Also called: Blood Sugar, Blood Glucose


Glucose is a sugar found in the bloodstream that is the body’s main source of energy. It is a simple sugar, meaning it cannot be broken down into other sugars. People obtain glucose by eating foods that contain carbohydrates, such as breads and cereals, vegetables, fruits and sugary sweets. Glucose is also manufactured in the body through the breakdown (metabolism) of protein and fats.

When food is eaten, it is broken down into glucose and absorbed into the bloodstream. As the amount of glucose in the bloodstream starts to rise, an abdominal gland called the pancreas reacts by producing insulin. This natural hormone helps carry the glucose from the bloodstream into the cells of muscle and fat tissue. Inside the cells, glucose – which is made up of carbon, hydrogen and oxygen – is broken down and converted into adenosine triphosphate, better known as ATP. This is the body’s primary fuel. 

Without insulin, glucose cannot be processed and builds up in the bloodstream, a condition called diabetes. This can occur when patients either cannot produce insulin (type 1 diabetes) or have cells that have become resistant to insulin (type 2 diabetes). People with type 2 diabetes may find that their ability to produce insulin also wanes over time.

Medications and lifestyle changes such as nutritious diet, exercise and weight loss help those with diabetes to use insulin more effectively, thus controlling their glucose. This can help patients to live longer and healthier lives.

About glucose

Glucose (blood sugar) is the most important source of energy for the body. Most glucose comes from eating certain foods, but the body can also manufacture glucose between meals, at night and in other situations.

When a person consumes a food, it is digested and broken down into glucose before it is absorbed through the intestinal wall and into the bloodstream. As levels of glucose rise in the blood, an abdominal gland called the pancreas begins producing insulin. This natural hormone helps move glucose from the bloodstream into the cells.

Once inside the cells, glucose – which is made up of carbon, hydrogen and oxygen – is broken down and converted into adenosine triphosphate, better known as ATP. This is the body’s main fuel.  Excess glucose in the bloodstream usually ends up in the liver, where it is stored as glycogen. When blood sugar is low, a hormone called glucagon stimulates the liver to convert glycogen back into glucose.

People experience an increase in blood glucose during the early morning. Known as dawn phenomenon, this condition occurs naturally but in people with diabetes can cause hyperglycemia (excess glucose). Some diabetic patients experience the Somogyi effect, which is rebound hyperglycemia after a bout of overnight hypoglycemia.

Exercise generally burns glucose, but physical activity can have varying effects on glucose in people with diabetes.

Glucose is a monosaccharide, meaning it cannot be broken down into a simpler sugar. The glucose that is used by the body comes from several sources, including:

Simple carbohydrates. These are monosaccharides and disaccharides (which contain two monosaccharides):

  • Monosaccharides. Include glucose, fructose (found in fruits) and galactose (found in milk products). The liver converts fructose and galactose into glucose.
  • Disaccharides. Include lactose (found in dairy products), sucrose (found in table sugar) and maltose (found in some vegetables and beer). Glucose is a molecule in each of these carbohydrates. Enzymes in the digestive tract convert these disaccharides to their monosaccharide bases.

Simple carbohydrates are quickly digested and absorbed into the bloodstream through the intestinal lining. This can cause blood glucose levels to rise rapidly, requiring greater amounts of insulin to be produced to move these sugars from the blood into the cells.

Some foods with simple carbohydrates are healthier choices than others. For example, vegetables and fruit contain simple carbohydrates but also contain fiber, essential vitamins and minerals important for bodily functions.

Other foods made up of simple carbohydrates contain few or no nutrients. Processed sugars such as those found in candy and cakes contain mainly empty calories. Consuming excessive amounts of simple carbohydrates, especially processed sugars, can cause a glucose roller-coaster effect in the body.  

Simple carbohydrates include:

  • Fruits
  • Milk and milk products
  • Some vegetables
  • Candy
  • Table sugar
  • Syrups (except natural syrups such as maple syrup)
  • Nondiet carbonated beverages

Complex carbohydrates. Commonly known as “starches,” these are made up of many glucose molecules linked together. Plants create glucose and chain the molecules together to form starch, which stores energy.  

The body breaks down starches into glucose. They are digested much more slowly than simple carbohydrates. Thus, they are less likely to cause a sudden spike in blood glucose levels. This is particularly true of carbohydrates containing soluble fiber. Eating complex carbohydrates that contain soluble fiber will result in a slower rise in glucose levels.

Complex carbohydrates include:

  • Grains (wheat, corn, oats, rice)
  • Cereals
  • Potatoes
  • Plantains
  • Legumes (peas, beans, lentils)

Carbohydrates are an essential source of energy for the body because they are a major source of glucose. The choice of carbohydrate source is especially important for people with diabetes. Foods such as whole grains, fruits and vegetables contain fiber, vitamins and minerals that release glucose at a slower rate and provide better control over blood glucose.

