Ketones appear in your urine when your body switches from burning glucose to burning fat for energy. This shift happens whenever your cells can’t get enough glucose, whether from fasting, intense exercise, diabetes, pregnancy, or several other triggers. Small amounts of ketones in urine are common and often harmless, but high levels can signal a serious medical problem that needs immediate attention.
How Your Body Produces Ketones
Your body’s preferred fuel is glucose, which comes mainly from carbohydrates. When glucose runs low or your cells can’t absorb it properly, your fat stores step in. Fat tissue releases fatty acids into your bloodstream, and your liver breaks them down through a process that generates a molecule called acetyl-CoA. Normally, the liver feeds acetyl-CoA into its main energy cycle. But when too much acetyl-CoA builds up faster than that cycle can handle it, the liver converts the excess into ketone bodies instead.
This process is tightly controlled by hormones. Insulin is the main brake on ketone production: it keeps fat breakdown in check and helps cells absorb glucose. When insulin drops, as it does during fasting or in uncontrolled diabetes, stress hormones like glucagon, cortisol, and adrenaline take over. These hormones ramp up fat breakdown, flood the liver with fatty acids, and accelerate ketone production. The ketones then circulate in your blood and eventually spill into your urine, where a test strip can detect them.
Diabetes: The Most Common Medical Cause
Uncontrolled diabetes is the most frequent medical reason for high ketone levels in urine. In type 1 diabetes, the body produces little or no insulin. Without insulin, cells can’t take in glucose even when blood sugar is extremely high. The body reads this as starvation and shifts aggressively to fat burning, producing ketones at a dangerous rate.
This can escalate into diabetic ketoacidosis (DKA), a life-threatening emergency. DKA is diagnosed when blood sugar reaches 200 mg/dL or higher, ketone levels rise significantly, and the blood becomes too acidic. It’s most associated with type 1 diabetes, but people with type 2 diabetes can develop it too, particularly during illness, infection, or surgery. Certain diabetes medications called SGLT2 inhibitors add an unusual wrinkle: they can trigger a form of DKA where blood sugar stays relatively normal (below 250 mg/dL), making it harder to recognize. This “euglycemic” DKA is uncommon but dangerous precisely because the normal glucose reading can be misleading.
Fasting, Low-Carb Diets, and Skipping Meals
You don’t need to have diabetes for ketones to show up in your urine. Mild ketosis typically develops after just 12 to 14 hours without food. If fasting continues for days, ketone levels can climb dramatically, reaching 8 to 10 mmol/L. This is the principle behind ketogenic diets, which restrict carbohydrates to roughly 20 to 50 grams per day to intentionally push the body into fat-burning mode.
For most healthy people, this mild to moderate ketosis isn’t dangerous. Your brain adapts to using ketones as fuel, and your kidneys filter the excess into urine. But prolonged fasting without medical supervision can tip into starvation ketoacidosis, a rare but serious condition documented even in otherwise healthy individuals after extended religious fasts or severe calorie restriction. Skipping meals regularly or eating very few carbohydrates over several days can produce enough ketones to show up on a urine test, even if you feel fine.
Pregnancy
Ketones in urine are a routine check during prenatal visits because pregnancy raises the risk in several ways. Morning sickness, especially the severe form called hyperemesis gravidarum, can cause repeated vomiting and dehydration that depletes glucose stores quickly. Skipping meals because of nausea has the same effect. The growing baby also increases your calorie and carbohydrate needs, so what might have been adequate nutrition before pregnancy may now leave your body short on glucose.
Gestational diabetes is another trigger. Like other forms of diabetes, it impairs your body’s ability to use glucose effectively, pushing metabolism toward fat burning. If ketones show up during a prenatal urine test, the usual approach involves eating more frequent meals, increasing carbohydrate and calorie intake, staying hydrated, and in some cases receiving IV fluids or anti-nausea medication for severe morning sickness.
Exercise, Illness, and Physical Stress
Intense aerobic exercise is a well-documented cause of urinary ketones. A large population study using South Korean national health data found that people who practiced regular aerobic exercise were significantly more likely to have detectable ketones in their urine. During prolonged or strenuous activity, your muscles burn through glucose reserves and your body increasingly taps fat for fuel, producing ketones as a byproduct.
Illness and fever create a similar metabolic shift. When you’re sick, stress hormones surge, appetite drops, and your body’s energy demands spike. The combination of eating less and burning more pushes your liver into ketone production. Infections, surgery, and any condition causing vomiting or diarrhea can compound the effect through dehydration, which concentrates ketones in your urine and makes them more likely to register on a test. Heavy alcohol use is another recognized trigger, as alcohol disrupts normal glucose metabolism in the liver and can lead to alcoholic ketoacidosis, particularly in people who drink heavily while eating very little.
Urine Strips vs. Blood Testing
Most people first learn they have ketones through a urine dipstick, either at a doctor’s office or with an at-home test strip. These strips are inexpensive and easy to use, but they have limitations worth understanding. Urine strips detect a ketone called acetoacetate, while blood meters measure a different one (beta-hydroxybutyrate) that rises earlier in ketosis. This means blood testing catches rising ketone levels sooner than urine strips do.
The reverse is also true: once ketone production slows down, blood levels drop before urine levels do. In a study of children with type 1 diabetes, ketosis was detected three times more often during urine monitoring (46.4% of tests) than during blood monitoring (14.8%). This gap largely reflects the time lag: urine can still test positive hours after blood ketones have returned to normal. For people managing diabetes, this delay can lead to unnecessary worry or overtreatment. That said, patients in the same study performed urine testing more accurately and more consistently than blood testing, likely because it’s simpler and less invasive.
Warning Signs of Dangerously High Ketones
Low-level ketones from skipping a meal or finishing a long run are generally not a concern. High ketones become dangerous when they accumulate enough to make your blood acidic, a condition called ketoacidosis. The symptoms to watch for include nausea or vomiting, abdominal pain, confusion, extreme fatigue or sleepiness, difficulty breathing, and a distinctive fruity smell on the breath. These symptoms can escalate quickly, and large amounts of ketones are life-threatening without treatment.
Anyone with diabetes who tests positive for ketones on a urine strip (especially at 2+ or higher) should take it seriously, particularly if blood sugar is also elevated. But even people without a diabetes diagnosis can develop ketoacidosis from prolonged fasting, heavy alcohol use, severe illness, or SGLT2 inhibitor medications. If you notice the symptoms listed above alongside a positive ketone test, that combination warrants emergency medical care.