Ketones are molecules your liver produces from fat when your body doesn’t have enough glucose for energy. They serve as a powerful backup fuel source, capable of powering your brain, heart, and muscles when carbohydrate intake drops or during fasting. Far from being just a byproduct of fat burning, ketones play active roles in energy production, brain function, and cellular protection.
How Your Body Makes Ketones
Your liver produces ketones through a process that starts when stored fat gets broken down into fatty acids. Those fatty acids travel to the liver, where they’re chopped into smaller units through a process called beta-oxidation. The resulting fragments are then converted into three types of ketone bodies: beta-hydroxybutyrate (the most abundant one circulating in your blood), acetoacetate, and acetone.
Beta-hydroxybutyrate and acetoacetate are the two that your tissues actually use for fuel. Acetone is a byproduct that forms spontaneously from acetoacetate and mostly exits the body through your breath, which is why people in deep ketosis sometimes notice a fruity or metallic taste in their mouth.
This whole system kicks in when glucose runs low. For healthy adults, ketone production typically ramps up after 3 or 4 days of eating fewer than 50 grams of carbohydrates per day, or during extended fasting. The liver essentially becomes a fat-processing factory, converting fatty acids into portable energy packets that other organs can burn.
Why Ketones Matter for Brain Function
Your brain is one of the most energy-hungry organs in your body, and it normally runs almost entirely on glucose. But it can’t burn fat directly. This creates a problem during starvation or prolonged fasting: where does the brain get fuel when glucose is scarce?
Ketones solve this problem. They cross the blood-brain barrier using specialized transport proteins and can supply up to 60% of the brain’s energy needs during periods of glucose restriction. This ability is likely why humans evolved the ketogenic pathway in the first place. Without it, the body would need to break down far more muscle tissue to produce glucose for the brain during lean times.
This brain-fueling capacity has also drawn significant interest from researchers studying neurodegenerative diseases, where the brain’s ability to use glucose efficiently appears to decline. The idea is that ketones could offer an alternative energy supply to struggling brain cells.
Ketones as a More Efficient Fuel
Ketones aren’t just an emergency backup. There’s growing evidence they may actually be a more efficient fuel than glucose in certain contexts. A 2020 study found that after 12 weeks of nutritional ketosis combined with exercise, participants showed a 19% improvement in mitochondrial efficiency and a 36% increase in ATP generation (the molecule your cells use as energy currency) compared to a control group eating a standard diet with the same exercise.
Ketosis also ramps up proteins involved in your cells’ energy-producing systems, including the enzymes that run the Krebs cycle, the central hub of cellular metabolism. On top of that, ketone metabolism increases production of uncoupling proteins, a family of molecules that sit inside mitochondria and help protect them from oxidative damage. Think of these proteins as pressure release valves that prevent your cellular power plants from overheating.
Medical Uses: Epilepsy and Beyond
The most established medical application of ketones is in treating epilepsy that doesn’t respond to medication. The ketogenic diet has been used for this purpose since the 1920s, and both beta-hydroxybutyrate and acetoacetate have demonstrated anti-seizure effects in animal models of epilepsy and neurometabolic disorders. For children and adults with drug-resistant seizures, a medically supervised ketogenic diet remains a validated treatment option.
Researchers are also exploring whether therapeutic ketosis, delivered through ketone supplements rather than strict dietary changes, could offer similar benefits. Ketone esters represent a potential way to raise blood ketone levels without requiring weeks of carbohydrate restriction, though human clinical trials for seizure disorders are still underway.
The Liver’s Fat Disposal System
Ketone production serves another purpose that often gets overlooked: it helps the liver manage fat. When fatty acids flood into the liver (from body fat breakdown during fasting, exercise, or low-carb eating), they get converted to a compound called acetyl-CoA. Ketogenesis takes that acetyl-CoA and packages it into ketone bodies that get exported to other tissues for fuel. This is a critical disposal pathway. Without efficient ketogenesis, excess fat can accumulate in the liver itself.
Research has linked impaired ketogenesis to metabolic-associated fatty liver disease, particularly in people with type 2 diabetes. In other words, when this fat disposal pathway breaks down, the liver essentially gets backed up with fat it can’t process efficiently.
Normal Levels vs. Dangerous Levels
There’s an important distinction between nutritional ketosis and a dangerous condition called diabetic ketoacidosis (DKA). In healthy people, the body tightly regulates ketone production and blood levels stay within a manageable range. DKA occurs primarily in people with type 1 diabetes (and sometimes type 2) when the body can’t produce enough insulin. Without insulin to act as a brake, ketone production spirals out of control, and the blood becomes dangerously acidic.
Blood ketone levels give a clear picture of where you stand:
- Below 0.6 mmol/L: Normal range for most people
- 0.6 to 1.5 mmol/L: Low to moderate DKA risk (this range overlaps with nutritional ketosis)
- 1.6 to 2.9 mmol/L: High DKA risk
- Above 3.0 mmol/L: Very high DKA risk, requiring immediate medical attention
For people without diabetes, reaching levels above 3.0 mmol/L through diet alone is extremely difficult because insulin keeps production in check. DKA is a medical emergency almost exclusively seen in the context of insulin deficiency.
How to Measure Your Ketone Levels
Three testing methods exist, and each measures a different ketone type. Blood meters measure beta-hydroxybutyrate, the primary circulating ketone, with accuracy around ±0.1 mmol/L. This is the gold standard for precision. Blood ketone readings reflect your current circulating ketone levels: what your body has produced minus what your tissues have already consumed.
Breath analyzers measure acetone, the volatile ketone that exits through your lungs. Rather than showing a snapshot of circulating ketones, breath readings correlate more closely with your overall rate of fat oxidation. One practical advantage: breath acetone stays more stable after meals, while blood ketone readings dip temporarily when insulin rises in response to food.
Urine strips are the cheapest and most accessible option, though they become less reliable over time. As your body gets better at using ketones efficiently, fewer spill into the urine, so the strips may show lower readings even when ketone production is steady. They’re a reasonable starting point but not ideal for ongoing tracking.