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Understanding High Blood Sugar: Causes, Symptoms, and Management

Katrina Larsen, MS, RDN, CDCES

Published in Glucose

14 min read

May 2, 2021
December 19, 2023
A person scanning a CGM sensor
A person scanning a CGM sensor

It’s a fact of life that blood sugar fluctuates throughout the day. These ups and downs depend on a handful of factors, like when you wake up, what you eat, the medications you take, and how you manage stress. So, some variation is normal, to the point that you might not even notice it. 

Ignoring blood sugar level changes altogether, though, means you’re ignoring a valuable marker of your health. Especially if you start to have new or unfamiliar symptoms like fatigue, thirst, or brain fog (to name a few). Learning these symptoms and their causes will give you the tools to better understand your own body, then take the right actions for better long-term metabolic health.      

What is High Blood Glucose?

To fully understand your blood glucose levels, it’s important to know what values are actually considered high, and which factors that can cause your elevated reading in the first place.

High blood glucose, also known as hyperglycemia, occurs when there is too much sugar in the bloodstream. Low blood sugar, or hypoglycemia, is the result of too little glucose in the bloodstream. Hyperglycemia usually occurs because your body doesn’t produce enough insulin or can’t properly use the available insulin to remove the glucose from the bloodstream.

Using milligrams of glucose per deciliter of blood (mg/dL) for measurement, high blood glucose readings after a meal indicating prediabetes can fall between 140 and 199 mg/dL. Levels reaching 200 mg/dL two hours after eating indicate you may already be insulin resistant or diabetic, though that diagnosis will need to come from your doctor.

By comparison, the typical standard for normal glucose readings is to remain under 140 mg/dL throughout the day and under 100 mg/dL after eight hours of fasting.

a glucose chart in the Nutrisense app

A range of lifestyle factors, habits, and health conditions can cause high blood sugar. To debunk common hyperglycemia myths, review causes and symptoms, and discuss the best ways to address them, we spoke with two experts on high blood sugar levels: registered dietitian and For The Love of Diabetes creator Lori Zanini and registered dietitian nutritionist and diabetes management expert Mary Ellen Phipps.

Myths About High Blood Glucose

There are a handful of misconceptions surrounding high blood glucose. Some you might have heard before, some you may not even know are myths.  

“Only Diabetics Get High Glucose Values”

While a high glucose value can indicate diabetes, nondiabetics can also have higher values than normal. When researchers studied people wearing a continuous glucose monitor who did not have a diabetes diagnosis, they found 93 percent of individuals reached glucose levels that are considered dangerous, with 10% spending over 2 hours per day in these dangerous levels. Traditional glucose measurements, like a single point in time blood glucose value, are unable to capture these abnormalities. 

There are actually several causes of high blood sugar unrelated to diabetes that the CDC recognizes. These include certain foods, like artificial sweeteners and coffee. Other factors like stress can do it, too. If you live with an endocrine or pancreatic condition, had surgery recently, or are experiencing intense physical stress (say, from a sunburn), you may also see your glucose value rise. 

“All Carbs Are Created Equal” 

Phipps explains that, despite containing similar amounts of carbohydrates, one serving of pasta could have a hugely different effect on your blood glucose levels than one serving of rice. Likewise, that serving of pasta may have an entirely different effect on your blood glucose levels than your friends’ or even family members’. “We’re all unique,” Phipps says.

a person choosing a healthy diet

“I Feel Fine, So I Don’t Need to Test My Levels”

Zanini points out that having high blood glucose can come as a surprise to anyone. “It's possible they didn't notice any symptoms or were simply feeling 'more tired than usual,’” she says. “It's easy to attribute being tired to many other things. . .so this is why regular physicals with your healthcare provider are important.” The bottom line? Listen to your body, take note of symptoms as they arise, and consider monitoring your continuous glucose values.

“High Glucose Isn’t Preventable”

According to the CDC, managing stress, staying active, and maintaining a balanced diet of fiber, protein, and fat, can help prevent the development of type 2 diabetes. You can also work with your doctor and a registered dietitian to devise a routine that's both preventative and sustainable. 

