Glucose is the primary energy source for the body's cells and the only energy source for the brain and nervous system. A regular flow of glucose must be available for use, and there must be a relatively constant level of glucose in the blood.
When we digest fruit, vegetables, bread and other food, carbohydrates are broken down into glucose and other nutrients and these are then absorbed by the small intestine before being circulated throughout the entire body. The use of glucose for energy production is dependent on insulin, a hormone produced by the pancreas. The hormone insulin transports glucose to the body's cells and directs the liver to store surplus energy as glycogen for short-term storage and/or as triglycerides in adipose (fat) cells.
Blood sugar levels normally rise slightly after a meal, and insulin is released into the blood from the pancreas in response. When the glucose is broken down in the cells, the blood sugar level falls again and the pancreas responds by slowing down the production of insulin before then ceasing to release insulin.
If the blood sugar level falls too low – which can happen between meals or after strenuous exercise – another pancreas hormone known as glucagon is secreted. This hormone directs the liver to turn stored glycogen back into glucose in order to increase the blood sugar level. If the glucose/insulin feedback mechanism is working correctly, this means that the amount of glucose in the blood remains relatively stable. If the balance is disrupted and the glucose level in the blood rises, the body compensates by attempting to restore the balance by both increasing insulin production and eliminating surplus glucose in the urine.
There are various conditions that can disrupt the balance between glucose and pancreatic hormones, resulting in high or low blood sugar levels. The most common cause of this is diabetes. Diabetes is a group of disorders linked with insufficient insulin production and/or resistance to the effects of insulin. People with untreated diabetes are unable to process and use glucose normally. If you are unable to produce sufficient insulin, or any insulin at all (and normally have diabetes autoantibodies), you have type 1 diabetes. If you are resistant to insulin and are unable to produce sufficient quantities, you may have prediabetes or type 2 diabetes.
Serious, acute changes in blood glucose levels, be they high (hyperglycaemia) or low (hypoglycaemia), may be life-threatening as they cause organ failure, brain damage, coma and – in extreme cases – death. Chronically high blood sugar levels can lead to progressive damage to the body's organs such as the kidneys, eyes, heart and blood vessels and nerves. Chronic hypoglycaemia may result in brain and nerve damage.
Some women may develop gestational diabetes, which is hyperglycaemia occurring during pregnancy. If this is not treated, these mothers may end up giving birth to large babies with low blood sugar levels. Women who have had gestational diabetes can then go on to develop diabetes.
Diabetics have to monitor their own blood sugar levels, often several times a day, in order to determine the extent to which their glucose levels are above or below where they should be and to determine which medications or insulin they may need.
The glucose test is used to:
There are two markers which measure blood sugar levels:
Signs and symptoms of high blood sugar levels are known as hyperglycaemia. High levels of glucose indicate diabetes in most cases, but many other diseases and conditions may also cause elevated blood sugar levels.
Other diseases and conditions that can lead to elevated blood sugar levels include:
Low glucose levels may indicate hypoglycaemia, a condition characterised by a reduction in blood glucose to a level which causes symptoms such as sweating, palpitations, hunger, tremors and anxiety. The condition then starts to affect the brain and causes confusion, hallucinations, blurred vision and sometimes even coma and death.
Low blood sugar levels (hypoglycaemia) can be seen in cases of: