Metabolic Health

What Is Glucose Intolerance and How Do You Know If You Have It?

5 minutes read

Glucose intolerance is a prediabetic state characterized by the inability to “tolerate” glucose — it’s an indicator of metabolic dysregulation and is often a precursor to type 2 diabetes [1, 3, 5]. Maintaining healthy glucose tolerance is important for your metabolic health, and there are several short- and long-term reasons why stabilizing blood glucose levels can improve your health, energy levels, and hormone production.

chart defining terms related to glucose intolerance

What is glucose intolerance?

Glucose intolerance, also called impaired glucose regulation, is a metabolic condition in which the body cannot effectively process glucose [1]. People with this condition have blood glucose levels that are too high to be considered normal but do not meet the threshold to be considered diabetic. 

Glucose intolerance can occur in states of prediabetes, impaired fasting glucose (IFG), and impaired glucose tolerance (IGT). However, having glucose intolerance does not necessarily mean you have diabetes. While you may see the term “glucose intolerance” used interchangeably with prediabetes, really it is a separate condition that can develop outside of a diabetic state [1, 2].

Glucose intolerance is also a characteristic of a diabetic state, but having glucose intolerance doesn’t necessarily mean that you already have diabetes.

IFG and IGT are both considered to be abnormal glucose regulation [3]. Impaired fasting glucose, as the name suggests, is higher than normal levels of blood glucose in a fasted state, whereas impaired glucose tolerance is higher than normal levels of blood glucose two hours after being given an oral glucose challenge [3]. 

A person can experience both IFG and IGT, or either one separately, but both types of glucose intolerance have negative effects on metabolic health and carry an increased risk of progressing to diabetes if there is no intervention.

What are the causes of glucose intolerance?

The exact causes of glucose intolerance aren't clear, but current research suggests that both genetic factors and lifestyle factors are at play [5]. Here are some potential causes that may lead to glucose intolerance:

Where does glucose intolerance fall on the metabolic health spectrum?

When looking at metabolism as a spectrum, glucose intolerance falls in the middle, between perfect health and a chronic metabolic condition diagnosis.

Optimal metabolic health is at the upper end of the metabolic spectrum – this is where we all strive to be and where healthspan is typically longest. When a person’s metabolic function begins to decline — due to changes in body composition, eating habits, physical inactivity, or stress — we start to see people experience insulin resistance, glucose intolerance, and diabetes.

Insulin resistance vs. glucose intolerance

Insulin resistance is a condition where cells become less responsive to the hormone insulin. This leads to lower effectiveness of insulin at allowing glucose to enter cells and higher levels of glucose in the blood. This condition can also be an early indication of developing type 2 diabetes and metabolic syndrome.

Glucose intolerance and insulin resistance can occur at the same time, but are not one and the same. Both conditions indicate metabolic dysfunction and increase a person’s risk of developing type 2 diabetes, as well as worsen cardiometabolic health and other health problems.

The difference between glucose intolerance and insulin resistance lies in their impact on blood glucose levels.

With insulin resistance, cells are not responding effectively to the action of insulin but blood glucose levels can remain normal (called normoglycemic) [4]. Levels can stay like this due to the overproduction of insulin to compensate for the decreased response. 

In contrast, glucose intolerance is a later stage where blood glucose levels are higher than normal (but not high enough to be considered diabetes) despite the presence of insulin [5]. This is the indicator that the body is no longer effectively using insulin and is not able to regulate blood sugar levels efficiently.

An analogy would be like passengers going to a train station, where glucose is the passenger and insulin is like the turnstiles allowing passengers into the station (i.e., allowing glucose into your cells). You can think of insulin resistance as malfunctioning turnstiles, making it slower and harder for passengers (glucose) to get into the station (cells). More turnstiles can be added to give passengers more chances to enter the station (like more insulin being produced to allow glucose into cells). However, if the turnstiles keep malfunctioning, even with added ones for entry, there will eventually be long lines of people waiting to get into the station – this is like the buildup of glucose in the blood with glucose intolerance.

Type 2 diabetes is the furthest from optimal health on the spectrum [6]. This is where insulin resistance progresses and insulin production decreases, leading to consistently high blood glucose levels (called hyperglycemic).

What are the symptoms of glucose intolerance?

Physical symptoms of glucose intolerance include [7, 8, 9]: 

  • Increased thirst
  • Increased hunger
  • Frequent urination
  • Fatigue or drowsiness
  • Blurry vision or irritated eyes
  • Unexplained weight loss, loss of muscle mass, or difficulty gaining weight

Other risk factors that may indicate glucose intolerance include [5, 9]:

You may notice an overlap between the symptoms of glucose intolerance and symptoms of insulin resistance. Some people who have glucose intolerance may not experience noticeable symptoms at all, making it important to test and check for glucose intolerance [5].

