What Does “Fat Burns in a Carbohydrate Flame” Mean?

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“Fat burns in a carbohydrate flame.”

Maybe you’ve heard it before, but what exactly does it mean? What implications does it have on health and weight control?

This expression has been resurrected lately with the resurgence in popularity of ketogenic diets (aka, low-carbohydrate, low-carb diets) and their role in ketosis.

I’ll discuss these in context below. [also check out my related article on carbohysteria]

If you’ve ever taken a university biochemistry class then maybe you can still recall some of this through a distant haze of pot smoke and hangovers (if you’re a university student reading this now, put down that joint!).

If you’re just a health enthusiast and curious to understand this carbohydrate flame thing a little bit better, then pay close attention and I’ll try to keep this as logical and understandable as possible.

Disclaimer: physiology and biochemistry are complex, so I have only given an extremely basic overview of metabolic pathways and cellular processes as it applies to answering the carb flame question.

“Carbohydrate flame” backstory

Before you can understand the “carbohydrate flame,” it’s important to set the stage by discussing two metabolic conditions:

1. Normal, well fed conditions (i.e., 24 hour supermarkets, fast food on every corner).
2. No-carb or low-carb intake, fasting, starvation conditions (i.e., you’re Tom Hanks stranded on a South Pacific island – WILSON!!).

Normal, well-fed conditions

Under normal conditions, you rely primarily on three energy-containing nutrients:

1. Carbohydrate
2. Fat
3. Protein

In digestion, carbohydrate is broken down into glucose, protein to amino acids, and fat to fatty acids.

These are absorbed and taken up to be stored for later or burnt now for energy.

Macronutrient	Broken down to
Carbohydrate	Glucose
Fat	Fatty acids
Protein	Amino acids

Carbohydrate is your central nervous system’s preferred high-octane fuel (i.e., your brain and nerves). Got it? Remember this because you’ll see it again.

Your body stores carbohydrate in the liver and muscles (called glycogen) and it can be broken down at rest (i.e., liver glycogen to maintain blood sugar) or in times of need (i.e., muscle glycogen to fuel exercise, slay the saber-toothed tiger, etc).

Fat is also used as a fuel source at rest and during prolonged exercise, as is protein to a lesser extent under certain metabolic conditions.

Energy producing metabolic pathways

There are two energy pathways by which your body liberates energy from the foods you eat:

1. Glycolysis
2. Krebs cycle (also called the tricarboxylic acid cycle or TCA cycle)

Glycolysis is the pathway you use to break down carbohydrate.

The Krebs cycle  is the pathway you use to fully oxidise (burn) glucose and fatty acids down to carbon dioxide and water (and amino acids, but not as a main fuel source).

For the purists out there, yes, I know oxidative phosphorylation in the electron transport chain also generates a lot of ATP, but I’m focusing on the Krebs cycle for simplicity’s sake.

These cycles both liberate the chemical energy stored in food and produce adenosine triphosphate (ATP), which is like your body’s chemical “money” it needs to “pay” for different chemical and physiological “transactions” (i.e., muscle contraction, cellular reactions, etc).

Glycolysis

Glycolysis translates to “glyco” which means glucose (or sugar) and “lysis” which means splitting or breaking down, so it’s a sugar-splitting or sugar-breaking down process. Note: you do not burn fat in the glycolytic pathway.

One molecule of glucose contains six carbons. It goes through glycolysis and is broken down into two 3-carbon molecules of pyruvate.

To completely oxidise glucose, pyruvate is transferred inside the mitochondria where it is further broken down to a 2-carbon molecule called acetyl coenzyme A (acetyl coA) – the other carbon is flicked off in carbon dioxide.

Acetyl coA then enters the Krebs cycle, combining with oxaloacetate to form citrate, and eventually passing through the entire cycle, down through the electron transport chain where it can be broken down into carbon dioxide and water.

Recall from above, the entire process generates energy in the form of ATP which is like a “chemical currency”  to “pay” for different metabolic “transactions” such as muscle contraction.

Krebs cycle for fat oxidation

What about burning fat? How does that work?

Enter the Krebs cycle.

Your body stores fat as triglycerides and can break them down for energy (such as when you go for a long run or bike ride).

