What Is Glucose?

Medically Reviewed by Zilpah Sheikh, MD on April 02, 2024
9 min read

Glucose comes from the Greek word for "sweet." It's a type of sugar you get from foods you eat, and your body uses it for energy. As it travels through your bloodstream to your cells, it's called blood glucose or blood sugar.

Insulin is a hormone that moves glucose from your blood into your cells for energy and storage. If you've been diagnosed with diabetes , you have higher-than-normal levels of glucose in your blood. Either you don't have enough insulin to move it through your bloodstream, or your cells don't respond to insulin as well as they should.

High blood glucose for a long period can damage your kidneys, eyes, and other organs.

Sugar is a type of carbohydrate that offers energy to your body. Fructose and glucose are both simple sugars, meaning that they have only one sugar molecule. They're found naturally in foods such as fruits, honey, and some vegetables.

Most sugars that are added to processed foods are a mix of glucose and fructose. For instance, high-fructose corn syrup is 45% glucose and 55% fructose. Agave nectar is sometimes 90% fructose and 10% glucose.

Your body breaks down both natural and added sugars in the same way. But added sugars have been linked to a range of health issues. Studies have linked fructose to obesity, diabetes, high blood pressure, and kidney disease.

The glucose molecules in your bloodstream mainly come from foods that are rich in carbohydrates, including bread, potatoes, and fruit. As you eat, food travels down your esophagus to your stomach. There, acids and enzymes break it down into tiny pieces. During that process, glucose is released.

From there, it goes into your intestines where it's absorbed and passes into your bloodstream. Insulin then helps glucose get inside your cells.

Your body's designed to keep the level of glucose in your blood constant. Special cells in your pancreas called beta cells monitor your blood sugar level every few seconds. When your blood glucose rises after you eat, the beta cells release insulin into your bloodstream. Insulin acts like a key, unlocking muscle, fat, and liver cells so glucose can get inside them.

Most of the cells in your body use glucose along with amino acids (the building blocks of protein) and fats for energy. But blood sugar is the main source of fuel for your brain. Nerve cells and chemical messengers there need glucose to help them process information. Without glucose, your brain wouldn't be able to work well.

After your body has used the energy it needs, the leftover glucose is stored in little bundles called glycogen in your liver and muscles. Your body can store enough to fuel you for about a day.

If you haven't eaten for a few hours, your blood glucose level drops. Your pancreas stops churning out insulin. Alpha cells in your pancreas begin to produce a different hormone called glucagon. It signals your liver to break down stored glycogen and turn it back into glucose.

That travels to your bloodstream to restock your supply until you're able to eat again. Your liver can also make its own glucose using a combination of waste products, amino acids, and fats.

Your blood sugar level should naturally rise after you eat. Then, it dips a few hours later as insulin moves glucose into your cells. Between meals, a healthy blood sugar level is less than 100 milligrams per deciliter (mg/dL). This is called your fasting blood sugar level.

But if you have diabetes, your body can't turn food into energy the way it should.

There are two types of diabetes:

  • In type 1 diabetes, your body doesn't have enough insulin. Your immune system attacks and destroys cells in your pancreas, where insulin is made, by mistake.
  • In type 2 diabetes, your cells don't respond well to insulin. As a result, your pancreas needs to make more and more insulin to move glucose into your cells. Over time, this can damage your pancreas so it can't make enough insulin to meet your body's needs.

Without enough insulin, glucose stays in your bloodstream. A level above 200 mg/dL 2 hours after a meal or above 125 mg/dL during fasting is high blood glucose, also called hyperglycemia.

Too much glucose in your bloodstream for a long period can damage the vessels that carry oxygen-rich blood to your organs. High blood sugar can increase your risk for:

If you live with diabetes, it's important to test your blood sugar often. Exercise, being careful about the foods you eat, and medicine can all help keep your blood glucose at a healthy level and prevent complications.

How to test blood glucose

The two most common ways to test your blood sugar are:

Fingerstick blood test. You'll use a special tiny needle called a lancet to prick the side of your fingertip. Then, you'll squeeze a small drop of blood on a disposable testing strip so it can be read by a portable device called a blood glucose meter. In just a few seconds, a screen will display your blood sugar level.

This method tells you what your blood glucose is at that exact moment.

Continuous blood glucose monitor (CGM). A small sensor is placed just under your skin, usually on your arm or stomach. It measures your blood sugar in real time by monitoring the fluid that travels between your cells. Then, it transmits that data to a device, like an app on your phone or a CGM receiver, where you can view it.

You may have a lot of questions when you first start using a CGM. There's a learning curve for most people. But this technology can give you a better sense of your blood sugar patterns. As most CGMs alert you to highs and lows, you have a window to treat them before they get worse.

Sometimes, your doctor may suggest that you use a CGM and still do fingerstick tests. This can help confirm that your CGM readings are accurate.

The A1c test is a blood test that can detect your average blood sugar levels over the past 3 months. It's also called the hemoglobin A1c, or the HbA1c test.

When glucose first enters your bloodstream, it attaches to hemoglobin, the part of a red blood cell that carries oxygen. An A1c test measures the percentage of hemoglobin in your blood that's glycated (coated with sugar). The more glucose in your blood, the higher your A1c. And the higher your A1c, the weaker your body's blood sugar control.

