What is High Blood Sugar

High blood sugar is also medically referred to as hyperglycaemia and is common in people who have diabetes.

Hyperglycaemia can affect those who have:

Type 1 diabetes

  • When the pancreas doesn’t produce insulin at all
  • Insulin is a hormone which regulates the metabolism of carbohydrates, proteins and fats
  • It encourages the absorption of glucose into fat, skeletal and muscle cells from the blood

Type 2 diabetes

  • Pancreas doesn’t produce enough insulin, or the cells don’t react to the insulin which is released

Gestational diabetes

  • Occurs only in pregnant women
  • High blood sugar during pregnancy which usually settles after birth

Normally, high blood sugar occurs in people with diabetes, however non-diabetic people can also have it. For example, people who have been ill particularly those who have recently experienced a stroke or a heart attack or those with a serious infection.

What are the symptoms of high blood sugar?

Hyperglycaemia occurs when there is a reduction in the effective action of insulin in the blood coinciding with an increase in counter-regulatory hormones such as glucagon, cortisol and growth hormones.

Because of the changes in hormones there is a greater production of glucose by the liver and kidneys and the body tissues are less likely to use glucose as their source of energy.

Hyperglycaemia therefore affects the body’s water balance and can result in a greater output of urine.

This means the body loses more water and important salts such as sodium and potassium than usual. These losses can lead to dehydration which can become progressively worse as the body’s hyperglycaemic state makes it harder to retain water.

If this situation gets worse the body can develop a condition known as hyperosmolar hyperglycaemic state (HHS), which is caused when the body works harder to rid excess sugar.

In people who have diabetes, the symptoms of high blood glucose levels tend to emerge slowly over a period of week or months. The symptoms may include:

  • An increase in thirst
  • A dry mouth
  • Needing to pass urine more often
  • Blurred vision
  • Recurrent infections
  • Unintended weight loss
  • Tiredness

Children who have type 1 diabetes have reported other adverse effects of high blood sugar including:

  • Nausea
  • Weakness
  • Headaches

Symptoms such as these may not even occur until the person’s blood sugar is extremely high. If left untreated or undiagnosed then excessively high blood glucose can lead to permanent damage to organs and body tissues including:

  • The eyes
  • Kidneys
  • Liver
  • Blood vessels
  • Nerve cells

Acute hyperglycaemia can have a detrimental impact on mental performance and mood.

Blood glucose and cognitive function

Experimental studies have explored the link between high blood glucose levels and cognitive function. Some have found some evidence to support the potential negative effect of acute high blood sugar levels.

To function appropriately, the brain requires a constant supply of glucose, its primary source of energy. If there is a continuous fluctuation in blood glucose level, the function of the brain can be affected.

In a study by Sommerfield et al., (2004), the effects of acute hyperglycaemia on brain function and mood in people who have type 2 diabetes were examined.

The results of the study confirmed that during periods of acute hyperglycaemia, cognitive function was impaired, and the subject’s moods worsened. The blood glucose level of the participants in the study was clamped at 16.5 mmol/l.

Individuals who were unaware of their blood glucose levels found their cognitive state became affected. Functions such as the speed in which they could process information, their memory and attention span were all affected. With regards to individual mood, many experienced increased agitation, and feelings of anxiety as well as tiredness and a decreased feeling of happiness.

A further study by Cox et al., also showed that hyperglycaemia affected cognitive function. In the study, participants were required to carry out some cognitive tests on a handheld computer before their blood glucose levels were measured.

The research showed that in individuals whose blood glucose level was above 15 mmol/l, verbal fluency and subtraction time was slower in individuals with type 1 diabetes. Whereas, individuals who had type 2 diabetes had a slowing in all of the cognitive tests they carried out.

Both studies show hyperglycaemia is associated with mild cognitive dysfunction alongside the other, more traditional symptoms. Yet, the link between impaired brain function in hyperglycaemic patients and their daily tasks is still not fully understood and more research needs to be carried out.

What level is high?

You can monitor your blood sugar levels at home with a blood glucose meter. If you do monitor your levels at home, then a normal blood glucose level before eating should be between 4 and 7 mmol/l and 2 hours after consuming a meal it should be under 9 mmol/l.

So, high blood sugar can be defined as:

  • Glucose levels above 7 mmol/l when you haven’t eaten
  • Glucose levels about 11 mmol/l 2 hours after a meal

What causes high blood sugar?

High blood sugar can be caused by a number of factors including:

Eating too much food

  • Having too many snacks between meals

Stressful situations

  • Work
  • Family issues
  • Bereavement


  • Common cold

Not taking enough diabetic medication

  • Incorrect dose
  • Missing a dose
  • Treating an episode of low blood sugar too much

Lack of exercise

  • Lounging

Not drinking enough fluids

  • You can become dehydrated
  • During periods of illness you’re more likely to become dehydrated due to:
    • Vomiting
    • Diarrhoea

Preventing high blood glucose levels

Every now and again we all love a sugary treat or a carbohydrate laden meal and there is nothing wrong with this, we are all human after all and deserve a treat occasionally. But consuming foods like this too often can have harmful effects on our health.

There are a number of steps that can reduce the likelihood of developing hyperglycaemia:

Eating a healthy balanced diet

Try to keep snacking between meals to a minimum and be aware of the amount of sugar and carbohydrates you are consuming.

Getting more exercise

Exercise doesn’t have to mean join a gym and follow a rigorous regime. One great way to help lower blood sugar is in fact walking. Exercising strenuously can cause the body to produce a stress response which in turn cause blood glucose levels to rise.

Monitor your glucose levels

You can do this at home or you can get a test from your GP or some pharmacies also offer a free service.

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Having always been intrigued by how body’s work, both in their day to day functions and also when things go wrong, Leanne is pursuing her passion for health. By combining her undergraduate degree in Biomedical Science and a Masters in Science communication with her love for writing, she is passing on her knowledge to Health Hub readers. Leanne strongly believes that science and health are something that should be talked about more and she hopes her articles will enable such conversations.