Ketones: what they are and why they exist

Why ketones turn up when insulin is short

Every cell in your body runs on glucose, but glucose can’t get in on its own. Insulin is the key that opens the door. In type 1, you don’t make your own insulin, so if the insulin you inject or pump runs short, glucose piles up in the blood but the cells stay hungry.

Faced with cells that can’t reach the glucose around them, the body does the sensible thing: it burns fat for energy instead. Ketones are what’s left over from burning that fat. A few ketones aren’t dangerous in themselves. The problem is what they signal, that your cells can’t get fuel the normal way, and that usually means there isn’t enough insulin on board.

This is why high glucose and ketones together are the combination that matters. It’s the body telling you the insulin isn’t reaching where it needs to go.

These aren’t the same as a 'keto' diet

You’ll have heard of ketones from low-carb and keto diets, where the aim is to run on fat on purpose. That’s a different situation. In someone who makes their own insulin, that process stays controlled: insulin keeps the ketones in a safe range even while the body burns fat.

In type 1, there’s nothing holding the brakes if your insulin runs low. Ketones can climb fast and turn the blood acidic, which is the “acidosis” in ketoacidosis. So a word that sounds fashionable elsewhere is a red flag here. When you see ketones with high glucose, treat it as an insulin problem to sort out, not a diet in action.

What the number is telling you

Blood ketone meters give you a number in mmol/L. Urine strips give you a colour, from trace up to large. The general pattern most teams use: a low or trace reading with things otherwise settled usually means recheck in a couple of hours and keep drinking fluids. One caveat worth knowing: illness usually pushes glucose UP and raises your insulin needs, which is the common pattern and the reason ketones build in the first place. Less often, an illness that stops you eating tips it the other way into a hypo (a low blood sugar below about 4 mmol/L, which can come on during the illness or straight after a missed meal and again hours later) even while ketones are present, which is why your sick day plan covers both directions. A moderate or large reading, or ketones with vomiting or feeling genuinely unwell, means contact your diabetes team or seek help now rather than waiting.

Your own team will give you a plan for what to do at each level, often called sick-day rules, including when to take extra insulin and how much. That part is specific to you, so it’s a conversation with them, not a number to copy from a page. What’s worth knowing now is that ketones are a prompt to act, and the higher they climb, the less time you want to leave it.

What changes how ketones behave

Ketones aren’t the same threat in every situation. When you’re ill, your body needs more insulin than usual, so ketones can appear even if you haven’t missed a dose. A pump problem can cut off your insulin without you realising, because there’s no long-acting injection as a backstop. And physical activity cuts both ways. Normal exercise with enough insulin on board is safe and won’t push ketones up. But intense exercise when your insulin is already too low, and your glucose is high, can actually raise ketones further, because the cells still can’t get their fuel. That’s why the advice is to check for ketones before hard exercise if your glucose is high, and not to train through it until you’ve sorted the insulin out first.

When ketones might show up

Examples, not instructions or doses.

Worth paying attention to

Questions that make an appointment useful

Similar articles