Glucose Guide

What is blood glucose?

Glucose is a type of sugar. It is the sugar that is in our blood.

Why do we need blood glucose?

Glucose is essential for life. The body needs glucose because it is one of the fuels that cells use for energy. Glucose is transported around the body in the blood.

How much glucose do we need in our blood?

A normal blood glucose level is about one teaspoon of glucose (4 to 5 grams) in all the blood in the body. This is a very small amount. When testing blood glucose, this one teaspoon would give a test result of about 3.8 to 5.5 mmol/L (70 to 100 mg/dl).

Blood glucose stability (homeostasis)

It is essential to have enough glucose in the blood to stay alive. It is equally important for the blood glucose level to not be too high, since high levels can cause many health problems. The body therefore has to keep control of the blood glucose level and maintain glucose stability. If the level falls, the body must raise it again. If it goes too high, the body must lower it. This is called blood glucose homeostasis.

Where does blood glucose come from?

There are only two places the glucose in our blood can come from. It can come directly from food, and it can come from our liver.

Glucose from food

Some foods contain carbohydrate, which can include either glucose sugar itself or starch that digests to become glucose. Most sweet foods contain some glucose. Starchy foods include anything made from wheat flour, as well as potatoes, rice and pasta.

The glucose we eat, whether from sugary foods or starchy foods, is absorbed from the gut into the blood, raising the blood glucose level. Typically, the more sugar and starch we eat, and the more refined (easy to digest) it is, the greater the rise in blood glucose.

Glucose from the liver

Our liver can store glucose. It does this by joining glucose molecules together to make glycogen. The liver can store about 100g of glucose in this way. When we need more glucose in the blood, the liver can break the glycogen back down into glucose and release it into the blood.

Our liver can also make glucose. It can take other non-glucose foods we have eaten, and foods we have stored in our body such as fat, and turn them into glucose. This is why we can maintain a normal blood glucose level even after not eating for a day.

How does the body lower blood glucose?

When blood glucose rises the body must take action to lower it again. A major player in reducing blood glucose is insulin. Insulin is a hormone, a messenger, that is released from an organ called the pancreas, which sits near the stomach and liver in the abdomen. People with type 1 diabetes do not produce insulin, so it must be injected.

When insulin is released into the blood it lowers blood glucose by “knocking on the door of cells”, instructing the cells to open their door and let glucose in. This allows the blood glucose level to reduce back to normal. Insulin also instructs the liver not to release glucose into the blood. This is important, because when blood glucose is already high the body does not want the liver adding more.

Interestingly, insulin also instructs the body to make and store fat, and not to burn fat.

Insulin resistance

In our current world, insulin resistance is a widespread problem. Put simply, insulin resistance means the cells do not want to open their doors when insulin comes knocking. When the body is insulin resistant the pancreas must release more insulin to lower blood glucose. Instead of cells receiving a polite knock on the door from insulin, there is a battering ram of insulin.

Insulin resistance and high blood insulin levels are thought to be a significant contributor to most modern diseases. Insulin resistance can cause blood glucose levels to become high, and stay high, after eating sugary and starchy foods.

How to measure blood glucose

Measuring blood glucose can be useful for assessing health, such as identifying, monitoring and improving type 2 diabetes. It can also be useful for understanding how the body responds to different foods and to other lifestyle factors such as movement, sleep and stress. There are several ways to measure the blood glucose level.

An instant blood glucose test

Blood glucose can be measured with a simple blood test. This could be done on a blood sample taken from a vein, or by using a finger prick sample. Testing blood glucose in this way provides a snapshot of the glucose level, a bit like taking a glance at the speedometer in a car. It tells you what speed the car is doing at a moment in time.

HbA1c, a blood test of average blood glucose levels

Another blood test commonly used to measure and monitor blood glucose is called haemoglobin A1c (HbA1c). Haemoglobin is the oxygen-carrying molecule in red blood cells. Glucose in the blood can attach to haemoglobin, and a greater amount of glucose in the blood will cause a greater amount to attach. A red blood cell has a lifespan of about three months, so the HbA1c test provides a measure of the average blood glucose level over about a three-month period.

Using the car analogy, the HbA1c test is a bit like driving a car for three months and then calculating the average speed over the entire three months.

Continuous glucose monitoring

Devices called continuous glucose monitors (CGM) were initially used in healthcare to assist with blood glucose control in type 1 diabetes, and more recently for all forms of diabetes. CGMs can also assist people without diabetes to assess and improve their health.

A CGM provides insights into the body's response to different foods, as well as other lifestyle factors such as movement, sleep and stress. By using a CGM a person can get instant and longer-term feedback on what effect their diet and lifestyle is having on their glucose level, then adjust their behaviour to achieve their health goals.

