Why Blood Sugar Spikes When You're Sick: The Surprising Science Behind It

You wake up feeling under the weather. Maybe it’s a sore throat, a fever, or a stomach bug. You don’t have much of an appetite, so you eat less than usual. But when you check your blood sugar, it’s through the roof. How is that even possible? Shouldn’t eating less mean lower glucose levels?

This can feel confusing—and even scary—especially if you’re trying to manage your blood sugar for health reasons. But the truth is, it’s totally normal for glucose levels to rise when you’re sick. It doesn’t necessarily mean something is wrong. In fact, your body is doing exactly what it’s supposed to do.

Let’s break down the science of why this happens, what it means for your health, and what you can do to manage blood sugar when you’re sick.

The Body’s Stress Response: How Illness Affects Blood Sugar

When you get sick, your body jumps into action to fight off the infection or illness. One of the first things it does is activate your stress response system. This involves releasing a wave of hormones like cortisol and adrenaline.

These hormones are part of your "fight or flight" response, and their main job is to keep you safe and energized during tough situations. But here's the twist: they also tell your liver to release more glucose into your bloodstream.

Why? Because your immune system needs fuel to work. White blood cells and other immune fighters rely on glucose to power through the battle. So your body floods your system with sugar to give your cells the energy they need.

At the same time, these stress hormones make your cells more resistant to insulin. Insulin is the hormone that helps move sugar from your blood into your cells. If your cells become insulin resistant, sugar hangs around in your bloodstream longer—leading to higher glucose readings.

So even if you haven’t eaten much, your blood sugar can still spike. It’s a side effect of your body trying to protect and heal itself.

Inflammation and Immune Response: Hidden Drivers of High Glucose

There’s another piece to this puzzle: inflammation.

When you’re sick, especially with an infection, your immune system releases signaling proteins called cytokines. These cytokines help your immune cells communicate and coordinate their attack on the invader. But they also come with a price.

Cytokines can make your cells more insulin resistant. That means it’s harder for your body to use glucose properly, and again, sugar builds up in your bloodstream.

This is especially noticeable in people with chronic inflammation, like those with autoimmune diseases or long-term infections. Their glucose levels often run high, even when their diet hasn’t changed.

Inflammation is like a double-edged sword: it helps fight illness, but it also messes with your metabolic system. When inflammation goes up, insulin sensitivity goes down.

Common Illnesses That Can Cause Glucose Spikes

You don’t need a serious condition for this effect to show up. Even mild illnesses can cause temporary blood sugar spikes. Some of the most common ones include:

• Colds and the flu
• COVID-19
• Sinus or respiratory infections
• Urinary tract infections (UTIs)
• Gastroenteritis (stomach flu or food poisoning)

In some cases, just feeling under the weather—without a specific diagnosis—is enough to trigger the stress response and raise glucose.

The more severe the illness, the more dramatic the spike can be. A mild cold might cause a small bump in your glucose levels, while a high fever or viral infection could lead to more significant changes.

Why Glucose Levels May Rise Even if You’re Not Eating Much

This is one of the most confusing parts for people. When you're sick, it's common to lose your appetite. Maybe you skip meals or eat just a little soup or toast. But despite eating less, your blood sugar climbs.

How is that possible?

It all comes down to something called gluconeogenesis. That’s a fancy word for how your body makes glucose out of non-carb sources, like protein or fat.

When you’re not eating enough, your body thinks it might starve. To keep your brain and immune system running, it turns to your liver and says, "We need more glucose. Now."

So your liver starts converting stored glycogen (a form of sugar) into glucose. If those stores run low, it creates sugar from scratch using other resources in your body. All of this adds up to rising blood sugar levels, even in the absence of food.

And remember, the stress hormones we mentioned earlier also tell the liver to dump more glucose into your blood. So now you have both gluconeogenesis and stress responses working together, pushing your blood sugar up.

Special Considerations If You Have Diabetes or Prediabetes

If you have Type 1 or Type 2 diabetes—or even prediabetes—getting sick requires some extra attention.

That’s because your body is already working harder to regulate blood sugar. Add in an illness, and the stress response can throw things way off balance.

Some people with diabetes might notice their glucose rising into dangerous territory, even with normal medication and diet. Others might experience big swings between high and low blood sugar, especially if they’re vomiting or unable to eat.

One major risk is diabetic ketoacidosis (DKA), especially for people with Type 1 diabetes. DKA happens when your body doesn’t have enough insulin and starts breaking down fat for energy, releasing harmful acids called ketones into the blood.

That’s why it’s so important to monitor your blood sugar closely when you’re sick. Use your glucose monitor or CGM (continuous glucose monitor) more often than usual. And if your numbers stay high or you feel worse, call your healthcare provider.

Tips to Manage Blood Sugar While You’re Sick

So what can you actually do when you’re sick and your blood sugar starts acting up?

First, don’t panic. Your body is doing what it’s designed to do. But there are some simple ways you can support your recovery and keep your glucose in check:

Stay hydrated. Dehydration can raise blood sugar and make you feel worse. Try sipping water, herbal teas, or electrolyte drinks.

