Ketones are often talked about like a diet trend. Depending on who you ask, they are the secret behind keto, a performance tool, a brain booster, or just another wellness buzzword. But before ketones were part of any trend, they were part of normal human metabolism.
At the most basic level, ketones are fuel. They are small molecules your body makes when carbohydrate (glucose) availability drops and fat use rises, such as during fasting, prolonged exercise, or a very low-carbohydrate/ketogenic diet. In those situations, ketones give the brain and body another energy option to work with.
Key Takeaways
- Ketones are small energy molecules your body makes mainly in the liver when carbohydrate availability is low, and the body starts to burn fat for fuel.
- The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone, with BHB being the main circulating ketone in most discussions of ketosis and ketone supplementation.
- Ketones can act as an alternative fuel for the brain and body, especially during fasting, prolonged exercise, and carbohydrate restriction.
- BHB is best understood first as a fuel, though it is also studied as a signaling molecule.
- Exogenous ketones can raise blood ketone levels without requiring fasting or a ketogenic diet.
What Are Ketones?
Ketones, more precisely ketone bodies, are small molecules produced mainly by the liver from fatty acids (released when the body breaks down or “burns” fat) when carbohydrate availability is low, and the body is relying more heavily on fat for fuel.
The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone. In most real-world conversations, BHB is the main character because it is the predominant circulating ketone and the one most ketone supplements (like Ketone-IQ) are designed to raise.
Your body can make ketones during:
- Overnight fasting
- Longer fasts
- Prolonged exercise
- Ketogenic or very low-carbohydrate diets
In all of these situations, the body is shifting its fuel strategy rather than suddenly “turning on” some unnatural pathway.
A useful way to think about ketones is that they are part of a fuel-export system. The liver makes them, releases them into the bloodstream, and then tissues like the brain, heart, and skeletal muscle can take them up and use them for energy.
Quick recap
- The three ketone bodies are beta-hydroxybutyrate (BHB), acetoacetate (AcAc), and acetone.
- BHB is the main circulating ketone, and the one most people mean when they talk about “ketones”.
- Ketones are produced mainly in the liver from fat-derived molecules.
- Your body makes ketones during fasting, carbohydrate restriction, prolonged exercise, and other low-glucose states, as well as when you take exogenous ketone supplements.
- Ketones act like a fuel-export system, supplying energy to other tissues like the brain, heart, and muscles.
How Ketones Work in the Body and Brain
A useful way to think about ketones is that they expand the body’s fuel options. They do not replace all other energy systems, and they do not mean glucose suddenly stops mattering. But when ketone levels rise, tissues that can use them gain access to another energy source.
At the biochemical level, the process is pretty simple. Ketones enter the brain, heart, and muscles through monocarboxylate transporters. Inside the mitochondria, BHB is converted to acetoacetate, then to acetoacetyl-CoA, and finally to acetyl-CoA, which feeds into the TCA cycle to make ATP. In plain English, ketones get turned into the same core fuel fragments the body uses from other fuels.
Ketones and the Brain
The brain is usually described as a “glucose-hungry” organ, and that is true under ordinary mixed-diet conditions. But the brain is also metabolically flexible. When ketones are available in the blood, the brain can take them up and use them as an alternative fuel.
Human evidence shows that the brain really does use ketones when they are available. In one human study, after fasting-induced ketosis, brain BHB rose from about 0.05 mmol/L in the nonfasted state to about 0.60 mmol/L after 2 days of fasting and 0.98 mmol/L after 3 days, showing that ketones were clearly entering the brain as blood levels increased. In a follow-up study, BHB infusion into the brain led to detectable ketone levels that were shown to be metabolized. It was estimated that at a blood ketone concentration of about 2.25 mmol/L, ketones accounted for roughly 6% of total brain energy production.
Ketones and the Heart
Most of the time, the heart gets a large share of its energy from fatty acids, but when ketones are available in the blood, the heart can take them up and burn them for energy rather than ignoring them.
In a study in healthy adults, the heart significantly metabolized acetoacetate, showing that the human heart takes up and processes ketones even under physiological conditions. In another human study, infusion of BHB altered how the heart used available fuels. These are studies where ketones were actively infused into the body—not generated by body fat breakdown or with exogenous ketones—but they demonstrate that the heart can use ketones as one available fuel source.
Ketones and Muscles
Skeletal muscle can also use ketones for fuel, but the story is a little different from the heart and brain. Muscle is a huge tissue system with very different energy needs depending on whether you are resting, walking around, doing intervals, or recovering from a hard workout. Ketones are not usually the dominant fuel in muscle, but when they are available, muscle can take them up and oxidize them as part of the broader fuel mix.
