyanidins Improves Mitochondrial Function and Reduces Oxidative Stress through an Increase in Sirtuin 3 Expression in EA.Hy926 Cells in High Glucose Condition.” Molecular Biology Reports 47, no. 5 (April 7, 2020): 3319–30.
  3. Herbst, Eric A.F., S. Paglialunga, Christopher Gerling, Jamie Whitfield, Koji Mukai, Adrian Chabowski, George J. F. Heigenhauser, Lawrence L. Spriet, and Graham P. Holloway. “Omega-3 Supplementation Alters Mitochondrial Membrane Composition and Respiration Kinetics in Human Skeletal Muscle.” The Journal of Physiology 592, no. 6 (March 15, 2014): 1341–52.
  4. Bagheri, Hossein, Faezeh Ghasemi, George E. Barreto, Rouhullah Rafiee, Thozhukat Sathyapalan, and Amirhossein Sahebkar. “Effects of Curcumin on Mitochondria in Neurodegenerative Diseases.” Biofactors 46, no. 1 (October 3, 2019): 5–20.
  5. Gibellini, Lara, Elena Bianchini, Sara De Biasi, Milena Nasi, Andrea Cossarizza, and Marcello Pinti. “Natural Compounds Modulating Mitochondrial Functions.” Evidence-Based Complementary and Alternative Medicine 2015 (June 18, 2015): 1–13.