Diabetes Sick Day Rules: Why Being Ill Raises Blood Sugar Even When You Cannot Eat

The instinct when you are ill and cannot face food is to take less insulin. For most people with type 1 that instinct is backwards, and it is the reason a manageable stomach bug occasionally turns into a hospital admission. Illness pushes glucose up, often sharply, and it does so regardless of what you have eaten.

This article is general education, not a sick day plan. Everyone should have a personal plan agreed in advance with their care team, including specific thresholds and who to call. What follows is background so that plan makes sense — it is not a substitute for one.

Why Illness Raises Glucose

Infection, injury and even significant stress trigger the release of counter-regulatory hormones — cortisol, adrenaline, glucagon and growth hormone. Their job is to mobilise fuel so your body can fight whatever is wrong, and they do it two ways: telling the liver to release stored glucose, and making your cells more resistant to insulin.

Both push glucose up. In someone without diabetes, insulin production rises to compensate and nothing much happens. In you, the compensation has to be deliberate.

This is why the two things move in opposite directions when you are ill: appetite falls while insulin requirements rise. Judging your dose by how little you have eaten is what gets people into trouble.

Never Stop Basal Insulin

If there is one thing to carry away, it is this. Long-acting or basal insulin covers the glucose your liver releases between meals — the exact process illness accelerates. It is not food insulin, and skipping it because you are not eating removes exactly the coverage you need most.

Stopping basal insulin during illness is a common route into diabetic ketoacidosis. Bolus doses for food may genuinely need adjusting if you are not eating, and that is a real conversation to have with your care team. Basal is a different thing, and it does not stop.

Ketones Are the Number That Matters

On ordinary days glucose is the number you watch. When you are ill, ketones matter more.

Ketones form when your body cannot get glucose into cells and burns fat instead. With not enough insulin present, they accumulate and turn the blood acidic — which is what diabetic ketoacidosis is, and it is a medical emergency rather than a bad day.

Two things about ketone testing are worth knowing in advance. Blood ketone meters are more accurate and more current than urine strips, which lag by hours. And ketones can be present even when glucose looks acceptable, particularly if you are dehydrated, eating little, or taking an SGLT2 inhibitor. A normal glucose reading does not rule them out.

Your care team should tell you the glucose level at which you start testing ketones, and what each ketone result means for you. Get those numbers written down before you are ill, because working them out while vomiting is not realistic.

Fluids, and Carbohydrate When You Cannot Eat

Dehydration makes everything worse. High glucose causes fluid loss through urine, and vomiting or fever accelerate it, which concentrates glucose further and impairs the kidneys' ability to clear ketones.

Steady fluid intake matters more than usual, and it matters even when you cannot keep much down — small sips frequently rather than a glass at once.

If you cannot eat solid food but still need carbohydrate to cover insulin, the usual approach is carbohydrate in liquid form, which is easier to tolerate. What and how much belongs in your personal plan, since it depends on your regimen.

Test More Often Than Feels Necessary

Illness changes glucose faster and less predictably than normal life. The gaps between checks that work fine on an ordinary day are too long when you are ill, and the same applies overnight.

If you use a CGM, be aware that dehydration and some medications can affect accuracy, so this is a time to confirm with fingersticks before acting on anything surprising — particularly before a large correction.

When to Call Someone

Your plan should name the person and the threshold. In general terms, the situations that warrant contacting your care team or emergency services without waiting include:

• Moderate or large ketones, especially alongside high glucose
• Vomiting that will not stop, or an inability to keep any fluid down — this removes your ability to manage at home
• Breathing that is fast or deep, breath that smells fruity, or abdominal pain — signs of ketoacidosis
• Confusion or drowsiness, in you or someone you are caring for
• Glucose that stays high despite corrections
• Repeated lows you cannot get on top of

Nobody has ever been criticised for calling too early about ketones. The failure mode runs the other way.

Preparing Before You Need It

The point of sick day rules is that they are decided in advance, because judgment is exactly what illness degrades. Worth having ready: in-date ketone strips, your written plan with its specific numbers, the phone number to call, and something that provides carbohydrate you can tolerate when nothing appetising will stay down.

SweetLife's emergency info card keeps your insulin types, doses, allergies and emergency contacts in one place, which matters when someone else needs to act for you. The education hub covers sick day management too. And logging through an illness — readings, doses, what you managed to keep down — gives whoever you call something concrete to work from instead of a recollection assembled through a fever.

The Short Version

Illness raises blood sugar through stress hormones even when you are eating nothing. Never stop basal insulin. Watch ketones rather than glucose alone, and know that ketones can be high while glucose looks fine. Keep fluids going, test more often, and call early.

Most importantly: get a written sick day plan from your care team while you are well. This article explains why the rules exist; only your plan can tell you your numbers.