Human studies suggest that skeletal muscle can take up and use ketones during exercise under some conditions. Furthermore, training improves the body’s ability to use ketones. However, ketones should be seen more as a supplemental fuel source during exercise—not the main one. Researchers are studying their potential benefits for exercise performance through reducing glucose utilization and reducing protein breakdown in muscle in some contexts.
Beyond Fuel: An Area of Ongoing Research
Ketones are best understood first as a fuel, but researchers are also studying their broader signaling roles in the body. And while BHB has biologically interesting signaling roles that may eventually be shown to play roles in healthspan and aging, the most established and least speculative part of the ketone story remains fuel metabolism.
Quick recap
- Ketones are not just a liver or blood phenomenon—when they are available, major organs and tissues like the brain, heart, and skeletal muscles can take them up and use them for energy.
- In all three tissues, ketones are converted into acetyl-CoA, which then enters the TCA cycle to help produce ATP.
- The brain can use ketones as an alternative fuel source, especially when blood ketone levels rise during fasting, low-carb dieting, prolonged exercise, or after exogenous ketone intake.
- The heart is highly metabolically flexible and can readily use ketones when they are circulating.
- Skeletal muscle can also oxidize ketones at rest and during exercise, but ketones are usually one fuel among several, not the dominant source of exercise energy.
- Ketones are best understood first as a fuel source, and researchers are also studying additional signaling roles.
Endogenous vs. Exogenous Ketones
A useful distinction when talking about ketones and ketosis is whether ketones are made by your body or consumed from outside it.
Endogenous ketones are produced mainly in the liver during fasting, carbohydrate restriction, or other low-glucose states. Exogenous ketones are consumed in a drink or supplement and raise circulating ketones.
One of the most common questions people ask about ketones is: how long does it actually take to raise them, and how high do they go?
The answer? It depends.
Endogenous Ketones
Fasting usually raises blood ketones faster and higher than a ketogenic diet, at least in the short term. A ketogenic diet can absolutely produce nutritional ketosis, but it usually happens more gradually and typically stays in a more moderate range. Exogenous ketones can raise blood ketones rapidly and potently, depending on the dose.
Fasting
Blood ketone levels generally stay below about 0.3 to 0.4 mmol/L after an overnight fast, which means most people are not in what would usually be called meaningful nutritional ketosis after simply skipping breakfast.
But once fasting extends beyond that, ketones rise much more noticeably. After about 2 to 3 days of fasting, blood ketones commonly move into the roughly 1 to 4 mmol/L range. With 5 to 7 days of fasting, plasma BHB has been reported around 2.5 to 4.5 mmol/L, and prolonged fasting studies suggest that ketones often plateau around 4 to 5 mmol/L by about day 8, then can remain elevated with continued fasting.
So the fasting pattern is usually: a small rise overnight, a clearer rise by day 1 to 2, and then a substantial climb over several days as glycogen availability falls and ketone production ramps up.
Ketogenic diets
Ketogenic diets can also raise ketones, but usually not as fast or as high as total fasting. In practice, meaningful nutritional ketosis often takes a few days, not a few hours, because the body has to work through glycogen stores and adapt to much lower carbohydrate availability. Nutritional ketosis is typically defined as circulating BHB levels above 0.5 mmol/L. After 7 to 8 days of a low-carb diet, blood BHB levels can reach close to 1.0 mmol/L.
But here’s a lot of individual variation. Not everyone reaches the same ketone levels on the same fasting window or the same ketogenic diet. Baseline glycogen stores, body fat, insulin sensitivity, total calorie intake, protein intake, exercise, and even how “keto-adapted” someone already is can all change the response.
Exogenous ketones
In contrast to endogenous ketosis, exogenous ketosis refers to elevating blood ketone levels using ketone supplements or ketone drinks. So, while the body isn’t producing its own ketones, ketosis is still occurring.
Different ketone products create different “fuel curves.” The form you take (e.g., ketone esters, ketone diols such as Ketone-IQ, and ketone salts) affects how high blood ketones rise, how quickly they rise, and how long they last.
When you take exogenous ketones, blood BHB rises for a period of time, creating a temporary window of increased ketone availability. Ketone esters produce the biggest and fastest rise in blood ketones, usually pushing BHB to roughly 2.5–3.5 mmol/L for a few hours; R-1,3-butanediol (e.g., Ketone-IQ) produces a slower, smoother rise that often lands around 1–2 mmol/L depending on dose; and ketone salts usually generate the smallest increase, often around 0.5–1.0 mmol/L.
For example, acute studies show that taking 1 serving of Ketone-IQ raises blood BHB to 1–2 mM within 30 minutes, and levels can stay elevated for several hours, or longer with continued supplementation (e.g., 1 serving or “shot” of Ketone-IQ every 30 minutes).
