Ketone bodies are three water-soluble molecules your liver produces from fat when glucose is in short supply. They serve as an alternative fuel source for your brain, heart, and muscles during fasting, prolonged exercise, or very low carbohydrate intake. The three ketone bodies are acetoacetate, beta-hydroxybutyrate (BHB), and acetone, and understanding what they do helps explain everything from ketogenic diets to diabetic emergencies.
The Three Types of Ketone Bodies
Your liver produces acetoacetate first. From there, it can go in two directions: it’s either converted into beta-hydroxybutyrate (BHB) through an enzyme reaction, or it spontaneously breaks down into acetone and carbon dioxide. Each of these three molecules behaves differently in your body.
BHB is the most abundant ketone body in your blood and the most useful as fuel. It’s technically not a “ketone” in the strict chemistry sense (it lacks a ketone chemical group), but it’s always grouped with the other two because it’s made from the same pathway. Acetoacetate is the central molecule of ketone production, the one the other two derive from. Acetone is the least useful of the three. It’s volatile, meaning it evaporates easily, which is why it escapes through your lungs and gives your breath a fruity or nail-polish-remover smell during heavy ketosis.
How Your Liver Makes Ketone Bodies
Ketone production, called ketogenesis, happens in the mitochondria of liver cells. The process starts when your body breaks down stored fat. Adipose tissue releases triglycerides, which get split into free fatty acids and glycerol. Those fatty acids travel to the liver, where they’re broken down through a process called beta-oxidation into a molecule called acetyl-CoA.
Normally, acetyl-CoA enters the main energy cycle of the cell. But when glucose is scarce and acetyl-CoA builds up faster than that cycle can handle it, the liver starts combining pairs of acetyl-CoA molecules into acetoacetate instead. This happens through a chain of three enzymatic steps, with the middle step being the bottleneck that controls how fast ketones are made. The liver itself can’t use the ketones it produces. It releases them into the bloodstream for other organs to burn.
While fat is the dominant raw material, amino acids and other metabolic leftovers can also feed into the process. These minor sources typically account for less than 10% of total ketone production.
What Triggers Ketone Production
The master switch for ketogenesis is the ratio between two hormones: insulin and glucagon. When you eat carbohydrates, insulin rises and suppresses ketone production directly by cutting off the fatty acid supply and inhibiting a key enzyme in the pathway. When you fast or restrict carbs, insulin drops and glucagon rises, flipping the metabolic switch toward fat breakdown and ketone synthesis.
This ratio is lowest during total starvation and highest after a carb-heavy meal. That’s why ketone levels climb during overnight fasting, multi-day fasts, ketogenic diets, intense prolonged exercise, or any state where your body has burned through its readily available glucose. Newborns also produce ketones at relatively high rates because they’re running on fat-rich breast milk and have limited glycogen stores.
Ketones as Fuel: More Efficient Than Glucose
Your brain is an energy-hungry organ that normally runs on glucose. But during fasting or carb restriction, ketone bodies cross into the brain and muscles through specialized transport proteins called monocarboxylate transporters. Once inside a cell’s mitochondria, they’re burned for energy.
Ketones are surprisingly efficient fuel. When BHB is broken down, it produces about 13 units of ATP (the cell’s energy currency) per molecule, compared to 10 ATP from the equivalent breakdown product of glucose. In heart muscle, this translates to roughly 30% greater efficiency of mechanical work compared to burning glucose alone. This efficiency is one reason researchers have studied ketones in the context of athletic performance, heart failure, and neurological conditions.
Signaling Roles Beyond Energy
BHB does more than provide calories. It also acts as a signaling molecule that influences gene expression and inflammation. In the brain, BHB can reduce neuroinflammation by shifting immune cells called microglia into a less inflammatory, more protective state. It does this by inhibiting enzymes that control how tightly DNA is packaged, which in turn changes which genes get switched on or off.
Animal studies show that BHB, along with the conditions that raise it (fasting, ketogenic diets), can prevent the brain’s immune cells from retracting their branches, a structural change associated with chronic inflammation and depressive behavior. When researchers blocked the signaling pathway BHB uses, these protective effects disappeared, confirming that the ketone itself was responsible. These signaling functions help explain why ketogenic diets have shown benefits in epilepsy and are being explored for other brain conditions.
Normal Ranges and Danger Zones
On a typical mixed diet, blood BHB levels hover near zero, usually below 0.5 mmol/L. Once you fast for 12 to 24 hours or follow a ketogenic diet consistently, levels rise into what’s called nutritional ketosis: 0.5 to 5.0 mmol/L of BHB in the blood. This is a normal, regulated metabolic state.
Diabetic ketoacidosis (DKA) is a different situation entirely. It occurs primarily in people with type 1 diabetes (or occasionally type 2) who produce little or no insulin. Without insulin to put the brakes on, ketone production runs unchecked. Blood becomes dangerously acidic. Research has identified diagnostic cutoffs for DKA at roughly 6.3 mmol/L of BHB or 8.0 mmol/L of total ketone bodies, both far above the range of normal nutritional ketosis. DKA is a medical emergency with symptoms including nausea, vomiting, abdominal pain, rapid breathing, and confusion.
The gap between nutritional ketosis and ketoacidosis is wide. A healthy person with normal insulin function essentially cannot produce ketones fast enough to reach dangerous levels, because even a small amount of insulin keeps the process in check.
How Ketone Levels Are Measured
There are three ways to measure ketones, each detecting a different molecule.
- Blood meters measure BHB from a finger prick, similar to a glucose test. This is the most accurate and timely method. Blood testing has been associated with reduced hospitalizations, faster recovery from DKA, and lower healthcare costs compared to urine testing.
- Urine strips detect acetoacetate. They’re cheap and easy to use, but they reflect what your kidneys excreted hours ago, not what’s in your blood right now. As your body becomes more efficient at using ketones, urine levels can actually drop even though you’re in deeper ketosis.
- Breath analyzers measure acetone in exhaled air. Maintaining a breath acetone level of about 2 parts per million (ppm) corresponds to roughly half a pound of fat loss per week. On the high end, levels can reach 8 ppm, corresponding to about 1.2 kilograms of fat loss per week. Breath testing is non-invasive but less precise than blood testing for clinical purposes.
For people managing diabetes, blood BHB testing is the clear winner in terms of accuracy and clinical outcomes. For people tracking a ketogenic diet, any of the three methods can provide useful feedback, though blood testing gives the most reliable snapshot of where you stand at any given moment.