What is ketosis?

Ketosis is a natural state for the body, when it is almost completely fueled by fat. This is normal during fasting, or when on a strict low-carb diet, also called a keto diet.

Ketosis has many potential benefits – related to rapid weight loss, health or performance – but there are also side effects. In type 1 diabetes and certain other rare situations excessive ketosis can even become dangerous.

On this page you can learn all about how to harness the benefits of ketosis, while avoiding any problems. It all starts with understanding what ketosis is.


1. Ketosis explained

The “keto” in the word ketosis comes from “ketones”, the name of small fuel molecules in the body.1 This is an alternative fuel for the body, produced from fat we eat, and used when blood sugar (glucose) is in short supply.

These ketones are produced when you eat very few carbs (the main source of blood sugar) and only moderate amounts of protein (excess protein is converted to blood sugar).

Under these circumstances, fat is converted in the liver to ketones, that then enter the blood stream. They are then used as fuel by cells in the body, just like glucose. They can even be used by the brain. This is extra important, as the brain can not be directly fuelled by fat, and it’s a very hungry organ.2

Maximizing fat burning

One way to enter the state of ketosis is to eat a ketogenic diet. Another is to do a period of fasting.

Under these circumstances, as soon as the body’s limited reserves of glucose starts to run out, your entire body switches its fuel supply to run almost completely on fat. The levels of the fat-storing hormone insulin levels become very low, and fat burning increases dramatically. You thus get easy access your fat stores, and can burn them off. This is great for losing excess weight. Studies prove that keto diets result in more weight loss, faster.3 There are also more potential benefits.

When the body produces enough ketones to measure significant levels in the blood (usually over 0.5 mM) it’s said to be in ketosis. The fastest way to achieve ketosis is by fasting – not eating any food – but that’s not possible to do forever.

A strictly low-carb or “keto” diet, however, can be eaten for any amount of time. It also results in ketosis. It achieves many of the benefits of fasting – like weight loss – without having to fast.
More about a keto diet

Ketones are brain fuel

It’s a common misconception that the brain needs carbs. The truth is that the brain happily burns carbs when you eat them. But if you don’t eat too many carbs, the brain is happy to burn ketones instead.

Food for thought: Does the brain need carbs?

Does the Brain Need Carbohydrates? – Answers to Common Questions
This is an absolutely necessary function for basic survival. As the body can only store carbs for a day or two, the brain would quickly shut down after a couple of days without food. Alternatively it would quickly have to convert our muscle protein into glucose – a very inefficient process – just to keep the brain going. That would make us waste away quickly. It would also ensure that the human race could hardly have survived all those millennia before we had 24-7 food availability.

Fortunately our bodies have evolved to be smarter than that. Normally we have fat stores that last so that we can survive for many weeks (if not months) without food. Ketosis is how the body makes sure that the brain can run on those fat stores too.

Bottom line: We do not need to eat any carbs at all. The brain can happily run on fat.

Many people even feel more energized and focused when the brain gets to run on ketones, made from fat. And it certainly speeds up fat loss, if you’re trying to lose weight.



2. The benefits of ketosis

There are many benefits of ketosis. By giving your body and brain an almost unlimited supply of energy, you can increase your mental and physical endurance. It also reduces hunger, facilitating effortless weight loss.

Furthermore, as getting into ketosis requires eating very few carbs, it can effectively reverse type 2 diabetes. Ketosis has also been used for a long time to control epilepsy – often even without drugs.

Even beyond this, it’s showing great promise for several other conditions, like improving acne, reversing PCOS and perhaps even treating brain cancer.



3. How to get into ketosis

To get into ketosis you need low levels of the fat-storing hormone insulin. The most important way to do that is to eat a strict low-carb diet, also called a ketogenic diet.

On top of the necessary ketogenic diet there are many ways to increase ketosis further. The most powerful is adding intermittent fasting.

Full guides



4. Symptoms & how to know you’re in ketosis

Symptoms of ketosis: dry mouth, thirst, frequent urination

How do you know you’re in ketosis? It’s possible to measure it by testing urine, blood or breath samples. But there are also other telltale signs, that requires no testing:

How to know you’re in ketosis

How do you measure ketosis?

ketone meters
There are three ways to measure for ketones, which all come with pros and cons:4

  1. Urine strips
  2. Breath ketone analyzers
  3. Blood ketone meter



5. Side effects, fears & potential dangers


Side effects

When starting a low-carb ketogenic diet and reaching ketosis it’s common to get some side effects during the first week. Possibilities include headache, lethargy, irritability, leg cramps, constipation and heart palpitations.

These side effects are usually relatively minor and transient, and most of them can be avoided by getting enough fluid and salt.

