With the gradual resurgence of low-carb diets in recent years, the word “ketones” is thrown around a lot. But many people aren’t really aware of the details. What are ketones, really? And what do they do in the body?

There can be a lot of misinformation regarding the answers to these questions, so read on for a full overview of ketones and their role in a ketogenic diet.


Ketones, also known as “ketone bodies,” are byproducts of the body breaking down fat for energy that occurs when carbohydrate intake is low.

Here’s how it works:

  • When there isn’t a sufficient level of available glucose — which is what the body uses for its main source of fuel — and glycogen levels are depleted, blood sugar and insulin are lowered and the body looks for an alternative source of fuel: in this case, fat.
  • This process can happen when a person fasting, after prolonged exercise, during starvation, or when eating a low-carb, ketogenic diet.
  • And when the body begins breaking down fats for energy like this, a process known as beta-oxidation, ketones are formed for use as fuel for the body and brain. This is known as ketosis.
  • People following a ketogenic diet specifically reduce their carbohydrate intake for this reason: to create ketones for energy.

Many people use the benefits of ketosis — less reliance on carbs and more burning of fat — to possibly help lower blood pressure, reduce cravings, improve cholesterol, increase weight loss, improve energy, and more.


So, what else about ketones do we need to know? To start, there are technically three types of ketone bodies:

  • Acetoacetate (AcAc)
  • Beta-hydroxybutyric acid (BHB)
  • Acetone

what are ketones

Both acetoacetate and beta-hydroxybutyrate are responsible for transporting energy from the liver to other tissues in the body.


During the process of ketogenesis, which is when ketone bodies are formed from the breakdown of fatty acids, acetoacetate is the first ketone that’s created.

Beta-hydroxybutyrate is then formed from acetoacetate. (It should be noted that BHB is not technically a ketone due to its chemical structure, but in the context of the ketogenic diet it’s considered to be one of the three ketones.)

Acetone, which is the simplest and least-used ketone body, is created spontaneously as a side product of acetoacetate.

If acetone is not needed for energy soon, it will break down and be removed from the body as waste through the breath or the urine. Acetone is the cause of a characteristic fruity smell on the breath when someone is in ketosis or ketoacidosis.



For thousands of generations, people have likely been relying on ketones for energy when glucose wasn’t always available.

For example, our ancestors likely had frequent periods of time when food wasn’t immediately available — not to mention the fact that most high-calorie and more nutrient-dense foods would need to be prepared before being readily available to eat. And still today, our bodies are amazing at adapting to the burning of ketones for fuel.

Other functional benefits of ketones can include:

  • A useful effect on mental performance, as a large part of the brain can use ketones as energy for mental function while on a ketogenic diet.
  • Energy for exercise. Normally, the body would use the most recently eaten food or any stored glycogen as energy during a workout. So when both of those fuel systems are gone, the body adapts to the ketogenic state of creating energy from protein or fat. This can be helpful for athletes or people who do strenuous workouts and need to continue efficiently burning fat for fuel.


Excess ketones that aren’t used by the body can spill over in a few different ways, showing up the urine, blood, and the breath. Many people who follow a ketogenic diet will do regular tests to determine their levels of ketones and ketosis.

There are several ways to test your body for ketones. You can get the tests done in a lab, but there are quicker and more affordable alternatives.

The levels of ketones in the body can be anywhere from zero to very high, and they are measured in millimoles per liter (mmol/L). Below are the general ranges, but just keep in mind that test results can vary by the individual.

  • Negative ketone level: less than 0.6 mmol
  • Low to moderate ketone level: between 0.6 to 1.5 mmol
  • High ketone level: 1.6 to 3.0 mmol
  • Very high ketone level: greater than 3.0 mmol

Now that the levels are defined, let’s go over the different methods for testing and the pros and cons of each:

Urine Testing for Ketones

Method: You pee on a urine strip, which indicates the level of ketones by color. Look for manufacturers that offer strips containing a special chemical for testing ketone levels.

Pros: You can buy the strips at most pharmacies for a very low cost. This is an affordable and easy option for someone new to ketosis and/or a ketogenic diet.

Cons: Urine strips aren’t always that reliable for testing ketone levels. This is often because the longer a person is in ketosis, the more efficient the body becomes at using ketones (especially acetoacetate) for function, and so it’s possible the test can indicate a lower level of ketosis than you’re actually in. Also, the readings can be affected by other factors, including the level of electrolytes in the body or how hydrated you are.


Blood Testing for Ketones

Method: With a blood glucose meter, you use a lancet pen to press onto your fingertip and draw a small amount of blood. The blood applied on a test strip that monitors ketone levels through the meter.

Pros: This is a very accurate method of ketone monitoring, since there are few factors that alter the results.

Cons: It can be expensive, especially if you test frequently. Cost is often $5-10 per strip!

Note: the ketone BHB is transported through the blood, so this is the best way to monitor your levels of it.


Breath Testing for Ketones

Method: You use a Ketonix breath meter to test the amount of acetone present on your breath.

Pros: It’s affordable after purchasing the meter. Once you buy it, you can use it continuously without extra expense.

Cons: It’s not the most reliable testing method, so it’s best used in conjunction with other methods.

How to test ketone levels

Ketones and Diet

When it comes down to the right level of nutritional ketosis and ketones in the body, a proper ketogenic diet is key. For most people, that means eating between 20-50 grams of carbs per day. How much of each macronutrient (including carbs) you need to consume will vary, so you need to use a keto calculator like this one to figure out your exact macro needs.

To do this means cutting back on or completely eliminating most carb sources in the diet, including:

  • Whole and processed grains
  • Candies and baked goods
  • Fruit juices and sugary soft drinks
  • Refined sugars
  • Fruits
  • Starches like potatoes and pastas
  • Beans and legumes

Besides cutting back on carbs, a ketone-centric diet also involves eating moderate amounts of protein and, most importantly, high amounts of fats to rev up your fat burning.

Ketone Side Effects

For those who are just beginning a ketogenic diet, there are possible short-term side effects from the process you might experience within the first week or so. These can vary greatly depending on the person, and some people might not have any at all.

Temporary effects of ketosis might include:

  • Feeling weak
  • Headaches
  • Feeling “foggy” mentally
  • Mild fatigue or irritability
  • Flu-like symptoms

Thankfully, side effects are temporary and alleviated quickly since the body adapts to the dietary changes.



It’s important for those with diabetes to be aware of diabetic ketoacidosis (DKA), which turns the blood acidic if ketones reach a very high, dangerous level. This can happen with diabetics who are hurt, sick, or not intaking enough fluids.

It’s important to know that DKA is different from nutritional ketosis, which is safe on a healthy, nutritious ketogenic diet. For most people, there should be no concern about the formation of ketones, as they are either used or eliminated from the body and are part of a healthy weight loss and fat burning process.

Ketones can have a very beneficial role in many aspects of life, including overall health, weight loss, energy efficiency, and maintaining a wholesome ketogenic diet. Understanding the details about ketones and how they fit within the scope of ketosis and a low-carb diet is key for success in all of these areas combined!

what are ketones