What is hyperglycemia? Learn about high blood sugar levels

Hyperglycemia, or high blood sugar, means that your blood sugar is higher than the healthy normal range. Insulin is a hormone released from the pancreas and it's responsible for lowering blood sugar. Having high blood sugar is a sign that you're not releasing enough insulin or your body is resistant to the effects of insulin.

Very high blood sugar levels can directly damage your blood vessels over time. This is why people with diabetes are at higher risk of heart disease, nerve damage and kidney problems. In the short-term, high blood sugar levels in diabetics can also lead to severe dehydration.

However, even if you don't have diabetes. Having higher blood sugar levels is a sign of poor metabolic health and is linked to an increase in the risk of heart disease, cancer and dementia. Of course, higher blood sugar levels also indicate a higher risk for going on to develop diabetes.

In this article we'll go through what high blood sugar levels are, the symptoms, causes and how you can lower your blood sugar.

What are high blood sugar levels?

Blood sugar levels naturally vary depending on whether you’ve been fasting or have recently eaten. To standardise interpretation of blood sugar levels, we look at levels wither when you're fasting or 2 hours after you've drunk a defined amount of glucose (an oral glucose tolerance test or OGTT).

According to the American Diabetes Association, the following levels indicate normal, pre-diabetes or diabetes¹.

Generally, fasting blood sugar levels over 6.9 mmol/l (125 mg/dL) and after-meal levels above 11 mmol/l (200 mg/dL) are considered hyperglycemia.

High blood sugar symptoms

Hyperglycemia may not cause obvious symptoms at first, but as blood sugar remains high, you might notice:

• Increased thirst (polydipsia)
• Frequent urination (polyuria)
• Fatigue or unusual tiredness
• Blurred vision
• Unexplained weight loss
• Slow healing of cuts or wounds
• Increased hunger (polyphagia)
• Recurrent infections such as urinary tract infections and thrush

What are the causes of high blood sugar (hyperglycemia)?

Eating refined carbohydrates will cause your blood sugar to increase but your body should be able to handle this, returning your blood sugar level to normal pretty quickly. If you have high blood sugar levels, it's a sign that your underlying metabolic health is not as good as it should be.

Common cause of high blood sugar include:

• Obesity: Excess fat, especially around organs, blunts the effect of insulin and increases blood sugar production by your liver.
• Insulin resistance or diabetes: Your body doesn't produce enough insulin or your cells don’t respond appropriately to it.
• Lack of physical activity: Inactivity reduces your muscles’ ability to absorb glucose effectively.
• Stress: Hormones released during stress (e.g. cortisol and adrenaline) increase blood sugar to fuel a fight-or-flight response.
• Poor sleep: Less than 6 or more than 9 hours of sleep can result in insulin resistance.
• Infections or illness: Your body raises blood sugar to provide extra energy when fighting sickness.
• Eating too many refined carbohydrates: Foods like white bread, pasta, and sugary snacks allow glucose to be rapidly absorbed into your bloodstream. If your metabolic health is not good enough to deal with this, your blood sugar level can spike.

How to lower your blood sugar

If you struggle with high blood sugar, there are several evidence-backed strategies to help bring it down:

• Exercise regularly: Aim for a mix of resistance training (e.g. weights, bodyweight exercises) twice a week and moderate aerobic activity (like brisk walking or cycling) 3-4 times weekly. Exercise improves insulin sensitivity, helping your body use glucose more effectively.
• Choose complex carbs: Swap refined carbs for whole grains, legumes and vegetables to reduce sugar spikes.
• Improve sleep quality: Target 7-8 hours of restful sleep nightly to regulate blood sugar hormones.
• Plan your meals: Eating earlier in the day and finishing meals 2-3 hours before bedtime can improve blood sugar handling.
• Quit smoking: Smoking impairs insulin sensitivity and raises diabetes risk.
• Manage stress: Use relaxation techniques like meditation or deep breathing to lower stress-related blood sugar spikes.
• Maintain a healthy weight: Losing even 5-10% of body weight can improve insulin sensitivity and reduce blood sugar.

For a deeper dive, check out our dedicated article on how to lower your blood sugar.

How to check if you have high blood sugar levels

Testing is the only reliable way to check if you have high blood sugar levels.

At Selph, we offer a Diabetes (HbA1c) Blood Test, which measures your average blood sugar levels over the past 2-3 months. This test uses a simple finger-prick blood sample you can collect at home and send to our lab.

Diabetes (HbA1c) blood test

• Order your test kit: Delivered directly to your home.
• Collect your sample: Use the finger-prick method to take a small blood sample. We also offer home visits or local clinic options for venous sampling.
• Send it back: Post your sample using the prepaid envelope included.
• Get your results: Receive detailed results within 2 days, with clear explanations of what your HbA1c level means.

Learn more about testing and how it works on our finger-prick blood test article.

Use a continuous glucose monitor

HbA1c testing shows your long-term blood sugar control. In contrast, you can use a small wearable device called a continuous glucose monitor (CGM) to monitor blood sugar fluctuations in real-time. A CGM gives you more detail than an HbA1c test because you can see how your lifestyle is directly impacting your blood sugar.

At Selph, we have a specific CGM programme that includes an oral glucose tolerance test, a report of your data and a consultation with one of metabolic health doctors to go through your results and help you improve your sugar-handling.