The Art of the Met: The amazing art of energy metabolism

We’ve covered quite a bit on our first few episodes of “What Does the Science Say?’ and there is so much left to discuss. Before diving even deeper into the physiology of exercise, I felt it would be helpful to discuss the basics of exercise metabolism and energy utilization. How do our bodies provide the energy we need for exercise? And ultimately, how can I use that to my advantage?

The basics

Broadly speaking, we can break down our energy utilization into three main categories: immediate energy (max effort), moderate intensity, and low intensity (endurance).

Many of us may remember (perhaps fondly) the days of high school or college (or even med school) biology and learning about ATP – adenosine triphosphate. ATP is the singular unit of energy for the body, commonly called the “energy currency”. 

Sound simple, right? Our bodies have ATP and we use it. Our bodies constantly produce and utilize ATP for everyday tasks (including non-exercise energy expenditure), yet with exercise our muscles can demand up to ONE THOUSAND times the amount of ATP utilized at rest (1).

This is the reason the body needs 3 modes of energy expenditure. Storing all of that ATP in our cells would be messy, inefficient, and create issues with volume and concentrations at the cellular level. Instead, we store our energy in more compact ways, to be made available later. Think of it like a zip file on your computer. While it can be messy to have dozens or hundreds of small files at once, these can be neatly compressed into a file that can be unzipped later for easier access. Our bodies do this with energy, storing it as glycogen and triglycerides (sugar and fats) to be ‘unzipped’ later.

Immediate energy 

We are constantly utilizing ATP for energy in the body. Because of this, we need to maintain some stores of this compound for immediate use. Our cells store about 5mmol of ATP per kg of muscle, meaning that our bodies have access to this much ATP for energy immediately (2). This means we don’t need to wait, take deep breaths, or break down fats or sugars, we can just go.

But what does that mean for you? That amounts to approximately 6-8 seconds of an all-out sprint. In a true, ‘max effort’ the body will start to fatigue and slow down, even in just a 100-meter dash.

Short-term (moderate-high intensity) – Anaerobic Exercise

So, what happens next? Our bodies will then attempt to keep up with the energy demands as much as it can by breaking down glycogen. The muscles store glycogen for this purpose, to break it down quickly and efficiently, though it is still a relatively messy process.

This type of exercise is considered ‘anaerobic’ exercise, meaning ‘without oxygen.’ Essentially, this means that your oxygen (i.e. breathing) is unable to keep up with the demands needed to efficiently create energy, so there are byproducts produced in this process, most notably lactate. 

Author’s Note: we will explore the concept of lactate and the lactate threshold in a future episode. In short, lactate itself can be further utilized for energy, either being resynthesized into glycogen for future energy storage, or entering another breakdown cycle to produce glucose (sugar) for active energy.

Endurance (low-moderate intensity) – Aerobic Exercise

After just a few minutes of exercise (approximately 4-6 minutes), our oxygen uptake increases significantly (4-5x that at rest). Here we can start to reach our steady-state, aerobic metabolism, or exercise with oxygen.

This is where things start to get interesting. When oxygen is available to be used for exercise and energy creation, our bodies can break down carbohydrates/sugars in a much more efficient way, yielding about 15x as much energy.

What’s more, with oxygen, we have the option to start using other compounds as well. We aren’t just limited to glycogen/carbs, we can burn fat for energy and even proteins (amino acids). In fact, fatty acids represent the most abundant source of energy for the human body (3). 30-50x as much energy is stored in our adipose tissue (fat tissue) than is stored as carbohydrates.

Roughly 2,000 calories of carbohydrates are stored as an energy reserve for the body, compared to 60-100,000 calories stored as fat. That’s over a month of a 2,000-calorie diet!

While there are complex interactions at play to determine the exact modes of energy metabolism -- and there exists significant overlap between them -- we do know that fat metabolism is a bit slower than carbohydrate metabolism. As our exercise intensity increases, we rely more on carbohydrate stores to provide that extra intensity. At lower intensities (up to 50-65% VO2 max) a large amount of the energy is coming from fat metabolism(4). It's worth noting here that there are several other determinants of energy storage, use, and metabolism, and not all can be covered on this episode (and not all are entirely understood by the sciences yet), and we can continue to explore them in later episodes.

Conclusion

Alright, now why is this helpful to know? Although the throwback to med school and memorizing every little detail of the Krebs Cycle (the energy breakdown pathways that create ATP from fats, carbs, and proteins) makes me nostalgic, there’s more to it than that. 

Last author’s note: Ask any physician you know about memorizing the Krebs Cycle. They may not remember the Krebs cycle, but they will certainly remember learning it.

Understanding the basics of this process is incredibly important as we learn to better prepare ourselves for the fitness and health goals we set. In later episodes, we will go into further detail on the specifics of energy use as it relates to performance, so understanding the foundations will be crucial.

Consider questions like:

1. Is my nutrition optimized so that my carbohydrate energy reserves are maximized for my next event?

2. Am I training my lactate threshold to better my endurance performance?

3. Am I training the right muscle types, to maximize the ATP utilization for my intended event?

These are all great questions, and questions we hope to answer for you, so stay tuned, stay active, and get out there.

Sources and Further Readings

1. Baker JS, McCormick MC, Robergs RA. Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise. J Nutr Metab. 2010;2010:905612. 

2. Bonora M, Patergnani S, Rimessi A, De Marchi E, Suski JM, Bononi A, et al. ATP synthesis and storage. Purinergic Signal. 2012 Sep;8(3):343–57. 

3. Wolfe RR. Fat metabolism in exercise. Adv Exp Med Biol. 1998;441:147–56. 

4. Rapoport BI. Metabolic Factors Limiting Performance in Marathon Runners. PLOS Comput Biol. 2010 Oct 21;6(10):e1000960. 

5. Mu WJ, Zhu JY, Chen M, Guo L. Exercise-Mediated Browning of White Adipose Tissue: Its Significance, Mechanism and Effectiveness. Int J Mol Sci [Internet]. 2021 Nov [cited 2024 Mar 13];22(21). Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8583930/

6. Plotkin DL, Roberts MD, Haun CT, Schoenfeld BJ. Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives. Sports. 2021 Sep 10;9(9):127.