Keto Adaptation. Are You Fat Adapted?

If you’re into keto, you should already be familiar with the terms “keto adaptation” or “fat adaptation”.

15-Jul 2022, by Lee Sandwith

In this article, I’m going to summarise what keto adaptation means, and give you some guidance on how to determine whether you’re fat adapted.

What you might not know is that there has been some debate about whether keto adaptation is a real thing, so I will discuss this briefly too.


When you first find the keto diet, in all likelihood you will have stumbled across a few terms such as “fat adapted”, “keto adapted”, or even “fat burning machine”.

But what does it all mean?

At the most basic level, fat adaption refers to the body’s transition from relying on using carbohydrates as its main fuel source to ketone bodies.

Most keto followers might associate fat adaption with things related to the negative effects of including sugar in the diet. And becoming fat adapted is related to how long it takes for these negative effects to subside after switching to keto.

We’re talking about things such as:

  1. An intense feeling of hunger, or even getting “hangry”
  2. The need to eat several times per day
  3. Sweet cravings
  4. Energy crashes after eating
  5. Mental fogginess

Once you comfortably make the transition, you should start to feel some of the common benefits of keto: less hunger, consistent energy levels, ability to fast for longer and mental acuity.

These are what I would consider to be the “subjective” or “behavioural” benefits, but is there something also going on at a metabolic level?

It turns out that there is a substantial amount of quality research which suggests that there is.

A quick search of “keto adaptation” on PubMed throws up many articles, but the most famous research on fat adaptation is that conducted by Steve Phinney & Jeff Volek from the Virta study which you can read about here.

One very clear metabolic difference between high carb and low carb diets is the fuel source.

If you’re following a high carb diet, glucose will be the main fuel source; if you’re following keto, whilst your body will always need glucose, your primary fuel source will switch to ketone bodies.

One thing that seems to be clear from the research is that having increasing levels of ketone bodies in the blood and fat adaption are not the same thing.

This pertains to a number of things which have been eloquently described by Phinney & Volek in their article “Keto-adaptation”.

It’s a pretty complex piece so let me try to summarise.


A number of studies have shown how, although blood ketone levels increase quickly after starting keto, it can several weeks or months for the body to catch up in terms exercise output.

Typically, after switching to keto, most people see a dip in performance, followed by a recovery where over time the body catches up to its previous level of performance.

Or, as Phinney & Volek state, “the process of keto-adaptation that allows for normal or increased exercise performance lags well behind the level of ketones in the blood”.


Next on the list is glycogen where the same researchers have shown how, after a period of “keto adaptation”, low-carb athletes demonstrate similar glycogen storage abilities to their higher carb counterparts.

As they put it, “Despite impressive weekly training mileage, their glycogen levels had come all the way back up to those of a matched group of athletes following a high carb diet. This implies that the body’s ability to produce and defend muscle glycogen via gluconeogenesis can become finely tuned, but that this takes much longer than 4-6 weeks to occur” (Volek 2015).

Blood Uric Acid Levels

Another biomarker that suggests the body embarks on a journey of keto adaptation after switching to a ketogenic diet is Uric Acid.

At the start of nutritional ketosis, many people see a spike in Uric Acid, but over time, this corrects and returns to normal, despite sustained ketone levels in the blood.

Mitochondrial density

Mitochondrial density in muscle, brain, and other tissues might also increase through the process of fat adaption.

The mechanism is thought to either mitochondrial biogenesis, or decreased mitochondrial damage and autophagy.

Again quoting Phinney & Volek, “It is understood that reactive oxygen species (ROS) cause structural and functional damage to mitochondria, and that nutritional ketosis decreases mitochondrial ROS production. This could result in a prompt increase in the lifespan of existing mitochondria”.


Although there a number of prominent scientists challenging whether keto adaptation is real, there is also high quality, quantitative research which suggests that fat adaption is a real thing.

In addition, there is sufficient first-hand, anecdotal experience from real people that it takes some time for the body to adjust to keto.

It seems that how long the process takes is, like most things, very much down to you as an individual and it could take a matter of days, weeks or even months for you to become fully keto adapted.

So, the takeaway is simple: if you’re looking to give keto a shot, you might need to buckle down for a while until you start to realise the full potential of the benefits.

And with everything in science, try to keep an open mind: be sceptical without being cynical, and always respect peoples’ opinions.

About the Author

Lee Sandwith holds a Masters Degree in Clinical Nutrition and is a registered nutritionist with the Association for Nutrition. You can book a free 30 minute consultation with Lee here.

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