Ultimate Guide to Ketones – What Are Ketones and How to Measure Them

Ultimate Guide to Ketones

If you recently became inspired to start a ketogenic diet, you’re probably beginning to ask some common questions. Primarily, "what are ketones?", "why are ketones important" and  “how do I measure them?"

Fortunately, we have some answers to all of those questions, and some suggestions on the best ways to measure your ketones.

What Are Ketones? The Ultimate Guide

You've probably heard that on the ketogenic diet, your body switches over to primarily burning fat as energy.  But if you're not burning carbs and therefore, glucose, then how does your body get energy?  That's where ketones come in.

Normally, on a high carbohydrate diet, your body would primarily use carbohydrates as an energy source.  In your body, the carbohydrates are broken down into glucose or glycogen (the storage form of glucose) as smaller components easier to use as energy in the body.

On a high fat low carbohydrate diet like the ketogenic diet, your body will primarily use fat as energy.  In your body, the fat is broken down into fatty acids, which are then broken down into ketones as the smaller components that are easier to use as energy in the body.

Ketones are molecules that take the place of glucose in the TCA/Krebs cycle which creates ATP (energy) for cells.  Ketones can be measured in the blood, urine or breath as a way to determine the extent of ketosis.  In short, ketones are molecules produced from fat sources to supply your cells with energy when they are starved of glucose.

Type of Ketones

There are three types of ketones, aka ketone bodies, that are produced in your body during ketosis.  These three ketone bodies are acetoacetate, beta-hydroxybutyrate, and acetone.

acetoacetate chemical structure
acetone chemical structure
betahydroxybutyrate chemical structure

Acetoacetate is the first ketone body produced from fatty acids.  From there, acetoacetate can be broken down into beta-hydroxybutyrate (BHB or beta-hydroxybutyric acid) and/or acetone.  Beta-hydroxybutyrate is the main ketone circulating in your body, making up about 78% of the circulating ketones.  Acetoacetate makes up about 20% of circulating ketones, while acetone only accounts for 2%.[i]

Beta-hydroxybutyrate and acetoacetate can be used directly to make ATP for your cells.  Acetone, on the other hand, is broken down from acetoacetate and is expelled from the lungs or in the urine.

Beta-hydroxybutyrate, acetoacetate and acetone are all chemically similar - and that is where their names are derived.  They all contain a ketone group - a double bonded oxygen atom.  See the pictures below to see the ketone group.


One last note: raspberry ketones are a popular fad weight loss supplement that actually have nothing to do with the ketogenic diet.

How Ketones Are Produced in the Body

This explanation of how ketones are produced in the body requires a little bit of a science background, but we've tried to keep it simple.

When your body is starved of glucose, it takes time for your body to switch over to producing and using ketones.  It first uses up all the available glucose and glycogen in the liver - as that is its preferred source of energy.

Next, as your body realizes it is running out of glucose/glycogen, it will begin to use fat as an energy source.  Fatty acids, derived from adipose tissue or fatty food, will be transported to the cells in your body for energy; however, fatty acids cannot cross the blood-brain barrier so cannot be used by your brain.  At this immediate time your brain will continue to utilize glucose through a process called gluconeogenesis - the breakdown of protein (ie, your muscle) into glucose.  Your body does not want to break down muscle as this can be detrimental, so once another source of energy is available for your brain, it will begin to utilize that - and this will soon be ketones.

As we noted before, your glucose and glycogen have been depleted.  When glucose levels are low, so are insulin levels.  This decrease in insulin levels (plus a few other body signals) triggers beta oxidation - the breakdown of fatty acids for energy.  In short, this begins the process of creating ketones.[ii]

Ketones are created in a process called ketogenesis.  Acetoacetate (one of the main ketone bodies) is produced in the liver through this process.  The full process of ketogenesis takes several steps, but it ultimately creates acetoacetate from fatty acids.  Acetoacetate can then be broken down into BHB and/or acetone.

BHB can then be shuttled by a mediator to the rest of the body to be used as energy through a process called ketolysis.  Ketolysis is where the beta-hydroxybutyrate is broken down into ATP to be used directly by cells for energy.

How Long Does it Take to Produce Ketones

Your body only stores enough glycogen in the liver for 12-14 hours.[iii]  So technically, your body actually runs out of glycogen overnight which initiates the process toward ketone production the next morning.  If you eat a high fat, low carbohydrate diet, your body will continue the process toward ketosis.

Gluconeogenesis (turning protein into glucose) does not last long because your body cannot afford to lose a lot of muscle to glucose.  Your body will start to produce ketones very quickly to avoid muscle breakdown.  In fact, once your blood ketone levels rise, your brain will begin to use ketones primarily over glucose.

