Ketones Explained: The Fuel Your Body Was Built to Burn
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Most people have been taught to think of ketones as a warning sign. They show up on a hospital lab report. They are associated with diabetes and emergency rooms.
That framing is incomplete — and in most contexts, it is simply wrong.
Ketones are an alternative fuel your liver produces from fat whenever carbohydrate intake drops low enough. They are not an accident or a malfunction. They are a deliberate, tightly regulated metabolic system — one that kept our ancestors functioning through long winters, failed hunts, and extended stretches without food. The modern high-carbohydrate diet has simply switched that system off for most people, for most of their lives.
As biohackers, we want it back online. Here is the full picture.
What Ketones Actually Are
Ketones, more precisely called ketone bodies, are small, water-soluble molecules made in the liver from fatty acids. When glucose is scarce, they take over as a primary energy source for the brain, heart, and skeletal muscle.
The brain is the key reason this system exists. Fatty acids cannot cross the blood-brain barrier in meaningful quantities, so the brain cannot run directly on fat. Without an alternative, it would depend entirely on a constant supply of glucose. Ketones solve that problem. They pass into the brain readily and, once the body has adapted, can cover up to roughly two thirds of its energy needs.
There are three ketone bodies.
Acetoacetate (AcAc). This is the first ketone the liver produces. It is either used directly for fuel or converted into the other two.
Beta-hydroxybutyrate (BHB). BHB is the most abundant and most stable ketone in circulation, and it does most of the work. It travels through the bloodstream to the brain, heart, and muscles, where it is converted back into usable energy. It also acts as a signalling molecule — a point we return to below.
Acetone. Acetone is a breakdown product of acetoacetate. The body cannot use it for fuel, so most of it is exhaled. It is the source of so-called keto breath, the fruity or slightly metallic smell many people notice in the first weeks of ketosis. It typically fades as the body adapts.
How Your Body Makes Them
On a standard diet, the body runs predominantly on glucose from carbohydrates. Insulin rises after every meal, fat is stored rather than burned, and ketone production stays close to zero.
Remove most of the carbohydrates and that changes.
Blood glucose falls, and insulin falls with it. Low insulin is the signal that unlocks stored body fat. Fat cells release fatty acids into the bloodstream, and the liver takes them up. Inside the liver's mitochondria, those fatty acids are broken down through beta-oxidation into a compound called acetyl-CoA. When acetyl-CoA is produced faster than the liver can burn it, the excess is converted into ketone bodies. That conversion is ketogenesis.
The liver then releases the ketones into the blood, and they are delivered to the tissues that need them.
The whole sequence fits on one line:
Fewer carbs → lower glucose → lower insulin → fat released from storage → fatty acids broken down in the liver → ketones produced → steady fuel for brain and body.
One detail is worth knowing. The liver manufactures ketones but cannot use them itself, because it lacks the enzyme needed to burn them. Ketones are produced entirely for export: fuel made by one organ specifically to power the others. That is not a design flaw. It shows how central this system is to human metabolism.
How to Enter Ketosis
Ketosis is the metabolic state in which ketone production is high enough to become a meaningful energy source. There are two reliable ways to get there.
Low-carbohydrate eating. For most people, ketosis requires keeping net carbohydrates between roughly 20 and 50 g per day. The exact threshold varies with body composition, activity level, and metabolic health. Someone with insulin resistance will typically need to stay closer to the lower end.
Fasting. With no incoming food, insulin drops to its lowest levels and ketone production rises steadily. People who are already fat-adapted get there faster. Combine fasting with low-carbohydrate eating and the transition accelerates — a subject we cover in more depth in our article on metabolic flexibility.
The critical qualifier is the same one we return to in almost every article: food quality. Hitting a carbohydrate number is not the goal. A low-carb diet built on processed meat, refined oils, and packaged "keto" snacks will produce ketones while starving the body of the micronutrients that make metabolism work.
At Biohackers Corner, the standard is real food. That means quality meat and fish, eggs, butter and other natural animal fats, and generous amounts of non-starchy vegetables, particularly cruciferous ones such as broccoli, cauliflower, and cabbage. Ketosis is the metabolic state. Nutrient density is what makes it worth being in.
How to Know Whether You Are in Ketosis
Keto breath, reduced appetite, and steadier energy can all suggest ketosis. None of them confirm it. If you want to know, measure.
There are three ways to do it.
Blood ketone meters. A single drop of blood from a finger prick gives a direct reading of BHB. This is the most accurate method available at home, and the one we recommend for anyone serious about tracking.
Urine test strips. Strips are cheap, easy, and useful for beginners in the first weeks. But they measure acetoacetate being excreted, not ketones being used. As the body becomes fat-adapted and uses ketones more efficiently, less spills into the urine. Strips can therefore show weak or negative results even when you are firmly in ketosis. Do not let a pale strip convince you that you are doing something wrong.