Learn more about low-carb side effects and how to cure them


Ketosis vs. ketoacidosis

Ketosis vs. ketoacidosis

There are many misconceptions about ketosis. The most common is mixing it up with ketoacidosis – a rare and dangerous medical condition that mostly happen to people with type 1 diabetes if they don’t take insulin.5 Even some health care professionals tend to mix up these two situations somewhat, perhaps due to the similar names and a lack of knowledge about the distinct differences.

Ketosis and ketoacidosis are not the same thing.

Ketosis is a 100% natural state, under full control by the body. It can be caused by a low-carb diet or by a brief period of fasting.

Ketoacidosis is a severe malfunction of the body, with excessive and unregulated production of ketones. This leads to symptoms like nausea, vomiting and stomach pain followed by confusion and finally coma. It requires urgent medical treatment, as it can potentially be fatal.6

This graph shows the vast difference in amount of ketones in the blood between ketosis and ketoacidosis:

ketone zone

Most people who eat a strict low-carb ketogenic diet never reach levels higher than 3 millimolar or so, in fact many people struggle to get it above 0.5. Long-term starvation – meaning a week or more with zero food – might bring the number up to 6 or 7. But ketoacidosis occurs at levels of 10+, most commonly 15+.

It’s like the difference between drinking a glass of water vs. drowning in an ocean. Both situations are about water – but they are not the same thing. Drinking a glass of water will not make you drown. Nor does ketosis result in ketoacidosis.

If you have a functioning pancreas that can produce insulin – i.e. you don’t have type 1 diabetes – it would be extremely hard or, most likely, impossible to get ketoacidosis even if you tried. That’s because high ketone levels result in release of insulin, that shuts down further ketone production. In other words, the body has a safety net that normally makes it impossible for healthy people to get ketoacidosis.7

How is it possible to get ketoacidosis?

There are 3 exceptions, where ketoacidosis is possible (even if rare) on a keto diet, and extra caution is warranted:

  • Type 1 diabetes: As your body lacks insulin you need to make sure you inject what you need. You’ll need less on keto, but still some. Learn more
  • Medication with SGLT-2 inhibitors8 for type 2 diabetes. Learn more
  • Breastfeeding: In very rare cases a keto diet when breastfeeding can result in ketoacidosis, so you need to adapt the diet to be safe. Learn more

Ketoacidosis makes you feel very sick, nauseous and extremely weak.

There’s a simple treatment if you suspect this may be happening: eat some carbohydrates right away (e.g. a couple of fruits or a sandwich or a glass of juice). If you have type 1 diabetes take more insulin. Then contact emergency medical services if you do not immediately start feeling better.




6. How to reach optimal ketosis

Getting into ketosis is not a black or white thing. It’s not like you’re either in ketosis, or out of ketosis. Instead, you can be in different degrees of ketosis, as this chart demonstrates.9 The numbers below refer to values when testing blood ketone levels.

The Ketone Zone

Learn more about optimal ketosis and how to reach it



Top ketosis videos


Dr. D’Agostino

You can truly learn a lot about ketosis from one of the world’s foremost researchers on the topic, Dr. Dominic D’Agostino:

10 years of ketogenic research
Nutritional ketosis: metabolic regulation and signaling


A ketogenic diet for beginners
Ketogenic diet foods – what to eat

  1. Three different ketone bodies are used as fuel by the body:

    1. Acetone
    2. Acetoacetate
    3. Beta-hydroxybutyrate


    Learn much more about ketones:

    Dr. Peter Attia: Ketosis – advantaged or misunderstood state? (Part I)

  2. The brain consumes about 20% of the body’s required energy every day, despite only representing 2% of the body’s mass.

    A good bonus for weight loss, if you can get your hungry brain to burn fat for you, 24-7.

  3. Two examples are Shai 2008 and Bazzano 2014.

    More studies

  4. Diet Doctor will not benefit from your purchases. We do not show ads, use any affiliate links, sell products or take money from industry. Instead we’re funded by the people, via our optional membership. Learn more

  5. Learn more about how to safely and effectively do a low-carb diet if you have type 1 diabetes:

    Starting Low Carb with Diabetes Medications

  6. In the most common case – type 1 diabetes – the treatment mainly includes insulin infusion and fluids.

    Ketoacidosis is potentially fatal, but provided adequate and timely medical treatment it fortunately has a more than 99% survival rate.

  7. There are a few situations besides type 1 diabetes that can – in rare and extreme cases – result in ketoacidosis. These include doing a strict keto diet or fasting while breastfeeding or on a new class of diabetes medications called SGLT-2 inhibitors (e.g. Farxiga, Jardiance, Invokana).

  8. E.g. Farxiga, Jardiance, Invokana and more

  9. The chart is from the excellent book The Art and Science of Low-Carbohydrate Performance by Professor Stephen Phinney and Jeff Volek. Highly recommended.