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Posted on Monday March 23, 2026 in Naked Heart
An article written by Dr Edward Leatham, Consultant Cardiologist © 2026 E.Leatham
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If you wanted to design the perfect engine, you would probably start with something highly efficient, endlessly adaptable, and capable of switching fuels depending on demand. You might imagine a sophisticated hybrid vehicle — one that uses electric power for rapid acceleration, fuel combustion for long-distance efficiency, and intelligent systems to optimise performance under changing conditions.
Nature has already built something better.
The human heart is the ultimate hybrid engine — a metabolic omnivore that continuously selects the best available fuel to keep us alive. Unlike any machine humans have designed, it does not rely on a single energy source. Instead, it flexibly switches between free fatty acids, glucose, and ketones, adjusting moment by moment to workload, oxygen availability, hormones, and nutritional state.
In many ways, the heart behaves like an advanced hybrid car — but with capabilities that would make any engineer jealous.
Before exploring fuels, it is worth remembering the scale of the task.
The heart beats roughly:
It must produce ATP continuously, with almost no rest periods. Energy failure is not an option.
Unlike skeletal muscle, the heart cannot pause and repay energy debt later. That means its energy system has evolved around two key principles:
This is where the hybrid analogy becomes useful.
Imagine a modern hybrid vehicle.
The heart behaves in a similar way.
Glucose is like the electric motor:
When the heart needs to work harder — during exercise, stress, or increased sympathetic drive — glucose metabolism becomes more prominent.
Why?
Because glucose oxidation produces more ATP per molecule of oxygen than fatty acids. In a situation where oxygen delivery becomes limiting, glucose offers a metabolic advantage.
In short:
Glucose is the heart’s “performance mode.”
Most of the time, however, the heart runs on something else.
At rest in healthy adults, approximately 60–80% of myocardial energy comes from free fatty acids.
Fatty acids resemble diesel fuel:
They provide massive ATP yield and support the heart’s uninterrupted workload across decades.
From an engineering perspective, this is logical:
So under normal everyday conditions:
The heart cruises on fatty acids.
The analogy becomes even more interesting because the heart goes beyond hybrid design.
A car has two modes — electric and fuel combustion.
The heart has more.
It can also run on:
This makes the heart less like a hybrid and more like an intelligent multi-fuel engine.
During fasting, prolonged exercise, or carbohydrate restriction, ketone bodies rise in the bloodstream.
The heart readily oxidises ketones and may even prefer them under certain conditions.
Ketones are:
Some research suggests that the failing heart increases ketone uptake — possibly because ketones offer a metabolically favourable fuel when energy efficiency becomes compromised.
In other words:
The heart can quietly switch to a third energy mode when conditions change.
No vehicle on the road today can do this.
The real brilliance of cardiac metabolism is not any single fuel — it is the ability to switch rapidly between them.
This flexibility allows the heart to:
A rigid engine would fail quickly in a variable environment.
The heart thrives because it is metabolically adaptive.
This table shows how the heart uses multiple fuels, each drawn from different body stores, much like a hybrid car switching energy sources. Fatty acids provide steady baseline power, glucose supports high demand, glycogen offers rapid reserve, and ketones act as an efficient alternative—highlighting the heart’s key strength: metabolic flexibility.
At the centre of this flexibility are the mitochondria — the tiny power plants inside cardiac muscle cells.
The heart contains more mitochondria than almost any other tissue:
These organelles constantly integrate:
The result is a continuously optimised fuel mix — like an onboard computer adjusting engine performance every second.
In metabolic disease, this balance begins to shift.
In insulin resistance and high visceral adiposity:
The heart becomes more reliant on fatty acid oxidation.
While fatty acids provide abundant energy, they are less oxygen-efficient, and excessive reliance may increase oxidative stress and mitochondrial dysfunction.
But the problem goes beyond fuel.
Visceral adiposity and insulin resistance also affect the delivery system:
In effect:
The fuel lines narrow while the engine becomes less efficient.
Visceral adipose tissue is not just a storage depot — it is metabolically active.
It releases:
This creates a state of chronic low-grade inflammation.
From an engine perspective:
It is like running the engine on contaminated fuel.
Consequences include:
So not only is the fuel mix suboptimal — the quality of the fuel is degraded.
High blood pressure adds a further layer.
Increased afterload means:
In engine terms:
The engine is working against higher resistance — like driving uphill constantly.
Or more mechanically:
Increased pressure load accelerates wear and tear on the system.
Over time this leads to:
Your analogy works well here:
Like a car engine under constant strain, the risk of “mechanical failure” increases — the biological equivalent being myocardial dysfunction and heart failure.
In cardiometabolic disease, three things go wrong simultaneously:
In VAT-driven metabolic disease, the heart is forced to run on poorer-quality fuel, through narrowed supply lines, while working against higher pressure — a perfect setup for energy inefficiency and eventual failure.
In metabolic disease, this balance can shift.
In insulin resistance and high visceral adiposity:
The heart becomes more “locked” into fatty acid use.
While FFA provide energy, they are less oxygen-efficient, and excessive reliance can increase oxidative stress.
This helps explain why cardiometabolic disease is not only about vessels — it also affects myocardial metabolism itself.
One of the most interesting observations in modern cardiology is that failing hearts often shift fuel preference again.
Rather than relying predominantly on fatty acids, the failing heart may:
This appears to be an adaptive attempt to improve energetic efficiency when ATP production becomes threatened.
It is almost as if the engine senses fuel inefficiency and switches to a more economical mode.
The comparison to a hybrid car helps illustrate several key principles:
The heart needs multiple fuels.
Problems arise not from fuel type alone but from loss of flexibility.
A healthy heart switches fuels effortlessly.
A metabolically unhealthy heart becomes rigid — and less efficient.
High visceral adipose tissue changes fuel availability at the system level.
VAT drives:
This influences not only lipid profiles but also myocardial fuel exposure.
In many high-VAT individuals:
Understanding this helps explain why improving metabolic health — through weight reduction, exercise, and therapies that improve insulin sensitivity — can have profound effects beyond cholesterol numbers alone.
Exercise is perhaps the best way to restore fuel flexibility.
Training improves:
In hybrid terms:
Exercise upgrades the operating software of the engine.
From an evolutionary perspective, this system makes perfect sense.
Humans historically experienced:
A heart capable of switching between fuels would have a major survival advantage.
Modern life — constant caloric availability and sedentary behaviour — challenges this system in ways it was not designed for.
When we think about cardiovascular risk, we often focus on:
But beneath these markers lies a deeper truth:
The heart is a metabolic organ.
Its function depends not only on blood supply but also on fuel availability and flexibility.
If a hybrid car represents the pinnacle of modern engineering, the human heart is something even more remarkable.
And unlike any machine we have built, it never shuts down for maintenance.
Perhaps the best way to think of the heart is this:
A self-optimising, multi-fuel engine designed for lifelong performance — provided we give it the metabolic environment it evolved for.
The heart is the ultimate fuel omnivore — a biological hybrid engine that usually cruises on fatty acids, switches to glucose for peak efficiency, and can even run on ketones, illustrating the extraordinary metabolic flexibility that underpins cardiovascular health.
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