What ketones are

Picture insulin as the doorkey for cells: glucose can knock all it likes, but without insulin the door stays shut. When insulin is absent or insufficient in type 1 diabetes, cells starve in the middle of plenty, and the body switches to breaking down stored fat for energy instead.

That fat breakdown produces three molecules collectively known as ketone bodies:

• Beta-hydroxybutyrate (BHB), the most abundant, and the one measured by blood ketone meters
• Acetoacetate, measured by urine ketone strips
• Acetone, a volatile by-product that produces the characteristic “fruity breath” sometimes noticed during high ketone states

Small amounts of ketones are entirely normal: the body produces them during overnight fasting, prolonged exercise, and low-carbohydrate eating, and uses them efficiently as an alternative fuel. The problem arises when ketone production accelerates beyond the body’s ability to use them, accumulating in the blood and making it progressively more acidic, a state that, if unchecked, leads to diabetic ketoacidosis (DKA).

Why ketones matter uniquely in type 1 diabetes

People with type 1 diabetes are uniquely vulnerable to dangerous ketone accumulation because they cannot produce their own insulin. Without insulin, two things happen at once: glucose cannot enter cells, so cells are starved of fuel even while blood glucose climbs; and the liver accelerates both glucose output and ketone production, because insulin normally suppresses both processes, and without it, both run unchecked.

This creates a vicious cycle: rising glucose, rising ketones, increasing acidity. In someone who produces their own insulin (type 2 diabetes, for example), even a small amount of residual insulin production acts as a brake on this cycle. In type 1 diabetes, that brake is absent.

This is the mechanism behind DKA. It is not simply “high glucose”; it is the combination of insulin absence, unrestrained ketone production, and the resulting acidosis that makes DKA dangerous.

Types of ketone tests

Two ways to test, two different molecules. Blood meters measure beta-hydroxybutyrate directly, the predominant ketone body during insulin deficiency, giving a real-time picture of what is happening now. Urine strips measure acetoacetate, which reflects what was happening hours ago when the urine was produced; they can also show positive during recovery (when ketones are actually falling), because the body converts BHB back to acetoacetate as it clears.

Understanding ketone levels

Blood ketone levels (beta-hydroxybutyrate, measured in mmol/L) are generally interpreted in four ranges. These are averages and general guidance; individual circumstances and clinical advice from a diabetes team always take precedence.

The specific protocol agreed with a diabetes care team always takes priority over general guidance.

Euglycaemic DKA, when glucose is not the whole story

One of the most important things to understand about ketones is that they can be dangerously elevated even when glucose is not particularly high. This is known as euglycaemic DKA, and it catches people off guard because the usual warning sign, persistent high glucose, may be absent or only mildly elevated.

Situations where euglycaemic DKA is more commonly seen include SGLT-2 inhibitor use (these medications lower glucose by causing it to be excreted in urine, masking the glucose rise that would normally accompany insulin deficiency); prolonged fasting or very low carbohydrate intake; illness with vomiting, where reduced food intake plus stress hormones drive ketone production without the expected glucose spike; and after heavy or prolonged exercise, where glycogen depletion and increased fat metabolism can drive ketones while glucose remains in range.

The key message: when feeling unwell, check ketones, not just glucose. Glucose alone does not tell the full story.

Ketones after hypoglycaemia

Some people notice elevated ketone readings after a severe hypo. The mechanism: during a significant hypoglycaemic episode, the counter-regulatory hormone surge (cortisol, glucagon, adrenaline) mobilises stored fat, and ketone bodies are produced as a by-product. Once glucose is restored, those ketones can still appear elevated for a few hours before falling back to normal. Worth knowing because the reading can look alarming but does not require the same response as ketones caused by insulin deficiency. Context matters.

When to test ketones

Many diabetes teams suggest testing blood ketones in the situations below. Tap each for the detail.

For the step-by-step action protocol when ketones are elevated, see the Hyper Treatment Explorer in the GNL app, which surfaces a population-average ketone correction estimate at your total daily insulin and routes paediatric high-ketones via ISPAD Chapter 13 sick-day rather than adult DAFNE or BERTIE pathways.

This content is for educational exploration only. It describes average responses and general principles. It is not medical advice and cannot replace individual clinical guidance from your diabetes care team.

Sick day rules: what the protocol above is built on

The Hyper Treatment Explorer gives the personalised numeric estimate; the principles below are what that estimate is actually built on, and they matter even before a single number is calculated. Illness changes the maths. The rule that catches people out is the opposite of what feels natural: when appetite drops, the instinct is to ease off insulin. That is usually the wrong direction.

The rule that matters most: insulin is never stopped

During illness, insulin needs typically rise, not fall, even when eating less or not at all. Infection and inflammation trigger counter-regulatory hormones (cortisol, adrenaline, glucagon, growth hormone), which increase insulin resistance and push the liver to release more glucose regardless of food intake. Stopping or significantly reducing basal insulin because a person is off their food is one of the most common routes to DKA, not a way to prevent a hypo. The dose may need adjusting; the insulin itself keeps flowing.

What tends to change while unwell

• Ketone checks step up. Testing every 2 to 4 hours is common practice when glucose is persistently above 14 mmol/L or a person feels unwell, using the level bands above to guide the response rather than guesswork.
• Correction decisions follow the ketone band, not appetite. The four-band table above is the same one used day to day; illness does not change the bands, it just makes checking them more often the sensible default.
• Basal or correction doses often need increasing. Many people find they need more insulin during illness, sometimes noticeably more, agreed with their diabetes team in advance where possible.
• Hydration matters more. Dehydration accelerates ketone accumulation, so fluids alongside insulin are part of the same response, not a separate one.

When to contact the diabetes team the same day

• Ketones are not falling after treatment and a retest
• Repeated vomiting, or any vomiting alongside raised ketones
• Blood ketones at 3.0 mmol/L or above (see the level table above)
• Anyone unsure what to do next

The best preparation happens before the next illness

A written sick-day plan, agreed with the diabetes team in advance and kept somewhere the whole household (or school, for a child) knows to find it, is the single most useful piece of preparation. Asking for one, or reviewing an existing one, is worth doing before the next bug arrives rather than during it.

This content is for educational exploration only. It describes general principles and accepted clinical practice (ISPAD 2024 Chapter 13; Diabetes UK adult sick-day guidance). It is not medical advice and cannot replace an individual sick-day plan agreed with your diabetes care team.