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You’ve likely heard of the three major macronutrients that provide us with energy: protein, carbohydrates, and fat. However, our bodies can use another substrate as a source of energy, a “fourth fuel” called ketones. Ketones are produced (and sometimes supplemented) when the body is in a metabolic state of ketosis; one where it’s breaking down or burning fat.

Interestingly, while ketosis is said to be an evolutionary adaptation that allows organisms to survive under conditions of low food availability, newer research is showing that ketones have several diverse and impressive biological roles in the body, and many health benefits. They’re more than just an energy source.

People are becoming aware of the many applications of ketosis—including weight loss, sports performance, and cognitive function. However, many people might not be aware of how to get into ketosis, how long it takes, and what happens when you do.

This article provides all the information you need about entering and staying in ketosis.

Endogenous vs. Exogenous Ketosis

Before we talk about the specific ways to achieve ketosis, let’s go over some basics. Ketosis can be classified in two ways; endogenous (meaning “within”) or exogenous (meaning from “outside”). We will only cover some of the basics here, but feel free to check out our in-depth article on the fundamentals of ketosis.

When we talk about endogenous ketosis, we are referring to the process of the body producing ketones on its own. This happens in the liver. When glucose levels in our body run low and insulin falls, our body begins to burn fat. Free fatty acids (FFAs)—the breakdown products of fat—are then transported to the liver and used to produce ketone bodies. These ketone bodies are then transported back out of the liver, where they travel through the circulation to organs and tissues to be used for energy.

Endogenous ketosis can be achieved through several means, which we will talk about later. These include low-carb diets, exercise, and fasting.

In contrast, exogenous ketosis is achieved through the use of exogenous ketone supplements or precursors to ketone bodies like medium-chain triglycerides (MCTs). In exogenous ketosis, your body isn’t producing ketones, but is still in ketosis. There are many examples of exogenous ketones, which we will cover below.

Regardless of the method used, ketosis is ketosis. However, there are benefits specific to exogenous and endogenous ketosis. Some of this has to do with how long it takes to actually enter ketosis through each route. Generally, endogenous ketosis is going to take much longer, while exogenous ketosis can be achieved within a matter of minutes.

Before going into specific methods for getting into ketosis, let’s take a look at how you can verify that you’re actually in ketosis.

Testing for Ketosis

There is only one true way to confirm that you’re in ketosis—measuring ketones. This will allow you to quantify your level of ketosis using commercially-available devices.

Traditionally, ketosis is considered to occur when blood ketone levels of the ketone body known as beta-hydroxybutyrate (BHB) are >0.5 millimolar (mM).

The only way to verify this is to test for BHB. Blood BHB can be measured using a blood ketone meter. This device is similar to a blood glucose meter, except it measures ketones instead of glucose. There are several great ketone meters on the market, which we reviewed in this post.

The main advantage of testing for blood BHB is accuracy. Furthermore, blood levels of BHB represent the ketones you’re actually using, and BHB is the ketone body most present throughout the circulation.

However, testing for BHB does require you to prick your finger and draw blood—which can be a downside for some people. If you’re testing often, blood testing can also get expensive, since you’ll need to continuously purchase ketone testing strips for the device.

Ketosis can be measured using two other methods. To measure acetoacetate (AcAc), urine ketone test strips can be used. These measure the amount of AcAc in your urine. Urine test strips are easy and non-invasive, making them attractive to people looking for a simple way to test ketones.

Acetone, the third ketone body, can be measured in the breath using a device known as a breathalyzer. This method simply involves breathing into the device and getting a reading on your level of ketosis.

Several new devices have appeared on the market that measure ketones in the breath.

Breath and urine testing do have a few downsides. These methods are far less accurate than blood BHB testing and only give a rough “estimate” of your level of ketosis, often only giving a qualitative output or a color that corresponds to your level of ketosis. These methods also become less reliable when ketone levels are high or you’ve been on a ketogenic diet for a long time.

While you can obviously enjoy the physical and mental benefits of ketosis without testing, it might be a good idea to measure ketones if you are new to ketosis. This way, you can know what gets you into ketosis, what takes you out, and how you feel at various levels of ketosis.

An image of a football player in action, with illustrations of how to test for blood ketones

How Long Does it Take to Get into Ketosis?

