How illness changes insulin needs
Illness usually pushes insulin needs up, often before you feel unwell, and whether or not you are eating. Why not eating is never a reason to stop insulin.
Being ill usually pushes insulin needs up, often before you feel properly unwell, and it does it whether or not you are eating. That last part is the bit that catches people: the instinct when you cannot face food is to take less insulin, and illness is precisely when your body needs more.
- Illness releases stress hormones that work against insulin, so the same doses do less than usual.
- Needs can rise before symptoms, so an unexplained run of highs is sometimes the first sign of something coming.
- Never stop your insulin because you are not eating. Background insulin is needed regardless, and stopping it leads to ketones.
- Some illnesses do the opposite, particularly ones with vomiting or diarrhoea, where lows become the bigger risk. A hypo can come on at the time or shortly after, and again hours later including overnight.
- Your team will have given you a sick day plan. This page is the why; that plan is the what, and it is worth knowing before you need it.
Why your body needs more insulin when it is fighting something
When you are ill, your body releases cortisol, adrenaline and other stress hormones to help it cope. Useful in general, and inconvenient here: one of the things they do is make your cells less responsive to insulin and tell your liver to release more stored glucose. So you get more glucose arriving and less ability to use it, at the same time.
The result is that your usual amounts genuinely stop being enough. Not because anything has gone wrong with your insulin or your technique, but because the target has moved. This can be a large effect, and it can arrive with an infection you have barely noticed yet, which is why a run of unexplained highs is worth treating as possible information rather than as a puzzle.
It is also why the two halves of insulin behave differently here. Mealtime insulin follows food, so if you are not eating there is less of that to give. Background insulin is covering your liver, which is not only still running but running harder than usual, so the need for it goes up rather than down.
Why not eating is not a reason to stop insulin
This is the single most important thing on the page. Feeling too rough to eat, and therefore skipping insulin altogether, is an understandable instinct and a genuinely dangerous one. Without background insulin your liver keeps releasing glucose with nothing to let it into your cells, your body starts burning fat for fuel, and ketones build. That is the route to DKA, and illness is already pushing you in that direction.
So the rule that diabetes teams give is consistent: never stop your background insulin, even when you are eating nothing. How much you take may change, but that is a decision made with your team and usually set out in advance in a sick day plan, precisely so nobody has to work it out while feeling awful.
Checking for ketones is the other half of this. During illness they can build even when your glucose is not dramatically high, so checking is more useful than guessing from the number alone. If you do not have ketone strips or a meter that reads them, getting hold of some before you are ill is one of the more useful pieces of admin there is.
The illnesses that push the other way
Not everything raises your needs. Anything involving vomiting or diarrhoea can push glucose down instead, because food is not staying in and not being absorbed while your insulin carries on working normally. Stomach bugs are the classic case, and they combine the two risks awkwardly: lows from not absorbing food, and ketones from not eating.
That combination is the reason vomiting with type 1 is treated more seriously than it would be otherwise. Being unable to keep fluids or sugar down removes your ability to treat a low as well as your ability to prevent ketones, and it is a recognised reason to seek help rather than wait it out.
The practical upshot is that “I am ill” is not one situation. An infection with a fever usually means more insulin; a stomach bug may mean less and closer watching. Which is another reason the sick day plan is worth having in writing rather than reconstructing from memory at two in the morning.
What changes the direction
Steroids deserve a specific mention because they raise glucose substantially and are prescribed for plenty of ordinary things, so a course for a chest infection or a flare-up of something else can change your needs sharply for its duration. Being stuck in bed removes whatever your usual routine would have contributed. And the length matters: a day of feeling rough is different from a week, and needs often stay raised for a while after you feel better.
How illness shows up in the data
Examples, not instructions or doses.
Two days of stubborn highs with no explanation, then a sore throat. The insulin resistance arrived before the symptoms did, which is common enough to be worth recognising.
You cannot face food, so you skip everything including your background insulin. By evening you are high with ketones, because the liver kept going and nothing was letting the glucose in.
Vomiting, nothing staying down, and glucose falling rather than rising. The opposite problem, and the one where being unable to keep sugar down is itself the reason to get help.
When it's urgent
Illness plus not enough insulin leads to ketones and DKA, which is a medical emergency. Never stop your insulin because you are not eating. Follow the sick day plan your team gave you, keep drinking fluids, and check for ketones. If you are being sick and cannot keep fluids down, have stomach pain, are breathing fast or deeply, or feel very unwell, contact your diabetes team or call NHS 111 straight away. If you are drowsy, confused or breathing very fast, call 999.