HIIT Workouts Turbocharge Our Mitochondria’s Superpowers

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About 2,400 years ago, Hippocrates, whom many consider the father of modern medicine, prescribed physical activity as a drug-free way to keep people healthy. He viewed exercise as a possible treatment for disease and is credited with saying, "Walking is the best medicine."

Even though a growing number of 21st-century doctors view "exercise as medicine," exactly how and why physical activity helps to offset and even prevent disease has been difficult for exercise physiologists to pin down.

Because we still don't have enough evidence-based knowledge about how best to prescribe Rx "doses" of cardio and strength-building workouts or how to tailor exercise regimens to treat specific conditions, the therapeutic potential of using exercise as medicine often falls short.

But thanks to recent advances in state-of-the-art technology, researchers are now able to map how an individual's cellular building blocks respond to different doses (duration, intensity, frequency) of exercise using a new technique called mass spectrometry.

Exercise Is Medicine: High-Intensity Interval Training (HIIT) Enhances Mitochondrial Bioenergetics

A potentially groundbreaking study (Granata et al., 2021) published on December 3 in the peer-reviewed journal Nature Communications decrypts how mitochondria in muscle cells—colloquially referred to as cellular powerhouses—respond to different high-intensity interval training protocols.

These findings could pave the way for creating personalized fitness prescriptives that optimize the health benefits associated with using exercise as medicine.

For this study, Granata et al. recruited 10 healthy men between ages 18-35 who were non-smokers and medication-free. These volunteers were "moderately fit" but didn't do more than four hours of unstructured aerobic activity per week.

Using mass-spectrometry equipment at the University of Melbourne's Bio21 Molecular Science and Biotechnology Institute, first author Cesare Granata and colleagues were able to identify how 185 differentially expressed mitochondrial proteins respond to three different "doses" of HIIT.

These three volume-based doses of HIIT were administered in three phases:

• First phase, normal-volume training (NVT)
• Second phase, high-volume training (HVT)
• Third phase, reduced-volume training (RVT).

Each HIIT session started with an eight-minute warm-up and consisted of a 2:1 work-to-rest ratio (four minutes very hard, two minutes super easy).

During a series of three experiments lasting from one to three weeks, each participant did different volumes (NVT, HVT, RVT) of high-intensity interval training on a stationary bike in an exercise physiology lab.

NVT, HVT, and RVT Represent Three Different Volumes of HIIT

1. NVT Phase: The normal-volume training sessions consisted of six HIIT sessions over the course of two weeks (three HIIT workouts per week ). NVT involved five to seven intervals of vigorous cycling for four minutes, followed by a two-minute recovery.
2. HVT Phase: The high-volume training sessions consisted of twice-daily HIIT workouts for 20 consecutive days. HVT training sessions involved either seven to ten intervals lasting four minutes interspersed with a two-minute recovery or 15–20 short intervals lasting two minutes with a one-minute recovery.
3. RVT Phase: The reduced-volume training phase consisted of six HIIT sessions over six days. Training volume was gradually reduced throughout the week; participants performed 10, 9, 8, 7, 6, and then 4 intervals lasting four minutes interspersed with a two-minute recovery.

Interestingly, after analyzing the mitochondria of each participant throughout these sequential HIIT protocols, the researchers unearthed an intricate and previously unrecognized network of differentially prioritized mitochondrial adaptations to high-intensity interval training. As the authors sum up in their concluding remarks:

"We identified 185 differentially expressed mitochondrial proteins, an ~10-fold greater number than previous studies. This increased depth of analysis enabled us to identify the time-dependent and complex remodeling of the mitochondrial proteome following different phases of endurance exercise training."

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As we get older, mitochondrial defects and the deterioration of our mitochondria's "energy powerhouse" capacity are implicated in many diseases. The latest (2021) research by Granata et al. untangles how high-intensity interval training can improve mitochondrial bioenergetics and how HIIT workouts may slow age-related decline or prevent illness.

"This study is an important step towards eventually updating current exercise guidelines for specific conditions with individualized prescriptions to best improve mitochondria," the authors explain in a December 2021 news release.

Beyond HIIT workouts on a stationary bicycle, more research is needed on how other cardio and strength-building exercise regimens affect mitochondrial bioenergetics. Nonetheless, the recent discovery that different "doses" of vigorous exercise enhance our mitochondria's ability to produce energy has clinical applications for helping patients achieve better health outcomes via physical activity.