Do We Burn Ketones for Energy?

Brain Uses Ketones for Energy
Many low carb dieters believe that ketones
fuel the entire body. They don't.
So, what are Ketones Really Used For?

When you say that:

“I am in ketosis,”

What does that mean?

Do your muscles use ketones to move?
Does your metabolism use ketones to keep your unconscious systems, such as physical digestion or your need to breath, running smoothly?

Are you burning ketones to fuel your everyday physical activities, such as getting out of bed, making breakfast, and going to work?

Most low-carb dieters would say: “Yes. We burn ketones for energy.”

However, that statement is misleading. And here's why:

The Truth About Ketone Oxidation

Ketones are only used to fuel your everyday activities in the very beginning of your low-carb diet.

Initially, when you drop your carbohydrate intake to 20-net carbs per day, or less, the body has no way to know how long that deprivation is going to last. 

In a glycogen-depleted condition, ketones are used to support the activities of almost every organ, system, and body function.

This initial adaptive response saves the available glycogen -- the storage form of carbohydrates -- for the central nervous system and a few other cells that don't have mitochondria.

And that is where the confusion comes in.

Most of the articles you'll read about the state of ketosis only talk about what happens during the first week or two.

I noticed that in some of the biochemistry texts available online.

They explained what initially happens to the metabolism during starvation, or at the very beginning of a low-carb diet, but they didn't go into what the liver does if the situation continues past a few days.

In this blog post, we're going to take a closer look at what ketones are actually used for and what happens when the carbohydrate stress doesn't let up.

What Does the Latest Scientific Research on Ketones Show?

Originally, this post was written in 2010, back when I still believed that a low basal insulin level was the magic behind low-carb diets.

Since then, personal experience and further research has laid that hypothesis to rest. Insulin level is not the magic bullet that makes a low-carb diet work.

Case in point:

Several popular theories were challenged recently by a scientific study funded by Gary Taubes and other low-carb influencers. The insulin hypothesis did not hold up within the study's design, but there was still a lot that we learned about low-carb diets.

The study saw no difficulty in overweight participants losing body fat when insulin levels were high.

In fact, fat loss slowed way down when the basal insulin level dropped due to carbohydrate restriction. 

This matches what you would find in an ordinary biochemistry textbook:

As the number of ketones in the blood goes up, so does the level of insulin, to help keep those numbers in check.

One of the interesting things brought out by the study was what actually happens during the first 3 weeks of a low-carb diet.

During the first few days, protein oxidation went up, blocking all fat loss, but within 7 days, the body had decided to do something different.

If muscle losses continued at the initial oxidation rate, the diet would be life threatening within only a few short weeks.

According to R. Paselk, at the Dept. of Chemistry, Homboldt University, you cannot lose more than 50% of your muscle mass and survive, so the body has alternative routes and paths to make sure that doesn't happen.

By day 7 of a low-carb diet, the body had implemented a new strategy to limit glucose utilization, and study participants began generously using fatty acids for energy without a hitch.

There really wasn't a need for “fat adaption,” as many low-carb experts call it, since moving into ketosis drastically lowered the body's need for gluconeogenesis.

The brain was able to derive up to 75% of its energy needs by way of ketones and the fats in the diet supplied almost all of the calories the body needed to fuel its other systems and organs.

What are Ketones Used For?

The brain's needs always take precedence over every other body function or system.


When it comes to your physical survival, the brain is King.

Initially, when we restrict carbohydrates, we are using glucose predominantly for fuel and fatty acids with a minimal amount of ketones to help us get through the night safely.

When the blood glucose level falls, due to carbohydrate restriction, the body doesn't have time to think about what's going on.

It simply responds to a drop in insulin by converting the glycogen stored in the liver to glucose, and assuming that the lack of glucose is only a temporary situation.

Within a few hours, as those glycogen stores begin to dwindle, the body has to adapt to the shortage of glucose by increasing protein oxidation from muscle tissue to handle the load demanded by the brain.

This muscle-stripping is a knee-jerk reaction to what's going on, but the body still assumes that the lack of carbs is temporary.

The carbon from a few amino acids can be used in the glucose-making process, but gluconeogenesis is very complex and not very efficient.

For example, it takes 200 grams of protein to get the 120 grams of glucose needed to fuel the brain for a single day. Those 200 grams of protein come from about 1 kg of muscle tissue (2.2 pounds).

Initially, the proteins oxidized by the body are what's called junk proteins. They are damaged or worn out cells, so the body simply gets rid of them.

Since ketones are produced from triglyceride breakdown and triglycerides contain a bit of glycerol, useful in making glucose, the body further adapts to the situation by beginning to break down triglycerides for glycerol and ketones.

The ketones drastically cut down on the amount of glucose the brain needs every day, and they can also be used to take up the glucose slack for almost all other body functions.

However, since ketones are a by-product of fatty acid breakdown, it is not efficient for the liver to continue supplying the body with ketones.

Within a week, the body starts to directly use fatty acids for energy. In this way, ketones can be saved for the brain since the brain cannot use fatty acids for fuel. Fatty acids cannot get past the blood-brain barrier, which is why the brain still needs to have 25% of its needs met by way of glucose.

Body Burns Fatty Acids for Energy

The state of ketosis occurs when the body begins making more than a token amount of ketones.

