Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle

TL;DR

? Different exercise intensities impact mitochondrial growth and capillary density in muscles, both essential for oxygen and nutrient delivery to cells.

? Sprint Interval Training (SIT) is significantly more efficient than Endurance Training (ET) or High-Intensity Training (HIT) in enhancing mitochondrial content per hour.

? Capillarisation improvements plateau after four weeks, with different training intensities affecting muscle growth differently.

I recently came across this interesting systematic review paper published in the Journal of Sports Medicine looking at a question which I often think about. What is the best way to train a person to achieve differentiated goals - like higher Vo2 max, endurance training or muscle growth. The reason this is important is that too many people wrongly apply the same protocol for everyone and this leads to a plateau effect very quickly.

In this review the authors study the global literature and develop a formula (at the end of the newsletter) for how you can change your fitness routine to match your goals. Happy Reading!

The Powerhouses of Health: Mitochondria and Muscle Capillarization

In the quest for longevity and optimal performance, understanding the biology behind muscle adaptation is key. Mitochondria and capillaries in skeletal muscle play fundamental roles in how efficiently our bodies produce energy, recover from exertion, and maintain overall health. While exercise is known to stimulate growth in these vital structures, recent findings reveal how different types of exercise—Endurance Training (ET), High-Intensity Interval Training (HIT), and Sprint Interval Training (SIT)—impact these adaptations and shape recommendations for health and performance.

What are Mitochondria?

Mitochondria are small, double-membraned structures known as the “powerhouses” of the cell. These organelles generate energy by converting nutrients from our food, primarily carbohydrates and fats, into adenosine triphosphate (ATP)—the cell’s energy currency. ATP powers nearly all cellular activities, from muscle contraction to protein synthesis, making mitochondria essential for both movement and overall health.

Inside mitochondria, a process known as oxidative phosphorylation takes place. Nutrients are broken down through metabolic pathways, where electrons are transferred across protein complexes embedded in the mitochondrial membrane. This transfer creates an electrochemical gradient, which ultimately drives the synthesis of ATP. Muscle cells, especially in skeletal muscles, require a high energy supply, so they contain a high density of mitochondria to meet their demands. Enhancing mitochondrial quantity and function has been associated not only with better athletic performance but also with reduced risk of chronic diseases, improved metabolic health, and increased longevity.

Exercise and Mitochondrial Biogenesis

Exercise acts as a trigger for mitochondrial biogenesis, the process by which new mitochondria are produced in cells. As muscles contract during physical activity, they consume large amounts of ATP, sending a signal that additional energy is needed. This activates signaling pathways that stimulate the replication and growth of mitochondria, enabling cells to meet increased energy demands more efficiently. Improved mitochondrial content and function support higher endurance, faster recovery, and better overall resilience to physical and metabolic stress.

This systematic review examined three exercise intensities—ET, HIT, and SIT—to determine their effects on mitochondrial growth, finding that:

? All three forms of training—ET, HIT, and SIT—boost mitochondrial content in muscle. ET showed a 23% increase, HIT a 27% increase, and SIT a 27% increase.

? SIT is highly efficient per unit of time, boosting mitochondrial content roughly 3.9 times more per hour than ET and 2.3 times more than HIT. This means a person short on time could maximize mitochondrial gains with SIT sessions, even with limited exercise minutes.

? Frequency matters: Training six days per week produced more significant mitochondrial gains than fewer sessions, suggesting that for optimal mitochondrial biogenesis, higher frequency is beneficial. Importantly, this increase was consistent across different ages, fitness levels, and even in people with health conditions such as cardiovascular diseases.

The Role of Capillaries in Muscle Health

Capillaries, the smallest blood vessels in the body, are critical for delivering oxygen and nutrients to muscles, especially during exercise. When we increase the density of capillaries within muscle tissue, we improve the ability of cells to uptake oxygen, remove waste, and support aerobic performance. Capillaries also provide essential support to mitochondria, ensuring they receive the oxygen needed to produce ATP.

