It is a common sight to see people slumped over or being one with the floor at the end of an exhausting workout. The inertia becomes too much to overcome. It’s difficult to stay up straight. If you’ve been in that position before, there are days when you would very much prefer to remain in that horizontal position rather than get up and get washed up. I’m not laughing at anyone, I’ve personally been in that state quite a few times myself.
But what exactly is happening inside our bodies when we exercise?
We often describe ourselves as being “tired” after an intense workout, but there is actually a fascinating chain of biochemical and thermodynamic processes taking place inside us.
Our muscles need energy to contract, and that energy ultimately comes from the nutrients that we consume. Carbohydrates and fats can be metabolised to produce acetyl-coenzyme A (acetyl-CoA), which then enters the biochemical pathways responsible for extracting energy from these nutrients.
Acetyl-CoA can be obtained from glucose, fatty acids or amino acids that we feed ourselves with. The cell operates the tricarboxylic acid (TCA) cycle to oxidise acetyl-CoA within their mitochondria, and from there this molecule known as adenosine triphosphate (ATP) is formed via oxidative phosphorylation (OXPHOS).
When acetyl-CoA gets shuttled into the TCA, both the reduced form of nicotinamide adenine dinucleotide (or NADH) and the reduced form of flavin adenine dinucleotide (or FADH2) are produced as byproducts of the oxidation reactions going on within the TCA.
These NADH and FADH2 molecules are then oxidised by the electron transport chain (ETC). Electrons are removed from NADH (to form NAD+) and FADH (to form FAD), and the electrons are transported along the chain by Coenzyme Q10 (CoQ10), which plays an important role in helping to shuttle electrons through the ETC, which eventually channels these electrons into molecular oxygen, converting it into water.
What's The Deal With CoQ10 In Our Bodies?
A majority of the human population in today’s modern society is tired and overworked. I feel like that too sometimes - there are all these deadlines at work to meet, mortgages to pay, and the crises that come seem to be neverending.
At the same time, our bodies are also producing heat as this energy is transformed and utilised. The oxidation of nutrients, the generation of ATP and the subsequent conversion of chemical energy into mechanical work are all accompanied by energy dissipation as heat.
A strenuous workout increases the rate at which our muscles consume ATP, forcing our cells to accelerate the metabolic processes that regenerate it. As our rate of energy expenditure increases, so does our rate of heat production.
Our bodies therefore face two simultaneous challenges: we need to continuously generate energy to keep our muscles moving, while also finding a way to dissipate the heat that is produced along the way.
From the perspective of the First Law of Thermodynamics, we can think of our bodies as an energy conversion system.
The chemical energy stored in the nutrients that we consume is transformed into different forms of energy. Some of it ultimately becomes useful mechanical work performed by our muscles, which allows them to contract and generate mechanical movement for our bodies. A significant portion, however, is ultimately dissipated as heat.
But where does all this heat go?
Our bodies have to find a way to dissipate this excess heat into the surrounding environment. One of the most important mechanisms during strenuous exercise is evaporation.
We could look at the overall biochemical energy conversion chain being that of:
Glucose (carbohydrates)/Fats —> Acetyl-CoA —> Carbon dioxide + water + energy
Interestingly, if we were to burn sugar over a fire, we would also get carbon dioxide and water. However, the bioconversions in our body are much more complex than just holding a sugar cube over a burning flame. Instead of releasing all the energy in one burst, our cells extract and capture the energy progressively through a series of controlled biochemical reactions, allowing some of it to be stored in ATP for later use while the remainder is ultimately dissipated as heat.
And this brings us to another interesting connection.
When our body temperature rises, our sweat glands release water onto the surface of our skin in the form of sweat. Some of this water ultimately originates from molecular oxygen that was reduced to water at the end of the ETC, contributing to the body's overall water pool.
The water in the sweat evaporates, removing heat from our bodies via the latent heat of vaporisation.
However, the key to sweating cooling our bodies stems from the fact that it has to evaporate. Sweat that simply drips off our bodies does not contribute to evaporative cooling. The water has left our skin, but it has not undergone the phase change from liquid water to water vapour at the skin’s surface. Consequently, it has not extracted the latent heat of vaporisation from our bodies.
This is particularly noticeable in the tropics, where high humidity means that the surrounding air already contains a large amount of water vapour. The reduced driving force for evaporation makes it more difficult for sweat to evaporate from our skin.