Using the car analogy, a CGM is like being able to look at the speedometer at any moment you wish, and also to look back in time to review what speed the car was going.

There are many different models of CGM now available. Typically they are worn on the arm and have a small filament that is inserted into the skin. After applying the CGM there is no needle left in the arm.

A CGM does not measure blood glucose directly. It measures the glucose in the fluid in the tissues near to blood vessels. As a result, the CGM reading lags changes in the blood glucose level by about 15 to 20 minutes.

How does continuous glucose monitoring help to improve health?

In type 1 and type 2 diabetes, a CGM can make it easy to measure the blood glucose level and the response to food and drink, and it can help direct medication use. It can also give people with and without diabetes insights into their current health, including their degree of insulin resistance.

For everyone, a CGM can support behaviour change by allowing personal insights into the effect of food and lifestyle on the blood glucose level. This feedback can support people to make choices that are likely to improve their health and achieve their goals. Examples of these insights include the following.

Glucose measurement in the morning

A glucose level on waking of more than 5.5 can be a sign of insulin resistance. If this is happening on most mornings, it may indicate that a person would benefit from making lifestyle changes to improve insulin resistance.

Seeing the effect of different foods

A CGM allows people to see instantly the effect of foods on their blood glucose, notably sugary foods and starchy foods, especially refined and ultra-processed ones. Depending on a person's goals, this feedback can help them alter their diet and perhaps reduce foods that cause a significant or prolonged rise in blood glucose.

Many people are using CGMs to adjust their diet and other lifestyle factors to improve insulin resistance, to reverse type 2 diabetes, to lose weight and waist, and to feel at their best throughout the day.

Reactive hypoglycaemia (a “hypo”)

Hypoglycaemia means low blood sugar. Sometimes eating food that leads to a high blood sugar, also known as a glucose spike, can cause a low blood sugar 1 to 3 hours after eating. This is called a reactive hypo, and it is because of the effect of insulin.

A rapidly falling blood glucose, and a low blood glucose, can make people feel hungry, anxious and sweaty, have difficulty concentrating, and feel sleepy. A CGM can help reveal whether glucose spikes and reactive hypos are happening, which can help people understand why they may be experiencing symptoms, and guide them away from problem foods.

Some CGM knowledge nuggets

When people first start using a CGM the information can be confusing. These knowledge nuggets may help.

The “normal glucose range”

Many monitors have a default normal glucose range. For example, the Freestyle Libre can come with a pre-set range of 3.9 to 10.0 mmol/L (70 to 180 mg/dl), shown as a green zone on the graph. This range was set with type 1 diabetes in mind. Depending on a person's goals, including for type 1 diabetes, it may be appropriate to adjust it in the app settings. For example, someone may decide it is useful to bring the upper limit down to 6.0 mmol/L (110 mg/dl).

Glucose spikes after eating

It is normal for blood glucose to go up, or spike, after eating sugar and starch. What people may wish to consider is how much the glucose goes up, how long it stays up, and how often these rises are happening.

Low blood glucose dips in the night

Overnight, when asleep, people may see their glucose level dip for short periods to below 3.9 mmol/L (70 mg/dl). These dips may show as red on the CGM graph, which can be worrying. However, for most people, especially those without type 1 diabetes and not taking drugs that force glucose down, this drop in the night is not a worry and is often normal or a simple error from lying on the sensor.

Exercise and blood glucose

Exercise can affect blood glucose. Steady and less intensive exercise can help lower a raised blood glucose level. Conversely, some people will see a rise in their glucose level with a bout of strenuous exercise. This rise is normal. It is a consequence of the body's hormonal and nervous systems instructing the liver to release glucose into the blood to fuel the exercise, and is usually not something to be concerned about.

Late night eating

A large meal in the evening, close to bedtime, can lead to a raised blood glucose level the next morning.

Alcohol

Blood glucose can fall after drinking alcohol, because alcohol blocks the liver from making glucose. This is perhaps one reason people can feel hungry after drinking alcohol, or the following morning.

Poor sleep

A poor night's sleep can lead to a raised blood glucose level the next morning. It may also increase blood glucose throughout the day. This is due to the hormonal and nervous system changes that follow poor sleep. Poor sleep can also increase the desire for sugary foods and refined starchy foods.

Stress

The hormonal and nervous system response to stress can increase insulin resistance. It can also increase glucose release from the liver. This can cause a higher blood glucose level throughout the day. Mental stress may also increase the desire for sugary foods in some people.

Infections

Infections, such as the common cold, will often increase the blood glucose level. As the infection resolves the blood glucose will reduce again.