Your body will actually overproduce ketones in the first few days of the ketogenic diet, which is the cause for things like keto breath (sweet smelling breath caused by an excess of acetone being expelled) and keto pee/urine (sweet smelling urine caused by an excess of acetone excreted in the urine).

How Long Does Keto Adaptation Take?

The process of producing ketones is actually very fast: starting at 12 hours after a meal.  However, what takes longer is the process of adapting your body to producing and using ketones efficiently and any initial side effects.

For example, the keto flu side effects take place usually in the first 3-4 days of the ketogenic diet, but symptoms can sometimes last 1-2 weeks.  Some of these symptoms are simply side effects of changing your body over to a completely different diet and metabolic process.  For example, headaches can be caused by a depletion of water and electrolytes as a side effect of depleting glycogen.  But this doesn't last long as your body adjusts to the loss of glycogen and stops depleting as much water.

In addition, it can take 3-4 weeks to become more efficient at using ketones for exercise.  Before this happens, your body can feel sluggish and tired during even the simplest of exercises.

For more intense exercise, it can take even longer, sometimes months to adapt to using ketones.  In some cases of extremely intense exercises, your body may never adapt and may require a targeted ketogenic diet (TKD) for high exercise intensities.

In summary, your body begins producing ketones in 12-14 hours following an overnight fast or low carb intake.  It will continue to produce ketones until you give it too many carbs, or in some cases too much protein, for it to run on fat.  Remember, insulin is a key factor in ketone production - so you want to avoid any sugar spikes that would cause insulin to shoot up.  This is especially important during the adaptation phases when your body is in ketosis, but is still not adapted to primarily using ketones.

Why You Should Measure Your Ketones

Some people begin a low-carb, ketogenic inspired diet, and never actually test to confirm that their body is in ketosis. Instead, they wait for symptoms of ketosis to arise, like reduced hunger, and sustained energy.

However, these symptoms aren’t entirely reliable. As you reduce carbohydrates, and increase fat, you’re going to naturally experience some changes in how you feel. But those changes don’t necessarily confirm the production and use of ketones.

To maximize the benefits of the keto diet, it’s best to measure your ketone levels, especially when you’re first starting out. By monitoring your ketone levels, you can assure that you’re doing the diet correctly and make dietary adjustments based on what you measure. People also respond to diet and exercise differently, so the best way to cater the keto diet to your own biology is to measure.

What are the Optimal Ketone Levels?

Before we discuss how to measure ketone levels, let’s set some guidelines for optimal ketone levels. Nutritional ketosis is detected when levels begin to read at 0.5 mmol/L, but your optimal ketone level will depend on your personal goals. For instance, if your goal is to lose weight, your target ketone level will be lower than someone who wants to improve mental performance.  The following table provides some general guidelines based on your goal.

Your Goal Target Ketone Level (in mmol/L) Target Ketone Levels (mg/dL)
I want to lose weight 0.5 mmol/L or more 9 mg/dL or more
I want to improve my athletic performance 0.5 mmol/L or more 9 mg/dL or more
I want to improve my mental performance 1.5 – 3 mmol/L 27 – 54 mg/dL
I’m treating an illness 3 – 6 mmol/L 54 – 108 mg/dL

Keep in mind, these are just guidelines. If you have type 1 diabetes, these will not apply to you, and you should seek medical consultation before starting a ketogenic diet. Also, if you’re eating a ketogenic diet to treat an illness, like epilepsy, consult with your medical provider for optimal levels.

The Best Ways to Measure Ketones

Don’t worry, you don’t need to go to a blood lab to get a detailed blood panel. You can test for ketosis at home, using a ketone monitor or test strips. The best ketone monitors or tests utilize one of three different methods for testing: urine, blood, or breath.

Remember that we previously discussed you can find three different ketones in the body: acetoacetate, beta-hydroxybutyrate and acetone.  But depending on the test you use, you'll be measuring a different ketone and can have different results.  Let's discuss which tests are best for your situation.

Testing Urine for Ketosis

This is one of the most popular methods for measuring ketone levels because it’s cheap, and easy. Urine ketone tests specifically measure for the ketone acetone.

When you first start to produce ketones, your body won’t be able to use them all. So, any excess ketones are excreted in the urine.  This is great at the beginning of the ketogenic diet, when you want a general idea that your body is creating ketones and you are doing the diet correctly.  And urine strips are a cheap way to do this.  However, there are a lot of factors that make testing ketones by urine inaccurate.

If you drink a lot of water, you’ll dilute your ketone levels, and the urine test will read low. If you’re dehydrated, your ketone levels will read higher. Also, as your body continues to adapt, it better utilizes ketones, and excretes less acetone into your urine. So, the more keto-adapted you are the less reliable urine tests become.

If you still want to try urine strips just to know you're in ketosis, you just need to buy a pack of ketone strips, which typically range from $10 - $15 for 100 strips. You then pee on a strip and wait approximately one minute for it to change colors, indicating your level of ketosis. You then simply compare your strip to the color key that comes with the test.