“If My A1C is Normal, My Glucose Is Good”

An A1C result that’s below 5.7% is normal by the CDC’s standards, but having a result below that number isn’t the end of the story. Pregnancy, hemoglobin variants, anemia, liver disease, and certain medications can cause inaccurate A1C results. 

Additionally, the A1C test is measuring your average glucose value over the past 3 months, but averages inherently do not capture highs and lows. So, you could have a normal average while also having abnormal glucose spikes. The A1C test should only supplement your regular blood sugar testing, not replace it completely.

Is Having High Blood Glucose Dangerous?

In short, it can be. Registered dietitian Lori Zanini says that untreated high blood glucose can lead to a wide range of health issues—some of the most common being chronic inflammation, heart disease, vision impairment, kidney disease, nerve damage, tooth decay, damaged blood vessels, and periodontal disease. 

Having high blood glucose also puts us at risk of mitochondrial dysfunction and oxidative stress. The former is a condition in which mitochondria fail to produce energy for cells. The latter occurs when free radicals outnumber antioxidants in the body and increase the risk of disease and other damage.

blood under a microscope

Phipps notes to avoid these risks, catching high blood glucose early on, then taking action to treat it is extremely important.

What Causes High Blood Glucose?

Experts are still learning about all the factors that can contribute to high blood glucose. With that in mind, these are the main known causes of high blood glucose.  

Causes Related to Personal Health

Insulin Resistance

This condition occurs when the cells in your muscles, fat, and liver are unable to use the glucose in your bloodstream for energy. Your pancreas responds to this increase in glucose by producing more insulin to help your body process it. This excess amount of insulin in the bloodstream can eventually cause your body to lose insulin sensitivity or build resistance to it, leading to higher blood glucose levels. 

Bodyweight and Body Fat

Research connects being overweight and having a higher body fat percentage with high blood glucose levels. In fact, a high body fat percentage might be a clearer indicator of high blood sugar and diabetes than weight or body mass index (BMI).

someone checking their weight on a scale

Other Health Conditions 

Certain conditions could make you more likely to have high blood glucose. These include Cushing’s disease, polycystic ovarian syndrome (PCOS), and gestational diabetes. Blood glucose may also rise as the result of common illnesses like a head cold or the flu.

Hormonal Changes  

It isn’t just stress and sleep that can cause fluctuations in hormones. Illness, physical pain and trauma, menopause or menstruation can as well. In any of these instances, your blood glucose levels may rise due to the changes in your hormone levels.


Certain medications, including steroids and beta-blockers, can disrupt insulin’s effectiveness. Others, like second-generation antipsychotics and certain antibiotics, may contribute to high blood sugar. However, research to learn why this happens is ongoing.

Gut Issues

A growing body of research links imbalances in the microbiota living in our guts to an inability to regulate glucose levels in the blood. This is sometimes due to antibiotic use or infection.


A family history of high blood sugar and specific genes may increase a person’s chances of having high blood glucose and developing diabetes. People with African American, American Indian, Asian American, Pacific Islander, or Hispanic heritage may face greater risks.

fries, a burger, muffins, a bar of chocolate, donuts and a glass of soda

Causes Related to Diet

Processed Foods and Sugar

Diet and how it relates to blood glucose values can vary immensely from person to person. But, research suggests that eating processed foods (particularly ones high in sugar and fat) can increase blood glucose.

Overeating or Grazing

It’s a common belief that snacking throughout the day (a.k.a. “grazing”) will keep blood glucose levels steady. But, a 2013 study found that eating two large meals per day helped reduce blood glucose more effectively than six small meals per day. It’s also important to keep our meals to the right proportions, as overeating can raise blood glucose levels as well.


Some research has found a connection between low water intake and a greater risk of hyperglycemia. This is most likely because less water in the body means the concentration of glucose in the blood is higher. 

Vitamin and Mineral Deficiencies

Research into this cause is ongoing. Yet, studies suggest diets lacking micronutrients like chromium, magnesium, or vitamin D may contribute to high blood glucose.

a person sitting in front of the laptop, looking tired

Causes Related to Lifestyle

Physical Inactivity

Exercising prompts your body to burn more energy than usual, and, as a result, consume more glucose. Maintaining a low level of physical activity, on the other hand, means more glucose will remain in the bloodstream. This raises your overall blood glucose values in the process. 

Exercise also makes our body more insulin sensitive, which means we will require less insulin for the rest of the day to control glucose levels. 


Part of the body’s fight-or-flight response to stress is to produce additional glucose. Another facet of that response is an increase in the hormone cortisol. High cortisol can reduce the body’s sensitivity to insulin. As a result, blood glucose levels may also increase. 

Poor Sleep

A lack of quality sleep can inhibit how much insulin your body can release. It can also cause the production of cortisol, which makes it harder for insulin to work. When your body’s insulin cannot properly metabolize the glucose in your blood, the glucose remains there and your glucose levels rise.

10 Main Symptoms of High Blood Glucose

While an individual can have no noticeable symptoms and still have high blood glucose, knowing what kinds of symptoms tend to accompany high blood glucose, such as frequent urination or brain fog can help you take the right action before things get worse.

For example, if you've ever wondered whether high blood sugar can make you sleepy, the answer is yes. High blood sugar levels can contribute to feelings of fatigue and sleepiness. When there is an excess of glucose in the bloodstream, the body may struggle to effectively utilize it for energy. This can lead to a sense of lethargy and drowsiness.

Other common symptoms of hyperglycemia to be aware of are:

  1. Thirst
  2. Frequent urination
  3. Blurry vision
  4. Headaches and brain fog
  5. Increased cravings for carbs and sweets
  6. “Hanger”
  7. Poor energy levels or midday crashes
  8. Fatigue
  9. Unintentional weight loss
  10. Slow-healing cuts

How to Diagnose High Blood Glucose

Doctors use four main tests to gauge patients’ blood glucose levels:

1) Fasting Blood Glucose Test

Conducted after fasting for eight hours; a reading of 100 mg/dL or more is considered high or a sign of prediabetes, while a reading of at least 126 mg/dL indicates type 2 diabetes. However, for optimal health, we recommend aiming for a fasting glucose value below 90 mg/dL

2) Oral Glucose Tolerance Test

Conducted after fasting for eight hours, drinking a high-sugar glucose solution, and waiting two more hours; a reading of at least 140 mg/dL is considered a sign of prediabetes. A reading of 200 mg/dL or higher is type 2 diabetes. For optimal health, you want to aim for a two-hour glucose of 110 mg/dL or lower

3) Random Blood Glucose Test

Conducted at any time throughout the day; a reading of at least 200 mg/dL indicates type 2 diabetes. 

4) A1C Test

Conducted at any time throughout the day; a result below 5.7% is considered normal. A result greater than 5.7% is considered an indication of prediabetes, and a result of 6.5% or higher indicates type 2 diabetes.

a diet plan and veggies, fruits and nuts

How to Treat High Blood Glucose

Zanini says high blood glucose treatment is a hyper-individualized process based on several personal details. These include age, pre-existing conditions, current medications, and current blood glucose value. But, several overarching lifestyle changes can help reduce high blood glucose.

Address a Poor Diet

Everyone can benefit from consuming a diet rich in whole foods. It’s further beneficial to minimize intake of processed foods, such as foods containing sugar, flour, and vegetable seed oil. While there is no “best” diet for everyone, this golden rule of nutrition can help any person get more from their meals.

Personalized nutrition makes room for your unique differences and takes your individuality into account. The best way to find your perfect diet is to test different approaches, experiment, work with a dietitian, and use data such as glucose monitoring to assess if something is working or not.

a person working out in the gym

Maintain an Active Lifestyle 

Take breaks during a sedentary workday, add a walk around the block to your morning routine, or try a new form of exercise. These activities bring great benefit in both the short and long term while helping reduce your overall stress. Including regular strength training and aerobic exercise will help to lower glucose values, as well. 