You can’t just snap your fingers and magically get into ketosis—it takes a bit of time. It is important to realize that lifestyle factors, aspects of your own body and metabolism, and even sleep and stress could influence how quickly (or not) you can get into ketosis.

In general, there are three primary ways to get your body into a ketogenic state: a ketogenic diet, exercise, and fasting.

Ketogenic Diet

Also known as “keto”, the ketogenic diet is a high-fat diet that drastically restricts carbohydrates intake while advocating a moderate protein intake. Reducing carbohydrates limits the amount of glucose you have available and thus, lowers blood glucose and glycogen in the body.

In general, someone on keto will get about 80% of their calories from fat intake, 15% from protein, and less than 5% from carbohydrates (generally less than 50 grams per day). The keto diet prioritizes the intake of healthy fats including those from coconut oil, avocados, and animal products like eggs and fatty meat.

Due to the high fat and low carb consumption, the keto diet creates the perfect conditions in the body for the breakdown of fat, while providing a substrate (fat) for the production of ketones.

The keto diet will get you into ketosis—but the timing will vary.

For some people, it might take a week or two of keto dieting to reach sustained levels of ketones of 0.5mM or higher. However, in some people, just 2 - 4 days of a keto diet can raise circulating ketone body concentrations to 1 - 2 mM.Pinckaers2017

How quickly ketosis occurs will depend on your level of carbohydrate intake as well as your physical activity levels. In general, lower carb intake and higher activity will allow you to enter ketosis faster.

Fasting

Fasting seems to be all the rage among those looking to lose weight, biohackers, and even some athletes. There are nearly unlimited variations of fasting—ranging from intermittent fasting to time-restricted feeding to 5:2 fasting.

However, for ketosis, intermittent fasting (IF) might be the most effective strategy. IF involves “intermittent” but generally prolonged periods of food restriction; such as fasting for a few days each week or month. These more prolonged periods without food are needed to effectively reduce blood glucose, glycogen, and insulin levels throughout the body.

How long do you need to fast for ketosis to occur?

After an overnight fast, ketone body concentrations can reach about 0.1 - 0.5mM—just below the official “ketosis threshold”. However, fasting for around 48 hours can elevate circulating ketones to around 1 - 2mM, and after 5 days, levels might reach 7 - 8mM.Pinckaers2017

If you’re looking to embark on a long-term fast—for ketosis or otherwise—it’s best to first consult your doctor or trusted health professional who can help advise you through the fast to ensure safety. It might be best to initially see how your body tolerates a restricted daily feeding window or a more moderate 24 hour fast before trying something more extreme.

Exercise

A final way to stimulate your body’s own ketone production is through exercise—particularly extended aerobic exercise which will reduce the body’s glucose and glycogen stores. Many studies in humans and animals have shown that exercise raises levels of BHB in the blood.Sleiman2016,Koeslag1982,Koeslag1980,Matoulek2014

The presence of ketone bodies in the blood after exercise is termed “post-exercise ketosis” and is a well-known phenomenon. Doing about 2 hours of exercise after an overnight fast can raise ketone levels to 0.5 - 1.0mM during exercise, with levels further increasing to 1 - 4mM in the period after exercise ends.Pinckaers2017

Ketosis during and after exercise depends on many factors including intensity and duration of exercise, glycogen availability, nutritional status during exercise, and even your training status. If you want to accelerate ketone production with exercise; do your workout fasted—that’s what the research says.

Exogenous Ketone Supplementation

While the methods listed above generally will require at least a few days to induce ketosis, other methods can result in a rapid and high elevation of blood ketones, without your body needing to produce them.

This involves the consumption of exogenous ketones.

Exogenous ketones are a form of supplement that contains a ketone body bound to some other molecule.

When you ingest these supplements, they raise circulating ketone concentrations in the blood, allowing you to get into ketosis fast.

Exogenous ketone supplements come in two main forms: ketone salts and ketone esters. Sources of fat like coconut or MCT oil are also sometimes categorized as ketone supplements.

Ketone salts are made from a ketone body (usually BHB) bound to a mineral like sodium, potassium, or magnesium. They’re often found in a powder form.

Ketone esters can come in the form of an AcAc diester or, most commonly, a BHB monoester.

Ketone salts and esters are both effective at raising blood ketones quickly, and don’t even require fasting or a keto diet. This makes them of particular interest to athletes.