How do you get tested for glucose intolerance?

There are several ways to test if a person has glucose intolerance [9]: 

  1. fasting plasma glucose (FPG)
  2. oral glucose tolerance test (OGTT)
  3. hemoglobin A1C (HbA1C) 
  4. random plasma glucose test

FPG and OGTT are used most often and can be used together as a two-step strategy [10]. The fasting plasma glucose test measures the level of blood glucose during a fast of at least 8 hours. A blood glucose result of 100-125 mg/dL (5.6 - 6.9 mmol/L) indicates impaired fasting glucose [9, 10]. 

The oral glucose tolerance test measures the level of blood glucose two hours after consuming about 75 grams of sugar. The results give information about impaired glucose tolerance, with blood glucose levels of 140-199 mg/dL (7.8 - 11.0 mmol/L)  indicating intolerance [9, 10].

Continuous glucose monitoring provides a way to screen blood glucose levels in a real-time and ongoing basis. As opposed to tests like FPG or OGTT, you can track trends in your glucose levels and identify how your blood glucose levels change in response, not only to fasting and high sugar, but to every aspect of life.

What should you do if you have glucose intolerance?

If you suspect you have glucose intolerance, you can ask your healthcare provider for a glucose tolerance test, or a fasting insulin test. With or without these tests, wearing a CGM can give you more information on your blood sugar trends and guide you in implementing changes that stabilize your blood glucose levels. 

While there are medications that may help in treating glucose intolerance, several lifestyle interventions can lead to improvement in symptoms and have the potential to reverse glucose intolerance [11]. Veri’s Four Pillars of metabolic health clearly outline research-backed ways to combat glucose intolerance and metabolic dysregulation:

  1. Nutrition plays a significant role in metabolic health, as what we eat directly impacts our glucose regulation. Food and glucose intolerance are tightly linked — studies show consuming processed foods is associated with impaired glucose tolerance, whereas fruits, vegetables, and whole grains can help control glucose tolerance [12].
  2. Exercise can help to regulate insulin sensitivity and blood sugar levels. Consistency is key, as regular physical activity benefits your overall metabolic health.
  3. Sleep directly impacts our glucose control as well as insulin levels — having a steady sleep routine can help regulate glucose intolerance. 
  4. Stress increases cortisol levels, which directly impact blood glucose. Poor sleep is associated with glucose intolerance and inflammation, making sleep habits and important part of metabolic health [13].

Key takeaways

  • On the spectrum of metabolic health, glucose intolerance is a prediabetic condition where the body has higher than normal blood sugar levels, but not quite high enough to be classified as diabetes. 
  • Glucose intolerance and insulin resistance are related but separate conditions that are both associated with metabolic dysregulation and can lead to type 2 diabetes if not identified and treated.
  • To test for glucose intolerance in a clinical setting, fasting plamsa glucose and oral glucose tolerance test are commonly used.

References:

  1. https://academic.oup.com/eurpub/article/26/4/699/2467539
  2. https://www.canadianjournalofdiabetes.com/article/S1499-2671(17)30813-4/fulltext
  3. https://diabetesjournals.org/care/article/30/3/753/25705/Impaired-Fasting-Glucose-and-Impaired-Glucose
  4. https://www.ncbi.nlm.nih.gov/books/NBK507839/
  5. https://www.ncbi.nlm.nih.gov/books/NBK499910/
  6. https://www.ncbi.nlm.nih.gov/books/NBK551501/
  7. https://www.diabetes.co.uk/glucose-intolerance.html
  8. https://www.hindawi.com/journals/nrp/2011/937038/
  9. https://www.verywellhealth.com/glucose-intolerance-symptoms-5116562
  10. https://www.ncbi.nlm.nih.gov/books/NBK532915/
  11. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6124975/#bibr1-1559827614554186 
  12. https://www.hindawi.com/journals/jobe/2023/2885769/ 
  13. https://www.sciencedirect.com/science/article/abs/pii/S0306453019304858

Written by: Sarah Jayawardene, MS
Reviewed by: Emily Johnson, MSc RD

Table of Contents

  • What is glucose intolerance?
  • What are the causes of glucose intolerance?
  • Where does glucose intolerance fall on the metabolic health spectrum?
  • What are the symptoms of glucose intolerance?
  • How do you get tested for glucose intolerance?
  • What should you do if you have glucose intolerance?
  • Key takeaways

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