Breath meters. These measure exhaled acetone. They are convenient and needle-free, but the least precise of the three methods.
Whichever method you choose, test at the same time each day. Ketone levels shift with meals, exercise, sleep, and stress. Consistent timing removes most of that noise and makes the trend readable.
Reading the Numbers
Blood ketones are measured in millimoles per litre (mmol/L). A reading below 0.5 mmol/L is normal for someone eating a standard, carbohydrate-based diet. Nutritional ketosis begins above that line, and most well-adapted people sit somewhere between 0.5 and 3.0 mmol/L, with considerable individual variation. Higher is not better. There is no additional benefit to chasing large numbers through diet.
Urine strips use a colour scale printed on the packaging: the darker the shade, the more acetoacetate is present. Breath meters report acetone in parts per million (ppm), and nutritional ketosis generally falls between 4 and 30 ppm.
Ketosis Is Not Ketoacidosis
This is the most important distinction in this article. It is also the one most often confused — sometimes even by medical professionals.
Nutritional ketosis is a controlled physiological state. The body still produces insulin, and that insulin acts as a brake. It keeps ketone production within a safe range and keeps blood chemistry stable. Ketone levels rise moderately, and blood pH does not change meaningfully. This is the state your body enters during fasting or carbohydrate restriction, and it is entirely normal.
Diabetic ketoacidosis (DKA) is a different thing altogether. It occurs when the body has a severe lack of insulin, most commonly in type 1 diabetes. Without insulin there is no brake. Ketone production runs out of control while blood sugar climbs at the same time. Ketones accumulate to levels many times higher than anything seen in nutritional ketosis, and the blood becomes dangerously acidic.
Early signs of DKA resemble those of very high blood sugar: frequent urination, intense thirst, dry mouth and skin, and headache. As it progresses, nausea, vomiting, abdominal pain, confusion, and deep, laboured breathing follow. DKA is a medical emergency and can be fatal without treatment.
DKA is not limited to type 1 diabetes. People with type 2 diabetes can develop it too, particularly those taking SGLT2 inhibitor medications, which can trigger DKA even when blood sugar is not obviously elevated. A separate form, alcoholic ketoacidosis, is associated with heavy alcohol use.
The practical rule is simple. Talk to your doctor before starting a ketogenic diet or extended fasting if any of these apply to you:
- You have type 1 diabetes.
- You take insulin or other glucose-lowering medication.
- You have a condition that affects insulin production.
Medication doses often need adjusting as carbohydrate intake drops. For a metabolically healthy person, nutritional ketosis is a natural state, not a danger.
What Burning Ketones Does for You
Shifting from glucose to ketones as a primary fuel changes more than which molecule your cells are burning. The downstream effects are what make ketosis worth pursuing.
Appetite control. A review published in Nutrients found that ketogenic diets reduce hunger and suppress ghrelin, the hormone that drives appetite — even during weight loss, when hunger would normally rise. That last point matters. Most diets fail because the body fights back with hunger. Ketosis appears to switch off much of that fight. Weight management becomes a matter of eating to satisfaction rather than resisting constant cravings.
Stable blood sugar and steady energy. Further research in Nutrients found that carbohydrate restriction significantly reduced blood sugar spikes. Fewer spikes mean fewer crashes. You lose the mid-afternoon fog and the irritability when a meal is late, and your energy stops depending on when you last ate. Ketones deliver a steady fuel supply drawn from fat reserves that, for most people, are effectively unlimited.
Lower inflammation. BHB is not only a fuel. Research published in Nature Medicine showed that it blocks the NLRP3 inflammasome, a key driver of inflammatory responses in the body. A review in the Journal of Child Neurology similarly found evidence that ketogenic diets reduce inflammation. The research here is still developing, but the mechanism is credible. It also fits what we see repeatedly: when the dietary drivers of inflammation are removed, the body calms down.
A steadier fuel for the brain. Ketones do not rise and fall with every meal the way glucose does. For the brain, the most energy-demanding organ in the body, that consistency matters. Many people describe sharper focus and clearer thinking once they are fat-adapted.
The Biohackers Corner Perspective
Ketones are not a hack. They are not a supplement trend or a diet gimmick. They are a native human fuel — one your body is fully equipped to produce, and one it relied on routinely for most of our evolutionary history.
What changed was the food supply. Constant access to carbohydrates keeps insulin elevated, keeps fat locked in storage, and keeps the ketone system dormant. Most people go their entire lives without ever running on it.
Turning it back on is straightforward.
Cut the carbohydrates. Prioritise real, nutrient-dense food. Use fasting strategically. Measure, so you know rather than guess. Understand the difference between ketosis and ketoacidosis, and see a doctor if your situation calls for it.
Fewer carbs → lower insulin → fat released → ketones produced → steady energy.
Your body already knows how to do this. Let it.
