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Here’s Why Your Glucose Levels Spike When You’re Sick

Written by: Yuchen He, MS

Reviewed by: Emily J., MSc RD

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2022-11-17

6 minutes

Experiencing high blood sugar when you’re sick is the body's natural response to stress. Learn why this happens and what you can do to manage your blood sugar.


We’ve all been there: you start to feel more tired than usual, with chills, fever, and maybe a sore throat. Sickness triggers a number of unpleasant physiological responses in your body, but did you know that it also causes a temporary spike in blood glucose levels? Don’t worry — it’s completely normal, and actually an important part of how your body overcomes illness. 

Why Illness or Infection Increases Blood Glucose Levels

There are many reasons why blood sugar could be elevated: high glycemic foods, eating late at night, dehydration, and insulin resistance. Beyond these familiar causes, why would blood sugar be elevated when you’re sick?

High blood sugar during illness is a sign that your body is under stress — the name for this is stress hyperglycemia. Stressors like fever, infection, and injury trigger a highly complex interplay between the nervous system, adrenal glands, and pituitary gland, which increases certain hormones in the body [1]. These increased levels of hormones can have a number of effects on your blood glucose levels.

Some hormones, including glucagon, epinephrine, and norepinephrine, increase glucose production in the liver, resulting in elevated levels of glucose in your blood. Other hormones — such as growth hormone and glucocorticoids — also contribute to decreased insulin sensitivity, especially in the muscles. 

The body also wants to recruit the immune system to fight the illness. To do so, it produces more cytokines, small proteins that signal the immune system to act. However, cytokines can make the body unable to utilize insulin properly (i.e., they promote insulin resistance). As a result of these shifts, your skeletal muscles and liver, the two major users of glucose in your body, can’t use up all the extra glucose circulating in the bloodstream, leading to higher-than-usual levels.

In general, the sicker you get, the more likely you’ll see an elevation in blood glucose. Blood glucose can remain high for several days while the levels of those hormones and cytokines remain elevated [2]. 

Resting is crucial when you’re sick, but we also know that inactivity, even if short-term, is linked to insulin resistance. Bed rest — during which your muscles use less glucose, even when you are not sick — also contributes to blood glucose elevation. A study found that 6 days of strict bed rest in healthy volunteers caused their blood glucose to rise [3]. When you add bed rest on top of your body’s normal stress responses during sickness, it makes sense why your your blood glucose levels are higher than usual.

Managing Blood Sugar Levels When You’re Sick

While an elevation in blood glucose is normal when you become ill, it’s better to keep your blood glucose in range, right? Not necessarily. A mild increase in blood glucose during sickness (140-220 mg/dL) is actually protective in the setting of an Intensive Care Unit (ICU), unlike the chronic elevation of blood sugar among people with diabetes [4]. And while it may not be as extreme when you’re home sick with a cold, there’s research to suggest that this increase in blood glucose actually has a protective effect — scientists have found that it provides fuel for the immune system and brain, allowing you to better fight off illness [5].

Because sickness can reduce blood flow, more glucose in the blood means that your cells, especially brain cells, are better able to pick up glucose. On top of that, macrophages and neutrophils, the immune system’s fighters against infection, primarily use glucose as energy, so elevated blood glucose provides abundant fuel to optimize their function. While it’s okay to try to balance your blood sugar if you’re dealing with a simple cold, studies have found that attempts to normalize blood sugar or even moderately low blood sugar levels in critically ill patients are associated with harmful outcomes [6, 7].

In other words, having high blood sugar levels when you’re sick is an adaptive, evolutionary response and might be better left untreated if you’re non-diabetic. That said, severe stress hyperglycemia (>220 mg/dL) may also be harmful [8], although it is more likely to occur in people with poor blood glucose control. Contact your doctor if this occurs.

Key Takeaways: How to Take Care of Your Body While Sick

While it’s better to let the body do its work in fighting infections, we can still take care of ourselves to aid recovery. We get it — self-care while feeling lousy is hard, so do your best with some of these simple tips to manage blood sugar while sick:

References:

  1. https://pubmed.ncbi.nlm.nih.gov/11219223/
  2. https://www.jci.org/articles/view/111650 
  3. https://www.sciencedirect.com/science/article/abs/pii/0026049588900182
  4. https://pubmed.ncbi.nlm.nih.gov/20727232
  5. https://pubmed.ncbi.nlm.nih.gov/23470218/
  6. https://pubmed.ncbi.nlm.nih.gov/18936695
  7. https://pubmed.ncbi.nlm.nih.gov/23062226 
  8. https://pubmed.ncbi.nlm.nih.gov/22863217
  9. https://www.cdc.gov/diabetes/managing/flu-sick-days.html
  10. https://pubmed.ncbi.nlm.nih.gov/28739050/
  11. https://diabetesjournals.org/diabetes/article/33/12/1150/5765/Demonstration-of-a-Dawn-Phenomenon-in-Normal-Human
  12. https://pubmed.ncbi.nlm.nih.gov/16227462/