How long does it take to enter ketosis using supplements? Some studies have shown that blood BHB levels jump up to 3mM after ingesting a BHB monoester supplement, reaching peak levels after just 10 - 30 minutes. Others have reported levels up to 6mM following ingestion of a BHB monoester.Stubbs2017,Cox2016,Clarke2012

In general, ketone esters seem more effective at raising blood ketones than ketone salts, and may come with far fewer side effects such as GI distress. As there are several types of ketone supplements available on the market, it might be best to experiment with a few to find what you tolerate the best.

Exogenous ketone supplements can allow you to enter ketosis quickly and achieve levels of ketosis that might only be feasible through multiple days of fasting.

An image of avocado toast, a person meditating on a mountain and a woman lifting a medicine ball, showing the three ways the body can get into ketosis

Keto Adaptation

You’ve probably heard the term “fat adapted” thrown around, but what exactly does it mean? Fat adaptation is the ultimate “goal” of the ketogenic diet—it’s a state in which your body has been in ketosis for a long enough time that it has transitioned from burning carbohydrates and sugar for energy to burning primarily fats. These fats can come from food or your own internal stores (body fat).

In keto adaptation, the metabolic machinery of your body changes, and you become a “fat burner”; able to efficiently burn fat and utilize it for fuel.

How Long does Keto Adaptation Take?

Given the lack of long-term research on humans and the ketogenic diet, we don’t really have a specific answer for how long the process of keto adaptation takes.

Even though ketosis can be achieved after a few days of fasting or a ketogenic diet, keto adaptation may take a bit longer—perhaps up to a month or longer. What this means is that your ability to actually use ketones doesn’t rise in parallel with the levels of ketones in your blood.

In other words, optimizing ketone metabolism doesn’t happen overnight.

What Happens in the Body After Keto Adaptation?

Keto adaptation occurs as a result of several changes throughout the body which allow for a greater production or a more efficient utilization of ketone bodies. These metabolic adaptations have been well studied, and can help give us an idea of the timeline for keto adaptation.

Increased Fat Burning

One of the main reasons that ketogenic diets are excellent for weight loss is the fact that they result in an increased ability to burn fat—a process known as lipolysis.Paolia2013 Over time, and especially once keto adapted, a greater fat-burning capacity will result in more fat loss for people looking to lose weight.

However, increased fat burning due to keto adaptation may have benefits for athletes; especially endurance athletes.

For instance, runners who were “keto adapted” (they had been on a keto diet for at least 6 months) were shown to burn twice the amount of fat at a higher exercise intensity compared to athletes eating a mixed diet.Volek2016 In another study, athletes who ate a high-fat diet for just 5 days were able to increase their fat oxidation rates during aerobic exercise.Leckey2018

A lower reliance on glucose—whether you’re an athlete or not—has its benefits. For one, our body can store about 30,000 - 100,000 calories worth of fat, but only about 1,500 - 2,000 calories worth of glycogen. Being able to tap into all of those fat stores means you can perform longer, without relying on external sources of energy once glycogen runs low.

Burning more fat may also stabilize energy levels. Rather than ride the highs and lows of glucose spikes throughout the day, keto adaptation can allow you to access a steady, consistent, and powerful supply of fat and ketones to fuel your body and mind. Just give it some time.

Glycogen is Conserved

It might seem obvious that reducing your intake of carbohydrates and glucose would lead to a reduction in stored glucose in the body, known as glycogen. Initially, this is true. Ketogenic diets naturally result in a reduction of skeletal muscle glycogen stores in humans.Phinney1983

Athletes might be fearful of this response, since muscle glycogen contributes to energy production during long-duration exercise, after glucose has been exhausted.

However, once keto adaptation occurs, there is evidence to support that muscle glycogen is preserved in humans and actually spared during exercise.

Endurance athletes who were long term keto dieters (i.e. keto adapted) had levels of muscle glycogen that were similar to athletes eating over 600g of carbohydrates per day!

Keto-adapted athletes also had the same glycogen levels as carb-consuming athletes after completing 3 hours of running, and were able to replenish their glycogen stores to a similar extent.Volek2016

How could they do this without eating carbs? The explanation is a process called gluconeogenesis—GNG for short. GNG is a process by which the body can create glucose from non-carbohydrate sources like fat and amino acids from protein, which helps to maintain blood glucose levels at the necessary amounts to power a variety of physiological functions in the body that do require glucose.