Manage Stress

Make stress-relieving practices like reframing your thoughts and soothing breathing exercises part of your daily routine. Doing so will reduce your overall stress and, in turn, help prevent spikes in cortisol. 

Get Adequate Sleep

Getting good rest will help regulate your hormones and reduce unwanted spikes in your blood glucose levels.

Maintain a Healthy Weight

According to the American Diabetes Association, maintaining a healthy weight can help reduce your risk of developing high blood glucose and diabetes. If you’re working toward a healthy weight, the ADA recommends creating a sustainable routine you can stick with and focusing your efforts toward realistic goals. 

Take Medication as Prescribed by Your Doctor

A doctor may prescribe anti-diabetes medication like metformin to help lower your blood glucose levels. If you’re at high risk of developing diabetes, taking this medication is crucial for lowering high glucose levels.

a nurse pricking a patient's finger to check their glucose level

When Should You See a Doctor?

Your yearly doctor’s appointment is an excellent time to check in on your blood glucose levels. This is especially true if your age (over 45 years old), weight, or family history put you at higher risk for developing diabetes in general.

If you suspect you may have high blood glucose already, Zanini says that any changes in your health count as cause to see your doctor. Make an appointment to undergo the diagnostic tests listed above and to discuss treatment options with your provider. If you have high blood glucose and experience trouble breathing, vomiting, confusion, extremely high thirst, or other symptoms of diabetic ketoacidosis, seek medical attention right away. This condition occurs when your body burns fat too quickly and converts it into ketones, which make your blood acidic. It can be life-threatening if left untreated. 

How To Tell If Your Blood Glucose Levels Are High

When learning more about your overall health and how your body responds to your routines, staying informed about your blood glucose levels should be a top priority. The following glucose monitoring options can help you get a better picture of your health: 

Blood tests

As previously described, your doctor can conduct a series of blood tests to determine your blood glucose level. These may include a fasting blood glucose test or a random blood glucose test.

Oral Glucose Tolerance Test

Your doctor may also conduct an oral glucose tolerance test. This procedure requires you to fast for eight hours, drink a high-sugar solution, and wait two more hours before your doctor tests your blood. From that test, they can identify your blood glucose level.

A person scanning a CGM sensor with a smartphone


Also known as a glucose meter, this device tests your blood glucose level with a finger prick. It can provide you with on-demand readings of your blood glucose levels.

Continuous Glucose Monitor (CGM)

CGM has a wealth of benefits. This option measures your levels through a small, painless device on the back of your arm, keeping you updated on your blood glucose 24/7 and showing your responses to food, exercise, and other routines in real time.

Prioritizing sleep, nutrition, and regular doctor’s visits are key for maintaining a healthy blood glucose level. They're also highly effective preventative measures. Keeping track of new symptoms or changes in your health, like weight loss, fatigue, or increased thirst and urination is also important.

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Engage with Your Blood Glucose Levels with Nutrisense

Your blood sugar levels can significantly impact how your body feels and functions. That’s why stable blood glucose levels can be an important factor in supporting overall wellbeing.

With Nutrisense, you’ll be able to track your blood glucose levels over time using a CGM, so you can make lifestyle choices that support healthy living.

When you join the Nutrisense CGM program, our team of credentialed dietitians and nutritionists are available for additional support and guidance to help you reach your goals.

Ready to take the first step? Start with our quiz to see how Nutrisense can support your health.

Find the right Nutrisense program    to help you discover and reach your health potential.
Kara Collier, RDN, LDN, CNSC

Reviewed by: Kara Collier, RDN, LDN, CNSC

Kara Collier is the co-founder and VP of Health at Nutrisense, one of America’s fastest-growing wellness-tech startups, where she leads the health team. She is a Forbes 30 under 30 recipient, frequent podcast guest & conference speaker.

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