Ketosis During Fasting: How to Recognize It and Which Testing Methods Are Worth It

Day two of your fast, just after seven. Maybe you know this moment.

If you know this scene, you are in good company. Many people start measuring themselves when they fast. That is understandable, because a color feels like feedback. But your breath, the strip and a blood value tell three different stories.

I am writing this as a physician who values fasting and at the same time knows where it can tip over. I am interested in three questions: What happens inside you when ketosis begins? What do the different measuring methods show? And when is ketosis no longer a sign of fasting, but a warning signal?

What happens in your body when ketosis begins?

In short: After your last meal, your body first lives on sugar from the liver and increasingly makes new sugar. At the same time, insulin falls and fat is released. From it, the liver builds ketone bodies. In a randomized trial, the most important ketone body in the blood reached an average of 0.5 mmol/L after about 21 hours.

Maybe you have pictured ketosis like a light switch. Off yesterday, on today. That is how it feels when the strip suddenly shows color. But your metabolism works more like a dimmer.

Imagine your body as a house with two heaters. One runs on sugar, fast and convenient. The other runs on fat, slower, but with a huge supply. After a meal, the sugar heater does most of the work. During fasting, your body turns it down step by step and turns the fat heater up.

The brain has a problem here. It hardly uses fatty acids directly. That is why the liver turns into a refinery. It turns fat into small, water-soluble molecules, the ketone bodies, and sends them through the blood to your whole body.

Ketones are nothing unusual in themselves. A review from the Joslin Clinic at Harvard Medical School describes that they are always present in the blood in small amounts. They rise during fasting and prolonged exercise. The two most important are called beta-hydroxybutyrate and acetoacetate. Acetone is the third and, in terms of quantity, the smallest ketone body.

With longer fasting, the shift continues. In a large cohort from the Buchinger Wilhelmi clinic in Überlingen, 1610 people fasted for between 4 and 21 days. From day four onward, ketone bodies were detectable in the urine of more than 95 percent. Those who received only vegetable broth, rather than juice and honey as well, took in 72 instead of 236 kilocalories on average and excreted more ketones. You can read how such a fasting week works in the article on Buchinger therapeutic fasting.

And the brain? In 1967, a team around Owen and Cahill studied three people with obesity who fasted for five to six weeks as part of a study. Measurements via catheters in the brain's blood vessels showed that beta-hydroxybutyrate and acetoacetate had replaced glucose as the brain's most important fuel. The Harvard physician Cahill later recalled that during his training, many regarded beta-hydroxybutyrate as a metabolic poison. In his retrospective, he describes it as the substance that, through the brain, made it possible for humans to survive long periods of starvation.

What does this mean for you? If you want to know whether you are already in ketosis, allow for some patience. After 14 or 16 hours of intermittent fasting, levels are still low in many people. You can read what happens in these first hours in the article Intermittent Fasting 16:8. And if you start a fasting day with exercise, the switch can come earlier.

And now you know why the strip often stays pale on the first day of fasting.

Signs of ketosis: what you can feel and what it means

In short: Common signs of ketosis are fruity breath or breath reminiscent of nail polish remover, a changed taste in your mouth and, in the first days, headaches, tiredness or lightheadedness. None of these signs is specific. Only a measurement shows whether you are in ketosis. Whether you are doing well is shown by how you feel overall.

A metallic taste. Breath that you notice yourself. A dull head in the afternoon. Many people read these signs of ketosis as proof that the fast is working. Others get scared that something is wrong. Both are understandable.

The breath is the best understood. Acetone forms when acetoacetate breaks down on its own. It is so small and volatile that it leaves your body through the lungs. That is why your breath can smell fruity or a little like nail polish remover. A review from the University of Washington gives a range from about 1 ppm of acetone in the breath of people not on a diet up to 1,250 ppm in diabetic ketoacidosis.

The metallic taste, on the other hand, has hardly been researched. I have not found any study that cleanly links it to ketone levels. Many people who fast still describe it, often in the first days. An interplay of acetone, a drier mouth and less chewing is possible. That is an assumption, not a finding.

Not every case of bad breath during fasting is acetone. A dry mouth and a coating on the tongue often play a part. You can read more about this in the article on tongue coating and bad breath.

The right-hand column lists signs described in case reports and reviews of ketoacidosis. They can also have other causes, but they should be checked by a physician.

Take the signs as an invitation to observe yourself. Drink throughout the day, stand up slowly and plan the first days more calmly. If nausea turns into vomiting or you feel ill rather than just tired, this is not the moment to push through.

And now you know why your breath reveals more than your sense of taste, but less than a measurement.

Urine strips, breath tests, blood tests: which method measures what?