If you’re interested in trying urine test strips, this is the top one we recommend.

[Check out our review of several tests here.]

Testing Blood for Ketosis

The best keto monitors work by detecting beta-hydroxybutyrate (BHB). As your body becomes keto-adapted, the levels of BHB in the blood stream increase. Blood tests are the most accurate method for measuring ketones, but precision comes with a price, and a little bit of pain.

BHB is measured in the same way diabetics measure blood sugar. You’ll clean your finger tip using alcohol, then use a lancet to prick and draw blood. You then place a drop of blood on a special test strip and insert it into an electronic ketone monitor. Many monitors are actually intended for testing both ketones and glucose, using different test strips.

Most keto blood test monitors come in a starter kit containing the monitor, a lancet, and some test strips, and they range from $30 - $80. As time goes on you’ll need to buy more test strips too, which average about $1 - $5 per strip - which really adds up.  However, this is definitely the most accurate way to go.

If you’re interested in this route, the Nova Max blood ketone monitor is the one we like.

Testing Breath for Ketosis

Finally, you can test your ketones through your breath.  Breath ketone analyzers are nice because you don’t need to keep buying test strips. You just pay a one time fee for the breath analyzer itself.

Ketone breath analyzers work by detecting the ketone acetone in your breath. You simply breath into the device and it will flash a different color to indicate the level of acetone in your breath. For the popular Ketonix brand test kit, the colors correspond to ketone levels as follows:

  • Blue = No Ketones
  • Green = Low Ketone Levels
  • Yellow = Moderate Ketone Levels
  • Red = High Ketone Levels

The process is straight forward, and more convenient than testing using blood or urine. However, breath tests aren’t as accurate as blood tests. You aren’t given a solid ketone number, like blood monitors will give you. Although you’ll save money on test strips, the initial investment can be somewhat steep. The Ketonix Bluetooth, a popular test kit, retails for $300. In addition, even if you have the extra cash to spend, they’re hard to get your hands on because they frequently run out of stock.

When is the Best Time of Day to Test for Ketones?

There are a variety of opinions on when it’s best to test for ketones. But the consensus is that you shouldn’t overthink it. The most important thing is that you consistently measure at the same time of day because ketones will fluctuate based on a variety of factors throughout the day.

Often people make the mistake of measuring their ketones at different times of day. So, one day they measure their ketones right when they wake up. The next day they wake up, eat breakfast, and then measure. Then, maybe the next day they get busy and don’t measure until after an evening workout. The problem with this approach is that your ketone levels will naturally fluctuate throughout the day, regardless of your diet.

Ketone levels naturally tend to be lower in the morning, and higher in the evening.  In fact, this study found that ketones in urine peak at 3am and gradually decrease until 10am, when they gradually increase again.  Blood ketones levels had a similar trend, however, on a much smaller scale.

Your ketone levels will also fluctuate immediately after a meal, or exercise. For instance, with aerobic exercise, your body more heavily relies on ketones for energy, so you’ll see your ketone levels increase. For this reason, it’s recommended to wait 2-4 hours after a meal or exercise to measure.

So, to accurately monitor your progress, you need to take consistently scheduled measurements, and compare those values. For a lot of people, the optimal testing time is upon waking up in the morning because it’s easy to remember, and there haven’t been any recent meals or activities to affect their levels.  However, ketones tend to be lowest in the mornings - so give yourself a break if they are lower than expected and possibly recheck later in the day.

The bottom line – pick a time of day that’s convenient for you to test your ketone levels. Then consistently measure at that same time of day.

When You Don't Need to Test for Ketones

After you’ve become keto-adapted, it becomes less important to measure your ketone levels. In approximately 2-4 weeks you should start to see your ketone levels (and blood sugar) stabilize. By tracking your progress over the first few weeks you’ll become more familiar with how your diet effects your ketone levels. As you develop this awareness, you’ll become less dependent on keto tests, and maintaining ketosis will become second nature. During that time, you’ll also start to feel the benefits of ketosis kick in. But, until then, ketone tests can provide helpful guidance, just like a pair of training wheels.


[i] Yue Qiao, et al.  " Breath Ketone Testing: A New Biomarker for Diagnosis and Therapeutic Monitoring of Diabetic Ketosis." Biomed Res Int. 2014; 2014: 869186

[ii] Maja Grabacka et al.  "Regulation of Ketone Body Metabolism and the Role of PPARα."  Int J Mol Sci. 2016 Dec; 17(12): 2093

[iii] G.F. Cahill.  "Ketosis" Trans Am Clin Climatol Assoc. 1973; 84: 184–202.

1 Comment
  1. Great post! Thanks for this informative article. This could be a good guideline to take into consideration before going to any desired goal for yours.

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