Ketones in Urine: What They Mean and When to Worry

Ketones are acids produced when your body burns fat for fuel instead of glucose. Small amounts in urine are normal during fasting, low-carb diets, or exercise. Moderate to large ketones combined with high blood sugar can signal diabetic ketoacidosis (DKA) — a life-threatening emergency that needs immediate care.

What Are Ketones

When carbohydrates are scarce or insulin can’t move glucose into cells, the liver breaks down fat into three ketone bodies: acetoacetate, beta-hydroxybutyrate, and acetone. The kidneys excrete some in urine, which is why urine strips can detect them. Acetone is also exhaled, producing the characteristic “fruity” or nail-polish-like breath of severe ketosis.

Why Ketones Appear in Urine

Insulin Deficiency (Most Dangerous)

Without enough insulin, glucose stays in the bloodstream and cells starve. The liver responds by breaking down fat at high speed, flooding the body with ketones and acid. This is the mechanism behind diabetic ketoacidosis, which most often affects type 1 diabetics but can also occur in type 2.

Ketogenic and Low-Carb Diets

Eating fewer than 50 g of carbs per day shifts the body into nutritional ketosis. Mild ketones (5-40 mg/dL) appear in urine, especially in the first weeks. This is the intended state of a keto diet and is generally safe in healthy people.

Fasting and Starvation

After 12-16 hours without food, glycogen stores deplete and fat burning ramps up. Trace to small ketones are typical after an overnight fast and rise with longer fasts.

Illness, Vomiting, or Dehydration

Sick days reduce food intake and can prevent insulin absorption. Both can trigger ketones quickly, especially in people with diabetes.

Pregnancy

Morning sickness, longer overnight fasts, and altered metabolism can produce ketones during pregnancy. Persistent moderate ketones in pregnancy need OB evaluation.

Heavy Alcohol Use

Alcohol blocks gluconeogenesis and can drive a condition called alcoholic ketoacidosis, especially after binge drinking with little food.

Prolonged Exercise

Endurance activity (a long run, hike, or ride) depletes glycogen and can produce small ketones, which clear with rest and food.

How to Test Ketones in Urine

1. Buy urine ketone strips at a pharmacy (Ketostix, Reli On, generic brands). Cost: $5-$15.
2. Either pass the strip through your urine stream for 1 second, or collect a fresh sample in a clean cup and dip the strip.
3. Tap off excess urine and start a timer for 15 seconds.
4. Compare the strip color to the chart on the bottle.
5. Record the result and the time.

Blood ketone meters (β-hydroxybutyrate) are more accurate but more expensive ($30-$60 for the meter, $1-$2 per strip). They are recommended for type 1 diabetics and people on SGLT2 inhibitors.

Ketone Level Chart

For blood ketones, less than 0.6 mmol/L is normal, 0.6-1.5 is mild, 1.5-3.0 is moderate, and over 3.0 mmol/L is severe.

Diabetic Ketoacidosis (DKA): The Real Danger

DKA happens when blood sugar is high (usually over 240 mg/dL) and ketones are moderate to large. Without enough insulin, the body cannot use glucose, fat breakdown floods the blood with acid, and organs begin to fail. Without treatment, DKA can be fatal within 24 hours.

DKA Symptoms

• Excessive thirst and frequent urination
• Nausea, vomiting, and abdominal pain
• Fruity or acetone-smelling breath
• Rapid, deep breathing (Kussmaul respirations)
• Dry mouth and skin, sunken eyes
• Fatigue and weakness
• Confusion or trouble staying awake
• Flushed face

Who Is at Risk

• People with type 1 diabetes (most common)
• People with type 2 diabetes during severe illness, infection, or surgery
• People taking SGLT2 inhibitors (canagliflozin, dapagliflozin, empagliflozin) — can have “euglycemic DKA” with normal blood sugar
• Newly diagnosed diabetics — DKA can be the first sign

When to Test Ketones

The American Diabetes Association recommends testing ketones if you have diabetes and:

• Blood sugar is over 240 mg/dL on two consecutive checks
• You feel sick — flu, cold, infection, gastroenteritis
• You are pregnant
• You have nausea, vomiting, or abdominal pain
• You feel unusually tired or thirsty
• You take an SGLT2 inhibitor and feel unwell, regardless of glucose

What to Do With Ketone Results

Ketones Without High Blood Sugar (Euglycemic DKA)

SGLT2 inhibitor medications can cause DKA at normal blood sugar levels. Symptoms (nausea, vomiting, fast breathing) can be missed because glucose looks fine. If you take one of these medications and feel unwell, test ketones regardless of your glucose reading and contact your doctor.

Preventing Ketones in Diabetes

• Take insulin and other medications as prescribed — never skip basal insulin.
• Have a sick-day plan from your care team.
• Test glucose more often when ill (every 2-4 hours).
• Keep ketone strips at home and in your travel kit.
• Stay hydrated — water is best, broth and electrolyte drinks help.
• Continue some carbs even when sick (15 g per hour if not eating normally).
• If you can’t keep fluids down for 4 hours, call your doctor.

For more on managing diabetes risk, see our guide to prediabetes basics.

Ketones from Keto Diets — Should You Worry

For healthy adults on a ketogenic diet, urine ketones of 5-40 mg/dL reflect the diet working as intended. As the body adapts (often after 4-8 weeks), urine ketones may decrease even when blood ketones remain in the nutritional range. People with diabetes considering a keto diet should consult their doctor first, especially if taking insulin or SGLT2 inhibitors.

The Bottom Line

Ketones in urine show your body is burning fat. Small amounts during fasting, exercise, or low-carb diets are usually fine. The danger is moderate to large ketones combined with high blood sugar — the hallmark of diabetic ketoacidosis. If you have diabetes, test ketones any time blood sugar is over 240 mg/dL or you feel sick, and call 911 for nausea, vomiting, abdominal pain, fruity breath, or rapid breathing. DKA is treatable when caught early.