The same problem can occur when we exercise under direct sunlight or in a hot environment. Our bodies may produce sweat very readily as our temperature rises, but if that sweat cannot evaporate efficiently, we may continue accumulating heat despite being covered in perspiration.
In other words, sweating and cooling are not necessarily the same thing.
What matters is not how much sweat our bodies produce, but how much of that sweat actually evaporates.
And that is why a person can finish a workout absolutely drenched in sweat, yet still feel incredibly hot.
But there is another interesting side to this entire process of energy generation.
We have just discussed how the electron transport chain ultimately channels electrons towards molecular oxygen, allowing it to be reduced to water. Ideally, this process should proceed smoothly and efficiently.
But biological systems are rarely perfect.
Some electrons can occasionally “leak” from the electron transport chain before they reach their intended destination. When these electrons react prematurely with molecular oxygen, they can produce reactive oxygen species (ROS), such as superoxide, which can subsequently give rise to other reactive molecules including hydrogen peroxide. As it is said in this book,
superoxide is the primary oxygen free radical produced in mitochondria via the slippage of an electron from the ETC to molecular oxygen during OXPHOS.
In other words, instead of the electron reaching its intended destination and ultimately helping to reduce oxygen into water, an electron can escape the chain and react with oxygen prematurely, producing a highly reactive molecule instead. The accumulation of these reactive molecules can contribute to oxidative stress eventually:
Oxidative Stress Is An Electrochemical Concept That Focuses On An Inappropriate Rate Of Electron Transfer.
The electron is a particle that is responsible for transporting electric currents. Whenever we flick a light switch on at home, we’re connecting up a circuit that allows for the flow of electrons to power up a device that can then convert that electrical energy into light energy, such that our houses get illuminated.
This is one of the trade-offs of OXPHOS. The process is essential for generating the ATP that powers our cells, but the electron transport chain is not perfectly efficient. Some electron leakage is inevitable, and the resulting ROS must be carefully controlled by our body’s antioxidant defence systems.
Antioxidant Protection In The Body Begins From Within The Cell.
Our body is constantly in a tug of war between reduction and oxidation (redox), which basically comprises multiple reactions where electrons are being transferred from one component to another.
Otherwise, we can look at elite athletes such as LeBron James, who has access to some of the world's best training, nutrition and recovery resources. Despite all these efforts to maintain his physical performance, however, even he cannot completely escape the effects of aging.
Why Our Cells Rust from the Inside: The Biochemistry of Aging.
Every morning, millions of people look into the mirror. Over the years, they gradually notice new wrinkles around their eyes, a few more strands of grey hair, and perhaps a face that looks subtly older than it did a decade ago. Aging is so familiar that we often accept these changes as inevitable without ever asking why they occur.
The mitochondria in our keep us alive by converting the food we eat into the energy that powers our cells. Yet the very process that sustains us can also generate reactive molecules that, over decades, may contribute to cellular damage and aging.
Perhaps the challenge of healthy aging is therefore not about stopping the clock, but about keeping our cellular machinery functioning as efficiently as possible for as long as we can.
So, the next time we find ourselves lying on the floor after an intense workout, drenched in sweat and wondering why we feel completely exhausted, we should bear in mind that there is actually an incredible amount of activity happening inside our body.
Consumed nutrients are being broken down and converted into ATP. Muscles are using that ATP to generate movement, while a significant portion of the energy involved is dissipated as heat. Sweat glands then help remove that heat, provided that the sweat is able to evaporate.
At the same time, deep within our mitochondria, the ETC continues to operate. Oxygen is ultimately reduced to water, allowing us to sustain the energy-generating processes that keep our cells alive. Yet the system is not perfectly efficient. Some electrons may leak, producing reactive oxygen species that our bodies must continuously neutralise.
Perhaps this is one of the great paradoxes of being alive.
The same biochemical machinery that gives us the energy to move, exercise and live also produces heat that we must dissipate and reactive molecules that we must keep under control.
Exercise therefore isn’t simply about burning calories or building muscle. It is a constant interplay between energy production, heat generation, water balance and oxidative stress.
And while we cannot stop the passage of time, understanding these processes gives us a better appreciation of what is happening inside our bodies as we age. After all, aging doesn’t suddenly begin when we turn 60 or 70. It is the cumulative result of countless biochemical processes taking place within our cells throughout our entire lives.
So perhaps the real question isn’t whether we can stop aging. It is whether we can give our cells the best possible environment to keep doing their jobs for as long as possible.
That’s where nutrition, exercise, recovery and the way we take care of ourselves may all have a role to play!
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