The body can convert protein into glucose by the body if necessary. This usually happens when there is no other source, such as carbohydrates, for the body to convert into glucose. Fat is not usually converted into glucose and excess food is stored as fat in the body tissues.

Glycogen. Excess glucose in the bloodstream is usually taken up by the liver, which stores it in a starchy form known as glycogen. When levels of blood glucose are low, the hormone glucagon breaks down the glycogen in the liver and returns it to its glucose state. This process primarily happens at night and during other periods between meals.

People with diabetes are unable to properly use all of the glucose in their bodies and are at risk of having high levels of blood glucose (hyperglycemia). People with diabetes either:

  • Cannot produce the insulin necessary to transport glucose from the bloodstream to the cells (type 1 diabetes, latent autoimmune diabetes of the young)
  • Have cells that have become resistant to insulin, a state that prevents the proper amount of glucose from reaching the cells (type 2 diabetes, gestational diabetes, maturity-onset diabetes of the young, most forms of secondary diabetes).

Consistently high blood glucose levels create a dangerous condition that can cause many associated complications, including:

  • Kidney disease (diabetic nephropathy)
  • Eye diseases (retinopathy, glaucoma, cataracts)
  • Nerve disease (neuropathy)
  • Heart conditions and stroke
  • Poor circulation
  • Foot problems and skin problems
  • Gastroparesis (delayed stomach emptying)
  • Sexual dysfunction
  • Yeast infections and other infections
  • Gum disease and tooth decay

Severe hyperglycemia can lead to diabetic coma. Poor control of diabetes can also cause low levels of glucose (hypoglycemia), which can result in insulin shock and diabetic coma.

Patients with diabetes need to consistently measure the amount of glucose in their bloodstream by regularly using a glucose meter at home and periodically having glucose tests performed by their physician. Such tests can reveal many things, including:

  • How foods are being broken down into energy
  • How efficiently the liver is functioning
  • How well certain organs are working, particularly those that regulate glucose
  • How well glucose has been controlled for the past few months (through the glycohemoglobin test)

Hyperglycemia and hypoglycemia

It is important that people with diabetes maintain proper levels of glucose (blood sugar) to ensure optimal health. For most people, unless otherwise instructed by their physician, blood glucose should fall into the following target ranges, with measurements given in milligrams per deciliter (mg/dL):

  • Fasting (upon waking): 70 mg/dL to 110 mg/dL
  • After meals: 70 mg/dL to 140 mg/dL

For someone with diabetes, the following target ranges are often recommended:

  • Before meals: 90 mg/dL to 130 mg/dL (plasma)
  • One to two hours after the beginning of a meal: 180 mg/dL and lower (plasma)
  • Bedtime: 110 mg/dL to 150 mg/dL

Levels of blood glucose levels can occasionally drop below 60 mg/dL or even 50 mg/dL and not necessarily indicate a serious problem. However, levels below 45 mg/dL almost always indicate a serious problem.

In some cases, glucose levels can slip out of the normal range and result in conditions such as:

  • Hyperglycemia. Abnormally high blood glucose occurs when the body has too little insulin or when the body cannot use insulin properly. All diabetic individuals occasionally have high blood glucose, but serious complications can develop when readings are unusually high or frequently high. Left untreated, hyperglycemia can lead to diabetic ketoacidosis, hyperosmolar hypoglycemic nonketotic syndrome or coma. Ways of preventing hyperglycemia include taking insulin or antidiabetic agents as prescribed and following the recommended diet and exercise plan.
  • Hypoglycemia. Abnormally low blood glucose occurs when a person’s levels of glucose and insulin are unbalanced. People with hypoglycemia unawareness (difficulty sensing low glucose) are especially vulnerable. Mild cases of hypoglycemia can cause dizziness or weakness. Severe cases can lead to fainting, convulsions, brain damage or coma. Eating a low-fat food with fast-acting glucose (such as fruit juice or at least five pieces of hard candy) or taking glucose medication in the form of tablets or gels, as recommended in a physician’s treatment plan, can help boost blood sugar quickly.
  • Insulin shock. This is a severe reaction in which hypoglycemia develops quickly and causes unconsciousness. Treatment may include an injection of glucagon or intravenous glucose.
  • Unstable diabetes. Chronic problems with high or low glucose may require changes in the treatment plan to avoid severe consequences.

Questions for your doctor about glucose

Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about glucose:

  1.  What is my target glucose range?
  2. What do my test results show about my glucose control?
  3. How often should I perform glucose monitoring?
  4. Should I test my blood sugar if I feel weak or anxious?
  5. What can happen if my glucose is not controlled?
  6. What are the best ways for me to control my glucose?
  7. How does my diet affect my glucose? Do I need to make any dietary changes?
  8. What exercises do you recommend to help me control my glucose?
  9. How can I avoid a hypoglycemic reaction from exercise?
  10. What action should I take if I experience a bout of low or high glucose?
  11. When are the best times to take my diabetes medication?
  12. Can alcohol cause problems with my blood sugar?
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