An A1c test can be used to diagnose prediabetes and diabetes.

Your result may show:

  • Normal blood glucose levels: Below 5.7%
  • Prediabetes: 5.7%-6.4%
  • Diabetes: 6.5% or higher

The A1c test values are based on guidelines from organizations such as the American Diabetes Association (ADA). But it's essential to consult with a doctor to get a proper diagnosis and understand your test results. Circumstances can vary.

For instance, some A1c tests may not give you a precise result if you are of African, Mediterranean, or Southeast Asian descent, or if sickle cell anemia or thalassemia runs in your family. You may have a different type of hemoglobin, called a hemoglobin variant, that gives a "false high" or "false low" number. If so, your doctor can order a different kind of A1c test to get an accurate reading.

An A1c test is also an important tool for managing diabetes.

Everyone has a different A1c target, based on their age and any other health issues they have. For many people, an A1c of 7% or less is ideal. But your doctor will explain the result they'd like to see for you and why. Then, if your A1c comes back higher than that, your doctor can adjust your treatment plan.

High A1c results can lead to severe health issues.

When to take an A1c test

How often you need this test depends on many factors. Generally speaking, your doctor may order it:

  • Once a year if you have prediabetes
  • Twice a year if you have diabetes, but your blood glucose is well-managed and you don't use insulin
  • Every 3 months if you have diabetes, but your blood glucose isn't well-managed and/or you do use insulin
  • More than four times a year if your diabetes treatment plan changes

Your blood sugar naturally goes up and down throughout the day. But many things can cause it to drop to a range that's unhealthy, such as:

  • Drinking alcohol (especially on an empty stomach)
  • Not balancing the fat, protein, and fiber in your meal
  • Getting overheated
  • Humid weather
  • High altitudes
  • Hormone changes (such as having a period)
  • Taking too much insulin or another diabetes medication
  • Working out more than usual
  • Skipping meals
  • Not timing insulin correctly
  • Eating more carbohydrates

If you live with diabetes, a reading below 70 mg/dL indicates hypoglycemia. Severe low blood sugar is 54 mg/dL. But your doctor may give you slightly different guidelines.

How to treat low blood sugar

Hypoglycemia can quickly become a medical emergency. It's important to get your blood sugar up as quickly as possible. Follow the American Diabetes Association's 15-5 rule:

  • Eat or drink 15 grams of fast-acting carbohydrates (such as 1 tablespoon of honey or 3-4 glucose tablets).
  • After 15 minutes, check your blood sugar.
  • If it's still below 70 mg/dL, have 15 grams more carbohydrates.
  • Repeat the above steps as needed.

Children may need fewer carbohydrates. Check with their doctor so you know what to do when their blood sugar is low.

If you live with diabetes, it's common for your blood sugar to climb too high. For most people, that means a reading above 180 mg/dL. But ask your doctor about your target numbers.

The reasons for hyperglycemia include:

  • Not using enough insulin
  • Not taking enough other diabetes medication
  • Not getting enough physical activity
  • Being injured
  • Being sick (for instance, with a cold, flu, or an infection)
  • Having a medical procedure
  • Stress in your personal or work life
  • Eating more than you planned

Your blood sugar can also rise too high for reasons that aren't clear.


How you treat your high blood sugar will depend on what's causing it. Your doctor might suggest:

  • A glucose-lowering drug
  • An extra dose of insulin -- but never take this without first checking with your doctor
  • Changes to your daily routine -- for instance, drinking more water to help your body flush out extra sugar, trying to better manage stress, or exercising more

If your blood sugar climbs to unhealthy levels, it raises your risk of severe health issues such as:

  • Joint issues
  • Bone damage
  • Heart disease
  • Nerve damage
  • Kidney failure
  • Vision loss
  • Gum infections

Because of these risks, it's key to treat high blood sugar right away.

When to go to the emergency room

Always call your doctor if you get two readings in a row of 300 mg/dL or higher.

Untreated hyperglycemia can also cause a condition called ketoacidosis (diabetic coma), which can be life-threatening. The signs to watch out for are:

  • Fruity-smelling breath
  • Nausea
  • Throwing up
  • Very dry mouth
  • Feeling short of breath

If you have any of these symptoms, go to an emergency room right away.

The American Diabetes Association recommends the following blood sugar ranges for most people who live with diabetes:

  • Between 80 and 130 mg/dL (4.4 and 7.2 mmol/L) before a meal
  • Less than 180 mg/dL (10 mmol/L) 2 hours after a meal

But many factors may change your target range. Among them are:

  • Pregnancy
  • Getting older
  • Other diabetes-related health issues

Checking your blood glucose at home is the surest way to keep track of your blood sugar levels. This can make you feel confident that your diabetes treatment is working. Take a reading as often as your doctor suggests. If that seems hard, let your doctor know. They can help you solve any challenges that you're facing.

To prevent severe health issues, your blood sugar should stay within a healthy range. If you live with diabetes, you'll need to monitor your blood glucose often. Follow your doctor's treatment plan and know the steps to take when your blood sugar gets too high or too low.