Rethinking Exercise Intensity: The Power of Aerobic Energy Expenditure
- Dr. Scott Christie
- May 16
- 5 min read
What if the way we've been measuring exercise intensity is missing a critical piece of the puzzle?
For decades, oxygen uptake (VO2) has been the gold standard for gauging how hard you're working during a workout. But emerging research suggests there’s a better way: aerobic energy expenditure. This metric could unlock new insights into performance, helping athletes, fitness enthusiasts, and even casual exercisers optimize their training like never before.
Let’s explore why this matters and how you can apply it to supercharge your performance.
The VO2 Paradigm: A Solid Foundation, But is it Enough?
If you’ve spent any time in serious training, you’ve probably come across the term VO2 Max. For those unfamiliar, VO2 Max refers to the maximum amount of oxygen your body can utilize during exercise. It’s a fundamental concept in exercise physiology, often used to set training zones and estimate endurance potential. But here’s the thing, VO2 Max doesn’t tell the whole story. A 2018 study by Beck, Kipp, and Byrnes published in the Journal of Applied Physiology challenges this assumption, showing that oxygen uptake alone isn’t the full picture.
The researchers point out a major flaw in VO2 Max: it fails to account for substrate utilization, the dynamic balance between fats and carbohydrates your body uses for fuel during exercise. This fuel mix shifts with exercise intensity: lower intensities burn more fat, while higher intensities rely heavily on carbohydrates.
Since VO2 Max overlooks these fuel shifts, it can lead to inaccurate estimates of actual energy expenditure. To overcome this limitation, the authors propose using aerobic energy expenditure, a metric that incorporates both oxygen intake and carbon dioxide output. They contend that this approach provides a more precise reflection of the body’s energy demands and offers a stronger prediction of performance. But is it really a better alternative? Ongoing studies are fuelling a thought-provoking debate that could redefine how we measure and optimize endurance training.
The Case for Aerobic Energy Expenditure
Building on Beck et al.’s work, recent studies further suggest that energy expenditure may be a more accurate measure than VO2 Max alone. For example, a 2018 study published in Frontiers in Physiology by Panissa et al.
revealed that VO2-based measurements tend to underestimate energy expenditure during high-intensity interval training (HIIT). The reason? HIIT relies heavily on anaerobic energy systems something VO2 Max doesn’t capture. By including factors like excess post-exercise oxygen consumption (EPOC), also known as the "afterburn" effect, and lactate production, the researchers demonstrated that total energy expenditure provides a more complete view of workout intensity. This is particularly important for athletes engaged in short, explosive efforts, such as sprinters or CrossFit enthusiasts.
Likewise, a 2024 study published in Scientific Reports by Liu et al. compared high-intensity interval training (HIIT) with moderate-intensity continuous training (MICT) in men with obesity. Despite both workouts being matched for total work, HIIT produced a significantly higher excess post-exercise oxygen consumption (EPOC) indicating greater post-workout energy burn. This finding highlights the advantage of energy-based metrics in capturing the full metabolic cost of exercise an aspect that VO2 alone fails to measure.
At ISHP, we view this as a breakthrough in performance training. Our Bio-PerformX Endurance program goes beyond traditional VO2 testing by incorporating advanced metabolic assessments that measure substrate utilization and total energy expenditure. This comprehensive approach allows us to pinpoint your unique energy profile and create personalized training zones that truly reflect how your body performs.
The Skeptics: Why VO2 Still Holds Strong
Not everyone is ready to move on from VO2 Max. Critics argue that while aerobic energy expenditure shows promise, it doesn’t consistently outperform traditional metrics. In a 2018 commentary in the Journal of Applied Physiology, Barnes et al. noted that for steady-state endurance events like marathons, VO2-based metrics combined with thresholds such as lactate or ventilatory thresholds remain highly reliable. The reason? Marathon runners typically maintain a stable respiratory exchange ratio, meaning their substrate utilization doesn’t fluctuate much offering minimal added value for predicting performance.
Another critique comes from a 2019 study in Experimental Physiology by Poole, which emphasizes VO2max, lactate threshold, and exercise economy as the trifecta for endurance performance. These metrics are well-established, backed by decades of data, and easier to measure than energy expenditure, which requires precise carbon dioxide measurements. For coaches and athletes, the practicality of VO2 often wins out.
So, who’s right? The truth likely depends on your goals. For steady-state endurance, VO2 remains a trusty guide. But for high-intensity or variable workouts, energy expenditure offers a richer view of what’s happening in your body.
Why This Matters for You
This debate isn’t just academic it has real-world implications for how you train. Understanding energy expenditure can help you:
Optimize Training Zones: By focusing on energy expenditure, you can fine-tune your workouts to target specific metabolic demands, whether you’re burning fat for weight loss or carbs for race-day performance.
Maximize Recovery: High-intensity workouts with high EPOC require more recovery time. Knowing your energy expenditure helps you plan rest days to avoid burnout.
Boost Performance: Energy-based metrics can reveal inefficiencies in your metabolism, letting you tweak nutrition or training to improve endurance.
At ISHP, we’re harnessing this science to empower our clients. Our Bio-PerformX Endurance Program uses state-of-the-art metabolic carts to measure your VO2, VCO2, and energy expenditure during exercise. We then create personalized training plans that align with your goals whether it’s running a faster 5K, cycling longer, or simply feeling stronger. Our approach bridges the gap between cutting-edge research and practical application, giving you an edge in performance.
Actionable Steps to Apply This Science
Ready to rethink exercise intensity? Here’s how to start:
Get Tested: Visit a facility like ISHP to undergo metabolic testing. This will reveal your VO2max, substrate utilization, and energy expenditure across intensities.
Train by Energy Zones: Use energy expenditure data to set training zones. For fat-burning, target lower intensities where fat oxidation peaks. For performance, focus on higher intensities where carbs fuel your effort.
Incorporate HIIT Wisely: High-intensity workouts burn more energy post-exercise. Include 1–2 HIIT sessions per week, but balance them with adequate recovery to avoid overtraining.
Track and Adjust: Use wearables or apps to estimate energy expenditure during workouts. While not as precise as lab tests, they can guide your efforts.
Fuel Smart: Your substrate utilization data can inform nutrition. For high-carb efforts, load up on complex carbs pre-workout. For fat-burning sessions, prioritize healthy fats in your diet.
The Future of Fitness Is Here
The shift from VO2 to aerobic energy expenditure is more than a scientific debate it’s a call to rethink how we approach fitness. By focusing on the energy your body actually expends, you can train with precision, recover smarter, and perform better.
At the Institute of Science, Health and Performance, we’re leading the charge with our Bio-PerformX Endurance Program, blending cutting-edge research with personalized metrics to help you achieve your best.
Want to see how this science can elevate your performance? Book a metabolic assessment today. Let’s personalize your training and unlock your full potential, one breath, one calorie, one breakthrough at a time.
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