These numbers depend a lot on dose, fed vs. fasted state, and the exact type of ketone you’re using.
Quick recap
- Endogenous ketones are made by your body, mainly in the liver.
- Exogenous ketones are consumed through supplements or drinks.
- Taking exogenous ketones can raise blood BHB without fasting or following a ketogenic diet.
- Overnight fasting usually produces only a small rise in ketones.
- Multi-day fasting can push ketones into a much higher range as glycogen becomes depleted.
- Ketogenic diets usually raise ketones more gradually over several days to about a week.
- Ketone responses vary from person to person based on factors like glycogen stores, activity, and insulin sensitivity.
Why People Use Ketones
People use ketones for three main reasons: steadier brain energy, exercise performance or recovery support, and broader metabolic-health goals—but the evidence is strongest for alternative fuel use and short-term metabolic effects, more mixed for exercise, and still preliminary for longevity.
Mental clarity and brain energy
One of the biggest reasons people use ketones is for mental clarity. In practice, this usually means taking them during a fasted morning, a long work block, a low-carb transition, travel, or any situation where someone wants steadier “brain energy” without the feel of something like caffeine or sugar. The logic here is straightforward: the brain can use ketones as an alternative fuel, so raising circulating ketones may help when fuel availability is constrained or when someone wants a non-stimulating support option.
That said, the evidence is more nuanced than “ketones make you smarter.” The most defensible version is that ketones may be helpful in specific contexts or situations of stress or low energy when an alternative fuel source might be desired (e.g., if you’re sleep deprived).
However, users of Ketone-IQ and other ketone supplements often describe the experience as steadier or smoother energy, though individual responses vary.
Exercise performance
Athletes often use ketones hoping for better endurance, steadier fuel availability, glycogen sparing, or better recovery. In the real world, that usually means taking them before long training sessions, races, or in the recovery window after hard exercise. Some of that interest came from early work suggesting ketone esters might alter substrate use in a way that could benefit endurance performance or recovery physiology.
So people definitely use ketones for sport, but the best current summary is that they are not a consistent all-around performance enhancer (it’s highly dependent on intensity, type, and duration of exercise), and many benefits—especially those starting to appear in the literature—may be more about specific recovery or exercise adaptation outcomes. That’s an area where research on professional cyclists is currently being conducted.
Metabolic health support
People use ketones here because ketone biology overlaps with some of the biggest themes in modern health science: fasting, metabolic flexibility, blood sugar control, and appetite regulation. This use case is less about feeling an immediate effect and more about supporting a broader metabolic state that people associate with better long-term health.
The important distinction is that metabolic-health support has better human evidence than longevity support. On the metabolic side, some studies have explored how ketones may influence short-term metabolic responses to meals and influence appetite-regulating hormones.
The longevity case is much more speculative. While ketones are biologically relevant to aging pathways, there is no direct human evidence that taking ketones extends lifespan.
In short, ketones have a promising metabolic-health use case, especially for glucose control and appetite signaling, but the longevity claim is still mostly mechanistic and animal-based, not something proven in humans.
Quick recap
- Many people use ketones to support steadier mental energy during fasting, low-carb transitions, or long work blocks.
- Athletes often use ketones in hopes of supporting endurance, fuel availability, or recovery.
- The performance literature is mixed and does not support ketones as a reliable universal ergogenic aid. Some studies suggest possible recovery-related or context-specific effects.
- Ketones are popular in the health and longevity category because they overlap with fasting, metabolic flexibility, and aging biology.
- The evidence is more convincing for short-term metabolic effects than for direct longevity benefits.
- Ketones may influence glucose responses and appetite signaling, but longevity claims are still mostly based on mechanisms and animal work, not direct human proof.
How to Measure Ketones
If you want to measure ketones, there are three main ways to do it: blood, urine, and breath. Each one measures a different ketone body, which is why the results are not interchangeable. Blood testing measures beta-hydroxybutyrate (BHB), urine strips measure acetoacetate, and breath devices measure acetone.
Blood ketone meters
Blood testing is the most accurate and useful way to measure your ketones right now. It uses a finger prick to measure BHB, the main circulating ketone in the blood, and is generally considered the best practical option when you want a real snapshot of current ketosis.
Pros
- Most accurate for current ketone status.
- Gives a specific number you can track over time.
- Best if you want to compare fasting, diet, or supplement effects.
Cons
- Requires a finger prick
- Test strips can be expensive
Urine ketone strips
Urine strips are the simplest and cheapest option. They measure ketones that have already been excreted into urine, so they tell you more about what was happening over the last few hours than what is happening in the moment. That makes them useful for beginners, but less useful when you want precision.