When someone says they are “in” ketosis, that doesn’t mean they are burning ketones for energy. The liver is using fatty acids to fuel the process of making glucose.

It is the brain's need for glucose that results in the synthesis of ketone bodies.

If the brain didn't need to run on a certain amount of glucose each and every day there would be no need for ketones because almost all body cells can use fatty acids or glucose for fuel.

The liver can’t use ketones, which is actually a safety measure to make sure the liver dumps them into the bloodstream, rather than hogging all of the ketones for itself.

After a few weeks, once the muscles and other body tissues have completely switched over to using fatty acids for fuel, they no longer use ketones for energy.

This is a preservation mechanism.

Its purpose is to save all available ketones and glucose for the brain.

Once the brain has switched to using ketones for 75% of its functions, ketone production will be tailored to fit that specific need. Extra ketones will no longer be made.

The average length of time it takes for the brain to completely switch to using ketones is about three weeks, but it can take as long as six or more, depending on your dieting background.

If you've used this alternative metabolic pathway before, or if you've previously been in some sort of starvation condition, the body will be more efficient at creating and using ketones, but it will also expect you to stop dieting at some point.

The body really doesn't “fight against” fat loss.

It just assumes that carbs are going to be available again soon, and as a result, it takes longer to make the necessary adaptions.

Your Brain Burns Ketones for Energy

Without Burning Ketones the Mind is Like a Dead Battery
Ketones are an alternative energy source the liver uses when there isn’t enough glucose available for the brain.

Once the body adapts to being in ketosis, the brain is the major body organ that burns ketones for energy. All other body organs and cells that have mitochondria will use fatty acids for fuel.

Cells that don't have mitochondria will continue to use glucose.

Since the body believes the carbohydrate stress is a famine situation, the liver will get more and more proficient in narrowing ketone production down to fit within the needs of the brain.

The more efficient the body is in utilizing ketones, the fewer ketones the body will make and toss away unused.

Being in ketosis is no guarantee that you'll lose weight.

While the alternative metabolic pathway predominantly burns fatty acids and ketones for energy, if you're eating all of the calories your body needs to survive the famine, you won't see much if any weight loss on the scale.

To shed those pesky unwanted pounds, you have to eat at a caloric deficit.

No deficit?

No weight loss!

However, metabolic processes are not that simple.

When eating very low carbs (less than 50 net carbs a day), the body will be in some degree of carbohydrate stress, so:
  • too little or too much fat
  • too little or too much water
  • too little or too much exercise
  • too few or too many calories
will all play a role in determining your current metabolic rate, hormonal balance, and energy output.

It's not just about the carbs, but it's not just about the ketones either.

Ketosis is simply a tool that can be used to help you get to where you want to be, but ketones do not erase the fact that reaching that target takes hard work, mindfulness, and plenty of dedication.

Additional Articles You May Find Helpful:

Are Ketones Bad?
Why Am I Not in Ketosis?
True Role of Ketosis in a Low-Carb Diet


  1. Thank you for this post. I've been reading "Why we get fat" and Wheat Belly and am getting confused about insulin and ketones.

  2. Truthfully, I haven't read either book. I'm a celiac, so I already don't eat wheat. But I do get a lot of letters from people who have tried eating very low carb, high-fat and gained weight from eating that way.

    I'm confused by those who believe that a high ketone blood count means one is burning body fat, because the only reason why ketones build up in the bloodstream is if you're not burning them for fuel.

    Thanks for your comment.

  3. Not necessarily a high ketone count, but pretty much any ketones in the blood or urine mean youre burning body fat as ketones are a by product of body fat being metabolized. People with high urine ketone counts are more likely to be dehydrated than burning huge amounts of body fat.

  4. William,

    Thank you for sharing that. I really appreciate your comments. I probably need to go back through my archive of posts and make sure everything is accurate. You're right about the dehydration.

  5. Wow so much different content from what I learned in graduate biochemistry. Anyway exogenous ketone supplements in combination with very low carbs feels good to me. I'm waiting to see the long term weight loss benefits. So far I'm optimistic.

    1. This one definitely needs a re-write, doesn't it. In 2010, I was still buying into Keto Magic. Given the fact that we now know that fat adaption occurs much quicker than the original low-carb theories suggest, I'm still not understanding how exogenous ketone supplements help with fat loss.

  6. Hi Vickie, you say "Fifty-eight percent of all dietary protein is broken down in the liver to make glucose". Can you very kindly say where the figure of 58% came from? Any info much appreciated. Thanks, Russell

    1. This post was written in 2010. Protein is NOT turned into glucose unless there is no other alternative. The latest research on low carb did show that protein oxidation goes up during the first week, but fatty acids were generously being used for fuel by week 2, and protein oxidation went down. I'm in the process of updating my posts, but haven't gotten to this one yet.

    2. Thank you very much Vickie for your kind reply. Incidentally, no matter how much dietary protein you eat none of it can convert to glucose. The only protein that converts to glucose is the protein that the body strips from muscles to make 'homemade' glucose in a process known as gluconeogenesis. and the body does this when glucose stores in the body run out as a result of insufficient dietary carbs.
      Keep well, Russell Eaton, author of The Lipo Diet,

    3. Thank you so much for sharing that. I went back and did a lot of current research today on this, and that was the new impression I got.


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