What happens to your body during ketosis?

Ketogenic diets are well known as a way to quickly lose body fat.  See “What is dietary ketosis?”, “What is a ketogenic diet?”, “What are the types of ketogenic diets?” and “What are the rules for ketogenic diets?” if you are new to keto. 

For those of you that are experienced ketogenic dieters, this article is to explain the science behind the different stages of ketosis and what happens to your body during each phase.

*The numbers used below are for blood ketone testing.  Urine ketone testing results are usually listed as milligrams per deciliter.  To convert, just move the decimal place to the right.  0.5 mmol/L for blood results would equal roughly 5 mg/dL on urine test strips. 

Ketogenic diets have different phases as your body first adapts to and then optimizes the ability to utilize fat as an energy source replacement for carbohydrates.  Many of the things you experience while on the diet are directly related to what is happening to your body as it makes the conversion. 

Although complicated at the biochemistry level, at a basic level, ketosis is just a switch in the fuel source your body uses. Without enough carbohydrates coming from your diet, and once you’ve burned through your body’s storage, your body starts making ketones from the fats you eat. 

Ketones become the new fuel source for the body. Without enough fat in your diet to meet all your caloric needs, your body starts burning through its fat stores, which of course is the whole point of the diet.

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Induction Phase

The first phase of ketogenic diets is called the Induction Phase.  It starts as soon as you dramatically decrease your carbohydrate intake.  Typically, it lasts around 4 days, but some people get into ketosis faster than others, while others take longer.  Additionally, the number of carbs you eat during the Induction Phase helps determine how fast you get through it and start the Transitional Phase. 

At the start of the Induction Phase, your blood ketone levels will be 0 mmol/L (see * quote at beginning of the post).  This is because your body does not normally produce ketones when you are eating enough carbohydrates. 

Once you start cutting the number of carbs to a level too low to supply your body with the needed energy, it starts breaking down and using stored carbohydrates for energy. 

Glycogen is the name of a polysaccharide that humans store excess glucose as.  Once carb intake drops and additional energy is needed, glycogen is hydrolyzed and it converts back to glucose for use as energy.  This isn’t important to remember, but it will explain somethings later.  

At this point, it is only important to know that the body stores extra carbohydrates and can access them when needed.  Over the course of the next couple of days, your body breaks down all of your glycogen stores for use as energy.  As these stores run low, your body starts to utilize its back-up option for energy… fat.  At this point, usually day 2 or 3, your body starts producing a few ketones, a type of fatty acid your body can use for energy. 

Most people during this stage are going in and out of low-level ketosis (measurable ketones in the blood stream) depending on the time of day, time since last meal, and activity levels.  Once all your stores are gone, you will start showing trace levels of ketones in your blood and urine.  Typically, the blood level at the end of Induction is around 0.4 mmol/L. 

Trace levels of ketones mean that you are in ketosis, but it may be transient and will fluctuate dramatically every time you eat any carbs.  Unfortunately, many people trying a ketogenic diet never advance much further than this phase.

During this phase, your body is going through a major change.  It is completely changing from a carbohydrate based metabolism to a fat based metabolism.  As a result, many people start having symptoms of what is commonly called “keto flu”.  Although there are multiple causes for the symptoms, by far the largest is dehydration.

When glycogen is broken down to glucose, water is released.  For every gram of glycogen burned, 2 grams of water are released and then excreted as urine.  The average person stores around 500 grams of glycogen in the muscles and around 100 grams in the liver.  That comes out to around 600 grams total to be burned, which will eventually yield around 1200 grams of water.  That is a little over 2 and a half pounds of water weight lost just from the burning of glycogen. 

While this is virtually immediate positive reinforcement for the dieter (with the glycogen and water combined, that is around 3.5 pounds lost in a couple of days), the resulting dehydration causes many of the symptoms of keto flu. 

As the body eliminates the water, salt and other electrolytes follow, which can lead to headaches, fatigue, muscle cramps, etc.  See “What is keto flu?” for additional symptoms and why they occur.  In order to overcome these symptoms, it is best to drink a lot of water during the Induction week as well as using salt liberally and possibly taking electrolyte replacements. 

Most people reach ketosis in the first five days of the diet.  Some in as few as a couple of days, typically due to active lifestyles, very low carbohydrate intake and high resting metabolism.  The majority, however, take 3-5 days just to reach ketosis.  This level of ketosis is very unstable and is easily broken by relatively few carbs. But once the switch from carbs to fats is complete, a different process starts.