Pros
- Cheap and easy
- No finger prick
- Helpful for a basic yes/no check
Cons
- Less accurate than blood
- Affected by hydration
- Often becomes less informative over time
Breath ketone meters
Breath devices estimate ketosis by measuring acetone in your breath. The big appeal is convenience: no strips, no urine cup, no finger prick. But breath testing is still less standardized than blood testing, and these devices need more study before they are considered fully reliable and accurate.
Pros
- Noninvasive
- Easy to repeat often
- Useful for tracking general trends
Cons
- Less precise than blood
- Device quality varies
Which method is best?
If you want the most accurate answer to “What are my ketones right now?”, use blood BHB. If you want the easiest and cheapest way to check whether ketosis is happening at all, use urine strips. If you want a no-prick option and are okay with a little more variability, breath can be useful for trends.
Practical tips
- Pick the method that matches your goal. Blood is best for precision, urine is best for simplicity, and breath is best for convenience.
- Test under similar conditions each time. Ketones move up and down with fasting, meals, exercise, and time of day.
- Do not compare blood, urine, and breath numbers like they mean the same thing. They do not. Each method measures a different ketone in a different place.
- Focus on trends, not one reading. A single number can be noisy; repeated measurements under similar conditions are more useful.
Final Takeaways
Ketones are best understood as an alternative fuel source that the body naturally makes and can also receive from supplements. Their most defensible benefits are tied to fuel availability and metabolic context, while broader claims around performance, cognition, and longevity require more nuance and are an area of ongoing research.
FAQs
What Type of Ketone Is Ketone-IQ?
Ketone-IQ is best described as a ketone diol product made with R-1,3-butanediol (R-1,3-BDO).
Here’s how it works: after you consume R-1,3-butanediol, your liver metabolizes it into beta-hydroxybutyrate (BHB), which is the main circulating ketone body your brain, heart, and muscles can use for fuel.
It is not a ketone salt, and it is not a traditional ketone ester. It is designed to raise blood ketones by giving your body the raw material to make BHB for you.
Do higher ketone levels mean I’m burning more body fat?
Not necessarily. Higher ketones mean you have more ketones circulating, but that does not automatically tell you where they came from or whether you are losing more body fat. Ketones can rise because you are fasting, eating very low carb, or using exogenous ketones. That is one reason “higher ketones = more fat loss” is too simplistic.
Ketones are a marker of altered fuel use, not a direct fat-loss score. Nutritional ketosis is often defined at about 0.5 mmol/L BHB or higher, but being deeper into ketosis does not guarantee faster weight loss.
Do exogenous ketones put you into ketosis the same way fasting or a ketogenic diet does?
They raise blood ketones, but they are not the same thing as achieving ketosis through carbohydrate restriction or fasting. Fasting and ketogenic diets raise ketones because your body is shifting its metabolism toward fat-derived fuel production. Exogenous ketones raise ketones because you are taking ketones from a supplemental source.
That distinction matters. A ketone supplement can temporarily raise blood ketone levels, but it does not automatically recreate the whole metabolic state that comes with low insulin, glycogen depletion, and increased fat-derived ketone production.
What’s the best way to measure ketones?
For most people who want the most accurate snapshot of current ketosis, blood BHB testing is the best choice. It’s the current gold standard for assessing ketosis.
Urine strips are cheaper and easier, but they measure acetoacetate being excreted, not current blood BHB, so they can be less useful once someone is more keto-adapted or more hydrated. Breath testing measures acetone and can be useful, but device quality and interpretation vary more.
Do I need to hit a specific ketone number to get the benefits of ketones?
Usually not. For general nutritional ketosis, 0.5 mmol/L BHB or higher is a practical threshold, but that is not the same thing as saying everyone needs to chase a high number.
The more useful question is what you are using ketosis for. If the goal is general low-carb eating or body-composition support, obsessing over a bigger number is often less helpful than diet consistency and sustainability. A specific number or ketone range may be more useful if someone is using ketosis for therapeutic or medical reasons, or targeting a specific blood ketone level to support post-exercise recovery.
Can I have ketones in my blood after eating carbs or after taking a ketone supplement?
Yes. Blood ketones reflect what is circulating at that moment. Exogenous ketones can raise blood BHB even without carbohydrate restriction, and some people may still have measurable ketones after a meal, depending on timing, dose, and metabolic context.
That is another reason ketones need context. A number on a meter tells you one piece of the story, but not the whole metabolic picture.
Learn More
- How Long Does it Take to Get Into Ketosis and Keto-Adapt?
- Beta-hydroxybutyrate or BHB – All You Need to Know
- Do Exogenous Ketones Improve Focus and Mental Performance?