Eat what you can, even in small amounts. Focus on easy-to-digest foods like soup, toast, bananas, or oatmeal. Try to include a little protein or fat to help balance your blood sugar.

Monitor more frequently. Your blood sugar may fluctuate more when you're sick, so check it often.

Don’t skip your medications unless advised. If you're unsure how your meds should change when you're sick, contact your doctor.

Rest as much as possible. Your body heals best when you give it time to recover. Avoid pushing through illness or exercising too hard.

When to Worry: Signs Your Blood Sugar Spike Needs Medical Attention

Most of the time, glucose spikes during illness are temporary and not dangerous. But there are some warning signs to watch out for.

If you have diabetes and your blood sugar stays above 250 mg/dL for more than a few hours, it’s a good idea to check for ketones (if you have a kit). High ketones can be a sign of diabetic ketoacidosis, which needs urgent care.

Other red flags include:

• Vomiting or diarrhea that lasts more than 24 hours
• Trouble breathing
• Feeling confused or very sleepy
• Dry mouth, sunken eyes, or signs of dehydration

In these cases, don’t wait—call your doctor or go to urgent care. Better safe than sorry.

Long-Term Takeaway: What Illness-Triggered Glucose Spikes Tell Us About Your Metabolism

Believe it or not, a temporary glucose spike during illness can tell you a lot about your metabolic health.

If your body handles the stress, keeps blood sugar within a reasonable range, and returns to normal once you're better, that's a good sign of metabolic flexibility. Your system is resilient.

But if your glucose levels swing wildly or stay elevated long after the illness is gone, it might be a sign that your metabolism is under strain.

This is where tools like a CGM can really shine. They give you a window into how your body responds to all kinds of stressors—not just food, but illness, lack of sleep, or even emotional stress.

It’s not just about avoiding sugar. It’s about understanding your body’s unique patterns and supporting long-term health.

Conclusion: Listen to Your Body and Watch Your Glucose When Sick

Getting sick is never fun, and seeing a blood sugar spike when you haven’t even eaten can feel frustrating. But now you know why it happens. Your body is actually trying to protect you—sending out stress hormones, boosting immune activity, and fueling your cells with extra glucose.

The key is not to panic. Stay hydrated, eat what you can, rest, and monitor your levels. And if something feels off, don’t hesitate to get help.

Understanding how your glucose responds to illness is a powerful tool. It helps you take control of your health, not just when you’re sick, but all the time.

Your body is talking. Keep listening.

Frequently Asked Questions: Blood Sugar Spikes During Illness

1. Can illness cause high blood sugar even if I don't have diabetes?

Yes. When you're sick, your body releases stress hormones like cortisol and adrenaline to combat the illness. These hormones can increase blood sugar levels, even in individuals without diabetes.

2. Why does my blood sugar rise when I'm sick, even if I'm eating less?

During illness, the body undergoes stress, prompting the liver to release stored glucose into the bloodstream to provide energy for healing. Additionally, stress hormones can make cells more resistant to insulin, leading to higher blood sugar levels despite reduced food intake.

3. What is stress hyperglycemia?

Stress hyperglycemia refers to a temporary increase in blood sugar levels due to physical stress, such as illness or injury. It's the body's natural response to stress and typically resolves once the underlying issue is addressed.

4. How can I manage my blood sugar levels when I'm sick?

To manage blood sugar during illness:

• Monitor your blood sugar levels more frequently.
• Stay hydrated by drinking plenty of fluids.
• Continue taking your prescribed medications unless advised otherwise by a healthcare professional.
• Consume easy-to-digest foods to maintain energy levels.
• Rest adequately to support your immune system.

If you have diabetes, consult your healthcare provider for specific sick-day guidelines.

5. When should I seek medical attention for high blood sugar during illness?

Seek immediate medical attention if:

• Your blood sugar remains above 240 mg/dL
• You experience symptoms like nausea, vomiting, abdominal pain, or confusion.
• You detect ketones in your urine, indicating potential diabetic ketoacidosis (DKA), a serious complication.

6. Can dehydration during illness affect my blood sugar levels?

Yes. Dehydration can concentrate glucose in the bloodstream, leading to higher blood sugar levels. It's essential to maintain adequate fluid intake during illness to help regulate blood sugar.

7. Are there specific foods I should eat when sick to help manage blood sugar?

Opt for easy-to-digest foods that provide energy and nutrients, such as:

• Broth-based soups
• Plain rice or toast
• Applesauce
• Bananas

These foods can help maintain blood sugar levels without overloading your digestive system.

8. Should I adjust my diabetes medication when I'm sick?

It's crucial to continue taking your diabetes medications during illness unless advised otherwise by your healthcare provider. However, you may need to adjust dosages based on your blood sugar readings. Always consult with your healthcare team before making any changes.

9. Can over-the-counter medications affect my blood sugar levels?

Some over-the-counter medications, especially those containing sugar or certain decongestants, can impact blood sugar levels. Always read labels and consult with a pharmacist or healthcare provider before taking new medications.

10. How often should I check my blood sugar when I'm sick?

During illness, it's recommended to check your blood sugar every 3 to 4 hours. Frequent monitoring helps in timely adjustments to your treatment plan and prevents complications.