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Transition Phase

The second phase of ketogenic diets is called the Transition Phase.  The main thing that happens in this phase is the optimization of the fat burning process.  Various biochemical processes are triggered by the switch to ketosis, including the up-regulation of genes that code for fat metabolism.  There are two main ways this occurs.

The first of these is adenosine monophosphate-activated protein kinase (AMPK), which is one of the metabolic regulators in your body that controls energy usage at the cellular level.  AMPK activation adapts rRNA synthesis away from a growth and storage state toward energy production and release state, common to fasting states. In short, activating AMPK stimulates the body to burn more fat and decrease energy storage as new fat or glycogen.

The second is something called peroxisome proliferator-activated receptor (PPAR), which regulates gene expression.  There are multiple subtypes, but subtype α and γ are the two most important ones for ketosis. The two main functions of PPAR-γ are to up-regulate genes that code for lipid uptake and adipogenesis (maturation of fat cells) and increased transport of glucose into the muscles for use as energy, thus removing glucose from the blood stream and decreasing the amount of insulin the body needs to produce.

Basically, the main purpose of the Transitional Phase is for the body to complete the conversion to a fat-based energy system by becoming more efficient at both the production of free fatty acids from body fat and utilization of fat as an energy source.  As this is happening, your body produces more ketones from the fat you eat and from the fat already in your body.

Typical blood ketone levels during this phase start around 0.5 mmol/L and reach around 1.5 mmol/L by the end of the transition.  For most people this occurs in a couple of weeks.  At this point, your body is fully capable of living off of fat instead of carbohydrates and carbohydrates are no longer needed in the diet at all. 

That is because of a process called gluconeogenesis, which is the process the liver uses to make the glucose that is needed for normal functioning. This is another process that can be altered to enhance ketosis (see the Enhancing Ketosis FAQ page). 

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Maintenance Phase

The final phase of the ketogenic diet is the Maintenance Phase.  Your body in now optimized to burn fat for fuel (although some processes may get even stronger with more time), it has completely adapted to the lack of dietary carbohydrates. 

As a result, this phase is where the body fat really starts to come off.  That is because any amount of fat that the body needs to burn, but doesn’t come directly from your diet, is taken from body fat. 

The normal blood ketone range that is optimized for weight loss is ~1.5 mmol/L to 3.6 mmol/L.  Although it is possible for your ketones to go above 3.6 mmol/L, no research has currently shown greater benefit by going above this concentration (for non-medical, strictly dietary purposes). 

The longer you stay in this phase, the more weight you will lose, so it is very important to keep your carbohydrate intake low enough to maintain it.  See “How do you stay in ketosis?”, “How does cheating affect ketosis?”, and “How can I get back into ketosis after a cheat day?” for more information on how to maintain ketosis and what happens during periods of increased carb intake. 

Exiting ketosis is usually either unintentional (not paying attention to carb intake) or intentional in the form of either a cheat day (with a plan to reenter ketosis) or to stop the diet altogether.

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Exiting Ketosis

One of the main issues with stopping a ketogenic diet is that your body has to revert back to a carbohydrate based energy source.  You can quit cold-turkey and leave the diet completely, but there are some consequences that come with that.  The most noticeable one is that once you switch back to carbs, your body starts re-storing carbs as glycogen.  Although unavoidable, it is still quite a shock to gain the 3.5 pounds or so weight back as a result. 

Additionally, the changes the body made during the Transition Phase have to be reversed.  That means that the body won’t be used to dealing with normal carb levels and will over-respond to the higher blood glucose with excess insulin.  This is a transient form of insulin resistance, quite similar to what Type II diabetics deal with, only it is short-term and resolves as your body compensates and converts back to using carbs for fuel. 

Unfortunately, during the time it is occurring, you are more likely to store excess energy as fat. Simply put, when you decide to quit a ketogenic diet, be prepared to gain back some weight.  It may be inevitable, but if you expect it, it won’t be as disheartening. 

The other option is to remain in ketosis, but increase your carbohydrates to a level that keeps you around 1.0 mmol/L to 1.5 mmol/L.  This will keep you in ketosis (somewhat), but the fluctuation will keep you from losing weight.  Which is still better than gaining, right?

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.