With the gradual resurgence of low-carb diets in recent years, the word “ketones” is thrown around a lot. But many people aren’t truly aware of the details. What are they, 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 as the primary energy source once you’re in ketosis.

What Are Ketones?

Ketones, also known as “ketone bodies,” are byproducts of the body breaking down fat for energy. This only happens when your carbohydrate intake is low and your body switches into a state of ketosis.

Here’s how it works:

  • When you go super low-carb, fast for a prolonged period, or exercise heavily, your body eventually burns through glucose (aka, blood sugar) and glycogen stores (aka, stored sugars).
  • Once you run out of glucose, your body starts to look for an alternative source of fuel. In the case of the ketogenic diet ⁠— it’s mostly fat.
  • At this point, your body will start to break down dietary fat and body fat for fuel ⁠— a process known as beta-oxidation. Your body can use fatty acids for fuel, plus other compounds called ketones, which are formed in your liver.
  • 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.

Types of Ketone Bodies

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

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

The different types of ketones

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

Ketone Formation

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 it’s considered a ketone because of its relation to the other metabolites and its function in your body.)

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, it will break down and be removed from the body as waste through the breath or urine. Acetone is the cause of a characteristic fruity smell on the breath when someone is in ketosis or ketoacidosis.

Why Does Our Body Use Ketones?

For thousands of generations, humans have relied on ketones for energy when glucose wasn’t available.

For example, our ancestors likely experienced frequent periods when food wasn’t immediately available, whether because of food preparation or availability.  And still today, our bodies are amazing at adapting to the burning of ketone bodies for fuel.

Other functional benefits of ketones can include:

  • An increase in mental performance — because ketones readily cross the blood-brain barrier to provide your brain with quick and efficient fuel.
  • Physical energy — once you’re not relying on glucose for fuel, your body will become more effective at burning fat during exercise. This means more fat burning and steady energy once you’re in ketosis[*][*].

Testing Ketone Levels

It is possible to produce excess ketones that your body won’t use. When this happens, ketones can spill over, showing up in your urine, blood, and breath. And this is how you can test your ketone levels at home.

Knowing your ketone levels is a good way to determine if your diet and lifestyle changes are working.

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.

Your ketone levels 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, depending on your diet, activity level, and how long you’ve been in ketosis.

  • 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

Method: You pee on a urine strip, which indicates the level of ketones by color.

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

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

Blood Testing 

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

Breath Testing

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 your diet, including:

  • Whole and processed grains
  • Candies and baked goods
  • Fruit juices and sugary soft drinks
  • Refined sugars
  • Fruits
  • Starches like potatoes, bread, and pasta
  • 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 that 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 change in dietary fuel source over time.

Ketone Level Warnings

It’s important for those with both Type 1 diabetes and Type 2 diabetes to be aware of diabetic ketoacidosis (DKA), which turns the blood acidic if ketones build up and reach a very high, dangerous level.

DKA can be life-threatening, so if you’re diabetic, you should never start this diet without medical supervision. This can happen with diabetics who are hurt, sick, or not intaking enough fluids.

It’s also crucial 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 ketone production, as ketones 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: All the details on ketone bodies