What Are Ketones?

Ketones — also called ketone bodies — are molecules produced by your liver when glucose (blood sugar) is in short supply. They serve as an alternative energy source for your brain, heart, and skeletal muscles. Under normal eating patterns your body runs primarily on glucose derived from carbohydrates, but during fasting or very low-carb diets it switches to burning stored fat, and ketones are the metabolic byproduct of that process.

The shift to ketone-based energy isn't a sign that something is wrong; it's a deeply evolved survival mechanism. Our ancestors routinely went long periods without food, and the ability to produce ketones meant the brain — an organ that can't burn fat directly — still received steady fuel.

Your liver produces three distinct ketone bodies, each with a slightly different role:

Beta-hydroxybutyrate (BHB) makes up roughly 78% of circulating ketones and is the primary fuel used by your tissues. It's also the ketone measured by most blood ketone meters. BHB is technically not a 'true' ketone in chemistry terms (it lacks a ketone functional group), but it's grouped with the other two for metabolic purposes.

Acetoacetate (AcAc) is the first ketone produced during fat metabolism. It can be used directly for energy or converted into BHB. Urine ketone strips detect acetoacetate, which is why early-stage ketosis is sometimes easier to pick up on urine tests.

Acetone is a byproduct created when acetoacetate spontaneously breaks down. It's volatile and expelled through your lungs, which is why some people notice a fruity or metallic breath during deep ketosis. Acetone is the least abundant ketone body and provides minimal energy.

The process, called ketogenesis, unfolds in the mitochondria of liver cells. When insulin drops and glucagon rises — typical during fasting — fatty acids are released from adipose tissue and travel to the liver. There, enzymes break them down through beta-oxidation into acetyl-CoA. When acetyl-CoA accumulates faster than the citric acid cycle can process it, the excess is funneled into ketone production.

Interestingly, the liver itself cannot use ketones for energy; it lacks the enzyme needed to convert them back into acetyl-CoA. Instead, ketones are exported into the bloodstream for other organs to burn.

Beyond serving as backup fuel, ketones appear to have signaling roles. Research suggests BHB can act as a histone deacetylase (HDAC) inhibitor, which may help regulate gene expression related to oxidative stress and longevity. Ketones also reduce the production of reactive oxygen species (ROS) in mitochondria, potentially lowering inflammation.

For the brain, ketones are a remarkably efficient fuel — producing more ATP per unit of oxygen consumed than glucose. Some researchers believe this improved energy efficiency is why many fasters report enhanced mental clarity during ketosis.

It's important to note that nutritional ketosis achieved through fasting or diet is entirely different from diabetic ketoacidosis (DKA), a dangerous condition where ketone levels skyrocket uncontrollably due to insulin deficiency. Nutritional ketosis keeps BHB in the 0.5–3.0 mmol/L range, while DKA pushes levels above 10 mmol/L.