The name “triglyceride” refers to fat’s chemical structure. A triglyceride has three (tri) fatty acids connected to a glycerol (glyceride) molecule.

When it’s time to burn fat, your body breaks down the triglyceride into free glycerol and fatty acids.

The fatty acids circulate in the blood to the target tissue, say muscle for example.

The fatty acid enters the muscle cell and is transported to the inside of the mitochondria.

There, 2-carbon fragments at a time are cleaved off the fatty acid to create one molecule of acetyl coA.

This process is known as beta-oxidation.

This acetyl coA feeds into the Krebs cycle and is processed the very same way as glucose as described above.

Quick recap 

Under normal circumstances, when you’re eating plenty of carbohydrate and fat, your body produces lots of acetyl coA from pyruvate (from glucose) or from fatty acids via beta-oxidation. 

Both glycolysis and the Krebs cycle tick over unimpeded and there is peace on Earth.

However, things can go a bit haywire under fasting or starvation conditions.

I’ll explain the “carbohydrate flame” in context in the next section.

Fasting or starvation conditions

Let’s say you decide to go on the high protein, high fat pig’s feet and mayonnaise diet and your daily calorie intake is hovering around a meager 500 calories.

What happens to your glycolytic and Krebs cycle energy pathways?

First, your body uses up its stored carbohydrate (glycogen) in your liver and muscles.

Glycogen holds a bit of water too so, as you deplete your glycogen, water will go with it.

You’ll experience a bit of quick weight loss on the bathroom scale.

You’re out of glycogen, so now it’s time to start robbing Peter to pay Paul – metabolically speaking.

Second, as you’ll recall, your central nervous system prefers glucose as its primary fuel source.

If carbohydrate is not readily available, then your body can make new glucose from non-glucose sources in a process known as gluconeogenesis (gluco = glucose, neo = new, genesis = formation).

Glucose can be made from amino acids, glycerol (from triglycerides), lactate, and even oxaloacetate. (Further reading on gluconeogenesis here).

Under normal metabolism with ample glucose availability, oxaloacetate is readily available to combine with acetyl coA to form citrate which then stokes the Krebs cycle. 

Oxaloacetate is normally replenished from within the Krebs cycle but can also be made from pyruvate.

But since you started the new low- or no-carb diet, you have depleted all your glycogen stores, slowed down the (glucose-fueled) Krebs cycle, and depleted your ability to make oxaloacetate from pyruvate and pyruvate from oxaloacetate.

In this particular scenario, you’re sort of boxing yourself in from both ends and creating a “metabolic traffic jam.”

Your body can send acetyl coA to the Krebs cycle (from fatty acids), but it needs to have oxaloacetate available to combine with Acetyl coA.

It is with this understanding that “fat burns in a carbohydrate flame.”

More specifically, it could be said that fat burns in an oxaloacetate flame but, since you’re unable to replenish your oxaloacetate from glucose, then your internal metabolic conditions favour the formation of ketones (or ketone bodies) for energy (ATP) production.

Ketosis and ketone formation

Glucose is your central nervous system’s preferred fuel source, but when you’re in starvation mode, gluconeogenesis is unable to produce adequate glucose from dietary carbohydrate so your body requires an alternative fuel source.

In response to either starvation (WILSON!) or a very low carbohydrate diet, hormone changes induce the breakdown of triglycerides from fat tissue and favour the formation of ketones (or ketone bodies) in your liver. 

The ketone bodies are then released by your liver into the blood and are taken up and oxidised by heart, muscle, and brain tissue which contain an enzyme to convert ketones (primarily beta-hydroxybutyrate) back to acetyl coA. 

Acetyl coA then feeds into the Krebs cycle to produce ATP (the “chemical currency” referred to earlier). 

When you finally get rescued from the island and have a meal again (or go off of your low-carb ketogenic diet), your body will down regulate ketone production and revert back to using carbohydrate as a fuel source again.

Ketogenic diets are quite trendy and popular these days and there is evidence that they provide sustained health benefits in some people.

And, on the other hand, there are reports that lower carbohydrate dietary patterns favouring animal-derived protein and fat sources were associated with higher mortality, compared to plant-derived protein and fat sources (vegetables, nuts, grains) which were associated with lower mortality.

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