Long story short; keto adaptation increases the body’s capacity for gluconeogenesis and therefore, is able to maintain blood glucose and muscle glycogen levels similar to those in someone eating ample carbohydrates.

More Ketones for the Brain

Our brains can run well on ketones, and may even prefer them as a fuel source.

Keto adaptation allows more ketones to become available for the brain. This happens because as tissues adapt to low-carbohydrate availability, they get better at using fats for fuel. This is especially true for skeletal muscle, which can directly oxidize fatty acids to produce ATP.

As a result of more direct fatty acid oxidation by the skeletal muscles, more of the body’s ketones are made available for the brain.

This has several benefits—increasing brain energy supply, reducing the brain’s reliance on glucose, and reducing the requirement for protein to help with gluconeogenesis.Volek2012

It could take anywhere from several weeks to months for muscle to become adept at using fatty acids for energy. Like all processes, optimization takes time.

Mitochondria Get a Boost

We’ve all learned it in high school biology class: mitochondria are the “powerhouses” of the cell. Without them, we’d be unable to create energy. Mitochondria keep us running—literally.

Fuel sources (carbohydrates and fat) are converted to energy (ATP) inside the mitochondria. In a simple sense, more mitochondria mean a greater ability to produce energy. Keto adaptation might have a role to play here, since several studies in animals have indicated that ketogenic diets actually increase the production of new mitochondria, a term known as mitochondrial biogenesis.Bough2006,Balietti2010,AholaErkkila2010

Producing ATP inside the mitochondria results in the generation of “toxic” byproducts known as reactive oxygen species—or ROS—which can cause damage to cells and tissues in the body. This is a normal process, but too much ROS activity can cause havoc.

Just like carbs and fat, ketones are used inside the mitochondria to produce ATP. However, compared to glucose, ketones result in a lower amount of ROS production.Volek2012 In addition, ketones might reduce the amount of ROS in the body directly, or by increasing levels of cellular antioxidants like glutathione; which protect against ROS induced damage.Veech2004,Kim2007,Jarrett2008

3 - 4 weeks of keto adaptation might be enough to start boosting mitochondrial capacity...in mice at least.Li2016,Srivastava2012 Humans might require a bit longer.

 

An image of a male speed cyclist, and illustrations of the different results of keto adaption

How Will I Know if I’m Keto Adapted?

Unfortunately, measuring or testing for ketones can't tell you directly whether you’re keto adapted, but, there is one way to get a picture of your level of keto adaptation using ketone testing methods.

Urine and breath ketone testing methods measure ketone bodies present in excretion products—breath and urine. For this reason, as you continue to use these methods while on a ketogenic diet, you might begin to notice that ketone levels actually begin to drop. While this is one disadvantage of these methods (they become less effective over time), it signifies that you’re becoming more keto adapted—utilizing more and excreting fewer ketones.Sapir1975

Subjective feelings may be just as valid for evaluating if you’re keto adapted. Making the transition from sugar to fat burning will come with a variety of bodily changes and feelings associated with the metabolic switch.

Once keto adapted, you’ll likely find it easy to go 4 - 6 hours, maybe even more, without food.

What’s more, you won’t get the “hangry” feelings you once did when low blood sugar levels ran low. On top of this, you’ll begin to feel increased and consistent energy levels throughout the day.

While some athletes report worse performance after initially transitioning to keto, after keto adaptation occurs, performance levels usually come back to normal or even improve. This occurs because your metabolic machinery will finally be able to meet the energy demands of exercising in the absence of a high-carb diet. This takes time, but many athletes find that the short term adaptation period is worth it in the long run.

Adapt and Advance

This may seem like a lot of information to handle, but there’s no need to be overwhelmed.

With a little self-experimentation, you can find out how your body responds to different methods of inducing ketosis, and how you feel while in ketosis. After a while, these concepts will become natural—a part of your lifestyle. Ultimately, that is the goal.

Ketosis can have several benefits, whether for a short period of time or throughout life.

If keto adaptation seems to be taking longer than expected, this isn’t a reason to ditch your diet or lifestyle regimen. Like everything, learning the ins and outs of ketosis will take time, but it’s worth it.

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