The study highlighted that exercise types impacted capillary adaptations differently:

? Endurance Training (ET) was found to be particularly effective in increasing capillary density in muscle tissue. Participants who engaged in ET experienced greater growth in capillaries per square millimeter of muscle, enhancing oxygen delivery across a larger area and supporting long-duration aerobic activities. This is especially beneficial for endurance athletes who rely on continuous oxygen supply to sustain long bouts of activity.

? Sprint Interval Training (SIT) and High-Intensity Interval Training (HIT) increased the number of capillaries per muscle fiber but did not achieve the same density as ET. This suggests that while SIT and HIT can improve capillary network within muscle fibers, they may be less effective at promoting capillarization across a broader muscle area.

? Training Frequency and Early Gains: Gains in capillary density generally occurred within the first four weeks of training, particularly in untrained individuals. This initial period seems crucial for building a solid vascular foundation in muscles, with minimal additional gains beyond this phase in the same intensity regimen.

VO2max and Energy Efficiency

VO2max, or maximal oxygen uptake, is a critical measure of cardiovascular fitness and reflects the body’s ability to supply oxygen to working muscles. Improvements in VO2max are associated with better endurance, faster recovery, and reduced fatigue.

Interestingly, the study found that while all three training types—ET, HIT, and SIT—increased VO2max, the efficiencies varied:

? High-Intensity Interval Training (HIT) showed the greatest improvement in VO2max, making it slightly more effective than both ET and SIT in boosting cardiovascular capacity.

? Sprint Interval Training (SIT) proved the most time-efficient for increasing VO2max, offering maximum gain per exercise minute. For those looking to quickly boost aerobic capacity with minimal time investment, SIT provides a powerful solution.

? Demographic Differences: Women showed more significant VO2max percentage gains than men, and individuals with lower initial fitness levels experienced greater improvements across mitochondrial content, capillarization, and VO2max than their more trained counterparts.

Summary of Practical Takeaways

Understanding how different training intensities impact mitochondrial and capillary adaptations allows for a more strategic approach to exercise:

1. Maximizing Mitochondrial Health in Less Time: For those with limited time, Sprint Interval Training (SIT) is exceptionally efficient. Short, high-intensity bursts (e.g., 4-6 sprints of 30-90 seconds) stimulate substantial mitochondrial growth and VO2max gains, making this approach ideal for high-impact results in minimal time.

2. Building Endurance and Oxygen Delivery: Endurance Training (ET) is best suited for those aiming to increase capillary density across muscle tissues. Longer, continuous, lower-intensity sessions (like jogging or cycling for 30+ minutes) support greater oxygen delivery across muscle groups, enhancing endurance capabilities.

3. Training Frequency Matters: Mitochondrial adaptations benefit significantly from higher training frequencies. For those aiming to boost mitochondrial function, exercising at least 4-6 times per week yields more pronounced effects, especially in untrained individuals who may see accelerated gains.

4. Capillarization Peaks Early: Initial training phases, especially in the first four weeks, are key to enhancing capillary density. Maintaining consistency through this period is vital, even as capillary growth begins to plateau. Use this phase to establish a robust vascular base before transitioning to maintenance routines or varying intensities.

5. Optimize VO2max with HIT and SIT: For individuals looking to improve cardiovascular fitness quickly, both HIT and SIT provide effective routes to boost VO2max, with SIT offering the most efficient results per exercise minute. Incorporating HIT sessions (like 4-minute high-intensity intervals with 3-minute recovery) is especially useful for sustaining long-term cardiovascular health gains.

By selectively combining these exercise types and adjusting frequency based on personal goals, you can biohack mitochondrial and vascular health, supporting longevity, resilience, and high performance in daily life and athletic pursuits.

If you would like to know more about how to apply these lessons to your own personalised longevity program then drop me an email at [email protected]. I will have a member of our clinical care team get in touch with you and assess your applicability to join our next cohort of participants starting in early 2025.

You can read the entire paper here:

https://link.springer.com/article/10.1007/s40279-024-02120-2