In short: Urine strips show acetoacetate excreted by the kidneys. Breath devices measure acetone. Blood meters measure beta-hydroxybutyrate, the most important ketone body in the blood. For a yes or no, the strip is often enough. For precise values and for people with diabetes, blood testing is the most reliable of the three methods.

Maybe you have already tried two methods. The strip is dark purple, the breath device shows a medium reading, and the blood value is rather modest. Which one is right?

Imagine a reservoir behind a dam. The blood is the lake, and beta-hydroxybutyrate is its water level. The urine strip measures what spills over the dam. The breath device measures the mist rising above the water. All three are connected. But only one of them measures the level.

This overview summarizes the studies and reviews in this section.

A team at Johns Hopkins University found a similar pattern in children on a ketogenic diet. As soon as the blood value was above 2 mmol/L, the strip already showed the highest level. Above that, it could no longer show any differences. The researchers therefore consider urine strips to be of only limited use for estimating ketosis in the blood.

That leaves the breath. Here the picture is more mixed.

A large study from West China Hospital in Chengdu used a laboratory mass spectrometer instead of a handheld device and measured breath, blood and urine in 4000 people. Breath acetone and blood values were only moderately related there as well. In people with normal blood sugar, acetone rose along with the blood value only up to about 3.28 mmol/L, and above that, the curve bent back down. Three members of the team worked for a technology company.

A review by the Diabetes Technology Society summarizes the state of the technology. For blood and urine ketones, there are self-tests cleared by the US Food and Drug Administration (FDA), but no breath devices for this purpose. It also describes a trap: as ketoacidosis subsides, beta-hydroxybutyrate is converted into acetoacetate. The urine test can then still rise even though the ketoacidosis is already easing. The review also mentions announced sensors that are intended to measure ketones in tissue.

And now you know why a purple strip and a modest blood value do not have to contradict each other.

Ketosis or ketoacidosis: where is the line?

In short: In fasting ketosis, your body brakes ketone production itself, partly through insulin. In ketoacidosis, this brake is largely missing. Ketones then rise unchecked, and the blood becomes acidic. This is an emergency that mainly affects people with diabetes, and rarely also pregnant or breastfeeding women or people taking certain medications.

Ketoacidosis. The word alone can be frightening when you are holding a purple strip in your hand. Maybe you have read that ketone bodies are acids. Now you are wondering whether your blood turns acidic when you fast.

Both states begin the same way: the liver builds ketones. The difference lies in the brake. Imagine a car rolling downhill. In fasting ketosis, it rolls, but the brake and the engine brake work. In ketoacidosis, the brakes have failed.

The most important brake is insulin. It keeps the fat stores closed and so limits the supply for ketone production. According to the review from the Joslin Clinic, the high ketone levels of diabetic ketoacidosis arise when insulin is lacking and opposing hormones such as glucagon are high. In healthy fasting, insulin is low, but it has not disappeared.

Added to this is a second, less well-known brake. Ketones can themselves dampen the release of fat.

The team at the Buchinger Wilhelmi clinic writes that fasting ketosis is often inappropriately compared with the unchecked ketone production of diabetic ketoacidosis. In their cohort of 1610 people fasting, ketosis never reached pathological levels. For context: the study comes from the clinic itself. A review from Oxford also describes starvation ketosis outside of pregnancy as rare and only rarely associated with severe acidosis.

Simplified comparison based on the cited reviews and cohort data.

For people with diabetes and high blood sugar, the Paris emergency department study offers a point of reference. There, a blood ketone value of 3 mmol/L or more was more strongly associated with ketoacidosis or hospital admission than a three-plus result on the urine strip. In 2024, five professional societies from the US and Europe presented a current framework for diagnosis, treatment and prevention in a joint consensus report. They include the American Diabetes Association and the European Association for the Study of Diabetes.

And now you know why the same color on the strip can mean something entirely different in two people.

Who should be especially careful: type 1 diabetes, SGLT2 inhibitors, pregnancy and breastfeeding

In short: With type 1 diabetes, while taking SGLT2 inhibitors, during pregnancy and while breastfeeding, fasting ketosis can more easily turn into ketoacidosis. In these situations, fasting belongs only within medically planned supervision, or it is not advisable from a medical point of view.

Maybe you are reading this article because you have diabetes and still want to fast. Or you are breastfeeding and want to lose weight again after your pregnancy. Your wish is understandable. That is exactly why a closer look is worthwhile here.

Type 1 diabetes: when the brake comes from a syringe

With type 1 diabetes, the pancreas makes little or no insulin of its own. The most important brake on ketone production then comes from an injection or a pump. If too little insulin is given during fasting, ketones can rise unchecked. If too much is given, there is a risk of low blood sugar. That is why fasting here belongs only in a plan made together with your diabetes team.

