Ketones are chemicals your liver produces when your body breaks down fat for energy instead of glucose. In small amounts, they’re a normal part of metabolism. Everyone produces trace levels of ketones during sleep and between meals. But when ketones build up in the blood beyond normal levels, they can shift your blood’s pH toward dangerous acidity, which is why understanding your numbers matters.
How Your Body Makes Ketones
Your cells prefer glucose as their primary fuel. But when glucose is scarce, whether from fasting, skipping carbs, or not having enough insulin to move glucose into cells, your liver pivots to burning fat. Inside liver cells, fatty acids are broken apart into a molecule called acetyl-CoA. When more acetyl-CoA is produced than the liver can use for its own energy, the excess gets converted into ketone bodies and released into the bloodstream. This process can handle as much as two-thirds of the fat entering the liver.
There are three types of ketone bodies: beta-hydroxybutyrate, acetoacetate, and acetone. Beta-hydroxybutyrate is the most abundant one circulating in your blood during ketosis, and it’s the one that blood ketone meters measure. Acetone is the least useful as fuel and gets expelled through your lungs, which is why people in deep ketosis sometimes have a distinctive fruity smell on their breath.
What Ketones Do in Your Body
Ketones aren’t waste products. They’re a legitimate backup fuel. Once released by the liver, they travel through the bloodstream and get absorbed by your heart, muscles, and brain, where they’re converted back into acetyl-CoA and burned for energy. The liver itself can’t use the ketones it produces, so every ketone body is destined for other tissues.
Your brain is especially interesting in this regard. Unlike fatty acids, ketones can cross the blood-brain barrier using specialized transport proteins. At low blood ketone levels (around 0.1 mmol/L), ketones supply roughly 5% of the brain’s energy. At 1 mmol/L, that jumps to 10 to 15%. During prolonged fasting, when blood ketones reach 5 to 7 mmol/L, they can cover 50 to 65% of the brain’s total energy needs. This is why humans can survive extended periods without food: the brain doesn’t depend solely on glucose.
During states of mild ketosis (around 1 mmol/L or higher), the heart, brain, and muscles all burn ketones in proportion to how many are available in the blood, up to a saturation point. This is a normal metabolic adaptation, not a sign of disease.
Normal Blood Ketone Levels
Blood ketone levels are measured in millimoles per liter (mmol/L). Here’s what the numbers mean:
- Below 0.6 mmol/L: Normal range. This is where most people sit day to day.
- 0.6 to 1.5 mmol/L: Slightly elevated. If you have diabetes, retest in two hours and contact your care team if levels stay up.
- 1.6 to 2.9 mmol/L: High. You’re at risk for diabetic ketoacidosis (DKA) and should seek medical advice promptly.
- 3.0 mmol/L or above: Very high. This level signals possible DKA and requires emergency care.
For people without diabetes who are fasting or eating a ketogenic diet, levels between 0.5 and 3.0 mmol/L are often referred to as “nutritional ketosis” and are generally not dangerous. The risk calculations above are most relevant for people with diabetes, particularly type 1 diabetes, where the underlying cause of high ketones is insulin deficiency rather than a deliberate dietary choice.
What Causes Blood Ketones to Rise
The most common non-medical triggers are everyday ones. Sleeping produces trace amounts of ketones because you’re essentially fasting overnight. Skipping meals or doing intermittent fasting pushes levels slightly higher. Prolonged, intense exercise can deplete your stored glucose, forcing your body to switch to fat burning and raising ketone levels temporarily.
A ketogenic diet is the most sustained non-medical cause. By restricting carbohydrates to roughly 5 to 10% of total calories and getting 70 to 80% of calories from fat, people on a keto diet keep their bodies in a consistent state of ketosis. This is intentional and, for most healthy people, self-regulating: the body adjusts insulin and other hormones to prevent ketones from climbing too high.
The dangerous scenario is uncontrolled diabetes. When someone with type 1 diabetes (or sometimes type 2) doesn’t have enough insulin, glucose piles up in the blood but can’t enter cells. The body reads this as starvation and ramps up ketone production aggressively. Without insulin to put the brakes on, ketones can spiral well past 3.0 mmol/L. Other triggers for dangerous ketone elevation include illness, infection, missed insulin doses, and certain medications.
Signs of Dangerously High Ketones
Mild ketosis from fasting or diet rarely causes noticeable symptoms beyond the fruity breath. But when ketones climb into dangerous territory, the signs escalate quickly. Early symptoms include nausea, vomiting, stomach pain, and headache. As levels rise further, you may notice fast, deep breathing (your body’s attempt to blow off excess acid through your lungs), dry skin and mouth, flushed face, muscle stiffness, and extreme fatigue.
Fruity-smelling breath is one of the hallmark warning signs of DKA. That smell is acetone, the same chemical found in nail polish remover, being exhaled in large quantities. If you have diabetes and notice this combined with any of the symptoms above, it’s a medical emergency.
Blood Tests vs. Urine Strips
There are two main ways to check ketone levels at home: blood ketone meters and urine test strips. They measure different things, and the difference matters.
Blood meters measure beta-hydroxybutyrate, the most abundant ketone in circulation, and give you a reading of your current levels in real time. Urine strips detect acetoacetate, a different ketone body, and reflect what was in your blood several hours ago rather than right now. Urine results can also be thrown off by dehydration, which concentrates the sample and skews readings. The color-matching chart that comes with urine strips is inherently less precise than a digital meter reading.
For anyone monitoring ketones regularly, whether for diabetes management or dietary purposes, blood testing gives more accurate and actionable numbers. Urine strips are cheaper and easier to use, which makes them a reasonable screening tool, but they shouldn’t be relied on when precision matters.
Why Ketones Matter Beyond Diabetes
Ketones play a protective role in normal metabolism that goes beyond simple backup fuel. Research published in the Journal of Clinical Investigation found that ketone production in the liver helps prevent fat from accumulating there. By converting excess fatty acid breakdown products into ketones and exporting them, the liver avoids becoming overloaded with fat. This process also helps regulate blood sugar by diverting carbon away from glucose-producing pathways in the liver.
The heart is a particularly enthusiastic consumer of ketones. Heart muscle cells readily absorb and oxidize beta-hydroxybutyrate, and research from the American Heart Association has explored how this fuel source may support cardiac function. During ketotic states, ketone oxidation becomes a primary energy contributor for the heart, with uptake increasing in direct proportion to blood ketone levels until the cells reach their processing capacity.
For most people, ketones in the blood are simply a sign that your body is doing what it’s designed to do: flexing between fuel sources based on what’s available. The numbers only become concerning when they climb past 1.6 mmol/L in someone with diabetes, or past 3.0 mmol/L in anyone, at which point the acid-base balance of the blood is at risk.