Blood sugar levels - what they are and what they mean

Blood sugar levels aren’t just important if you have diabetes. We all need to pay attention to our blood sugar levels because they’re an important sign of our metabolic health which affects the risk of heart disease, cancer and dementia^{1, 2, 3}.

In this article we’ll cover what blood sugar levels are, why they matter, the factors that affect them, what’s normal and how you can check yours.

Your “blood sugar level” is a measure of the concentration of glucose in your blood at a particular point in time. In the UK, your blood sugar or blood glucose level is typically measured in millimoles per liter (mmol/L). However, in the United States, it’s measured in milligrams per deciliter (mg/dL).

Glucose is one of the key “fuels” that your body uses for energy. In fact, your brain depends almost entirely on having a steady supply of glucose to function properly.

Blood sugar levels naturally change throughout the day and can be affected by many factors such as what you eat and the exercise you do. However, because blood sugar level is so important for your normal bodily functions, it’s kept under tight control by insulin - the main hormone that lowers your blood glucose level - as well as several other hormones.

There are two main reasons you should care about your blood sugar levels.

Firstly, high blood sugar can cause direct injury to your tissues. In particular, high blood sugar levels (so called “hyperglycaemia”) directly damages the delicate lining of blood vessels which contributes to atherosclerosis and heart disease⁴. Furthermore, high blood sugar levels lead to more insulin release and over time, this can cause insulin-resistance, where your body requires more insulin to deal with a given sugar load.

Secondly, high blood sugar levels are the first sign - the “canary in the coal mine” - of poor metabolic health. If your body is struggling to control your blood sugar, it’s a sign that your metabolism is not working as well as it should be. In turn, having poor metabolic health increases your risk of developing heart disease, cancer and even dementia. You may find it surprising, but even within the “normal” range, lower blood sugar levels are associated with a lower risk of chronic disease².

So, blood sugar levels are important. But what are the factors that affect them?

Carbohydrates, such as potatoes, pasta and sugary-drinks, are where we get sugar from in our diet.

Carbohydrates can be “simple” (also called “refined”) where the sugar doesn’t need to be broken down in the gut and is rapidly absorbed. Eating simple carbohydrates such as sugary-drinks can lead to rapid spikes in blood sugar.

On the other hand, complex carbohydrates, such as potatoes, need to be broken down in the gut to release sugars so they tend to lead to a steady absorption of glucose over time and less of a spike.

But there’s more to blood sugar levels than just what you eat. When you eat, the order in which you eat different foods and what you do around the time of eating can all have an impact:

• Blood sugar peaks can be higher and last longer when we eat the same meal in the evening compared to eating it in the morning.
• Eating fat, fibre or protein before carbohydrates slow sugar absorption and reduce spikes.
• Going for a walk after eating leads to a lower and shorter-lived glucose spike⁵.

Our muscles are the major stores for glucose after we eat a meal.

In the short-term, exercise has three beneficial effects on our glucose metabolism. Firstly, using our muscles increases their demand for glucose. Secondly, active muscles are more sensitive to the effects of insulin. And thirdly, active muscles can take up glucose independently of insulin. All these mean that exercise after a meal - even a short walk - reduces the size and duration of a glucose spike.

In the longer-term, exercise alters our metabolism such that we’re better able to handle glucose loads without having spikes.

Good sleep is essential for our metabolism. Studies have shown that poor sleep quality and reduced quantity results in higher glucose spikes after eating⁶.

When we’re under stress, our body releases the hormone cortisol which causes our body to produce more glucose from the liver in a process called “gluconeogenesis”.

Drinking alcohol can cause blood sugar to drop because it interferes with your liver's ability to release glucose. But drinking alcohol over the long-term may lead to higher glucose levels as it impairs your liver function and insulin sensitivity.

Certain medicines, like insulin for those with diabetes, lower blood sugar by helping glucose move from the blood into cells. Others, like steroids, can raise it by affecting how your body processes glucose.

When you're sick, your immune system triggers the release of certain hormones, such as cortisol, that raise blood sugar levels.

So what is a normal blood sugar level? Because blood sugar levels are impacted by food, we generally measure blood sugar levels in people after they have been fasting for 6 to 8 hours so that results are “standardised” (i.e. comparable between people or over time). Because fasting can make blood tests harder to do, we can also assess blood sugar levels with a different test called HbA1c (which we’ll cover in more detail below).

There are several ways to check your blood sugar levels and they all have their pros and cons.

This is the most common method for checking your blood sugar level. You prick your fingertip to collect a drop of blood that’s placed on a test strip and analysed by a portable machine called a “glucometer”. This gives an instant reading of your blood sugar level and is particularly useful before and after meals to help diabetics manage their glucose levels.

A CGM is a small device placed on the upper arm that has a hair-thin sensor that sits under the skin. A CGM measures glucose levels continuously, in real-time and sends data to your phone for you to see. This is really valuable when it comes to looking at the effects of diet, exercise and sleep on your blood sugar levels.

A CGM actually measures glucose levels in the fluid around your cells (called interstitial fluid), rather than in the blood itself. The two levels are very closely related but the glucose level in your interstitial fluid lags behind the glucose level in your blood by about 15 minutes.

Over time, glucose in your blood actually sticks to one of the proteins in your red blood cells called haemoglobin. This is called “glycated haemoglobin” or HbA1c. The higher your blood sugar, the higher the amount of haemoglobin with sugar stuck to it and the higher your HbA1c.

HbA1c gives us a way to estimate your average blood sugar over the last 3 months or so. This is because red blood cells last in the blood for 3 months. HbA1c is a simple blood test and we regularly use it to diagnose pre-diabetes and diabetes.

The most accurate way to measure your blood glucose level at a particular point in time is to take a blood sample from a vein. We usually do this when someone has been fasting for 6 to 8 hours to standardise the test.

We offer both the HbA1c blood test and the continuous glucose monitor.

With our HbA1c blood test, we’ll send you a kit so you can take a blood sample at home and send it to our lab for analysis. You’ll receive your result and doctors report the same day.

Our CGM programme includes the CGM itself, a glucose challenge test to stress your metabolism and a consultation with one of experts to go through your results in detail and discuss how you can improve your sugar-handling and your metabolic health.