If you have type 2 diabetes, you can read in the article Fasting with Type 2 Diabetes what is possible and what is not.

SGLT2 inhibitors: dangerous because the blood sugar looks harmless

SGLT2 inhibitors are tablets that make you excrete more sugar in your urine. You can usually recognize them by the active ingredient ending -gliflozin. Combined with fasting or very few carbohydrates, they can promote ketoacidosis with almost normal blood sugar. Experts call this euglycemic ketoacidosis.

An international consensus report from 2019 notes for people with type 1 diabetes that some of these cases of ketoacidosis occurred with almost normal or only slightly elevated blood sugar. This makes them harder to recognize and can delay treatment.

A case report from Stanford shows another trap. A woman with type 2 diabetes developed euglycemic ketoacidosis even though she had stopped the SGLT2 inhibitor before being admitted to the hospital. Few carbohydrates, the stress of surgery and an acute illness came together. According to the case report, the effect of these medications can last longer than their half-life would suggest.

What this means for you: if you take an active ingredient ending in -gliflozin, for whatever reason, discuss any longer fast with a physician beforehand. Do not stop or change anything on your own.

Pregnancy: a greater tendency toward ketosis

Breastfeeding: rare but well-described cases

If you nod at any of these points, that does not automatically mean no. It means: first the conversation with a physician, then the decision.

And now you know why the same fasting week is an experiment that can be planned well for one person and a risk for another.

Why higher ketone levels are not automatically better

In short: A ketone value shows how much ketone is in your blood right now, not how much fat you are burning. Ketones from outside can raise the value and at the same time slow your own fat release. And in one study, a long water fast with markedly rising ketones was accompanied by signs of inflammation.

In online forums, ketone values are sometimes traded like personal bests. Anyone who reaches 3 mmol/L seems further along than someone at 1. In my practice, I often see people reach for the meter first thing in the morning and only afterwards notice how they slept.

A ketone value is a level gauge, not a speedometer. It shows how high the water in the bathtub is. Whether a lot is flowing in and a lot is draining out, or little is flowing in and little is draining out, it does not reveal. A high level can mean that you are producing a lot of ketones. But it can also mean that your cells are currently using little of them.

Remember the infusion study from Copenhagen? There, ketones supplied from outside lowered the body's own fat release by 37 percent. For ketone drinks, this probably means that they can raise the reading without your body releasing more of its own fat. This is an extrapolation from an infusion study with six men, not a result from studies with drinks.

The Überlingen fasting cohort also shows a mixed picture. People with high ketone excretion lost more weight and waist circumference, and their blood sugar and long-term blood sugar marker fell more. At the same time, their uric acid rose more. So more ketones did not only mean more of what was desired. You can read what elevated uric acid can mean in the article High Uric Acid.

There is one important exception. In ketogenic therapy for epilepsy, better seizure control in children was linked to higher blood values, especially above 4 mmol/L. But that is a medically supervised therapy with a clear indication, not a fasting goal for healthy adults.

On top of that, ketone values fluctuate strongly over the day. In the Freiburg study, the blood value averaged 0.70 mmol/L at 3 a.m. and 0.33 mmol/L at 10 a.m. So if you compare values from different times of day, you are often comparing times of day rather than progress.

If you measure, do it with a clear question, at the same time of day and not several times a day out of restlessness. And plan the end of your fast as carefully as the beginning. You can read how this can work in the article Breaking a Fast Properly.

And now you know why, when it comes to ketone values, I would rather ask how you feel than ask about your record.

How does KPNI view ketosis?

Clinical Psychoneuroimmunology asks how the nervous system, hormones, metabolism and immune system work together. Ketosis touches all four. The evidence is not equally strong everywhere, and that is exactly what I want to make visible.

I see a fifth level as well: mindset. In Nonviolent Communication, Marshall Rosenberg coined a principle that fits here too: observing is different from evaluating. A purple strip is an observation. “I'm doing everything right” or “I've gone too far” are evaluations. When you keep the two apart, measuring becomes what it can be again: information, not a grade.

And now you know why I see ketosis as a conversation between four systems and not as a single number.

Three levers if you want to keep an eye on your ketosis while fasting

You don't have to measure any of this. If you do, these three directions can give you orientation. They are not a treatment plan.

If you would rather not assess your situation on your own: below this article you will find the option to book an appointment.

And now you know why the best measuring method is the one that fits your question.

Frequently asked questions about ketosis during fasting

Where this topic touches other areas

Ketosis touches on nutrition, blood sugar, uric acid, exercise and even your breath. If you want to go deeper, these neighboring topics are worth a look.