Insulin secretion may sound fairly simple – the process of insulin release into the body’s blood and cells, which is triggered by eating and drinking, and occurs as part of the processes needed to achieve control of raised blood glucose levels.
Yet, this is a very simplistic way to describe the process of glucose-induced insulin secretion. Several chemical processes occur involving complex chemical pathways.
In this article, we will review an overview of those processes while aiming to simplify the concept and relate it to the management of diabetes.
What is insulin secretion?
Insulin is a hormone secreted by the pancreas. A hormone can be defined as a regulatory substance that stimulates specific cells or tissues into action. In the pancreas, insulin is synthesized and secreted by the beta cells in response to high blood glucose levels. Pancreatic beta cells are cells within the pancreatic islets of Langerhans (1,2).
The primary job of insulin secretion is to facilitate insulin entry into the blood and trigger the import of insulin into muscles and other insulin-sensitive tissues (3). Dietary carbohydrate is the main driver for insulin secretion.
In the process of insulin secretion, a glucose transporter called GLUT2 allows glucose to enter the pancreas’ beta cells. Essentially, the glucose transporter GLUT2 triggers glucose uptake into the pancreatic b cells. Within the beta cells, complex chemical processes occur. Glucose will undergo glycolysis, the metabolic pathway that converts glucose into pyruvate and a hydrogen ion.
Following glycolysis, the Krebs cycle and oxidative phosphorylation will also happen. Glycolysis is a complex process involving several steps, complex reactions, and enzymes, such as pyruvate kinase. Glycolysis also leads to the production of Adenosine Triphosphate (ATP). ATP is an organic compound that is the energetic drive for many cellular processes and includes essential messengers such as cyclic amp.
The amount of ATP produced in the beta cell is relative to the amount of glucose that enters the beta cell via GLUT2. Seeing as the carbohydrates that we eat are all broken down into glucose, the amount of carbohydrate in our diet (that can come from food or drink) is directly related to the process of insulin secretion (1,2).
Yet, it is not only the consumption of glucose from dietary carbohydrates that stimulates insulin release. Amino acids from dietary protein consumption can also stimulate insulin release from pancreatic beta cells, although the effect on the amount of secreted insulin is less extreme.
There are some more critical steps in the process of insulin secretion, including a potassium channel and a calcium channel, which controls the influx and efflux of potassium and calcium via the plasma membrane and within the beta-cell. Although it is important as it is calcium 2influx across the plasma membrane, this is a complex process that will eventually trigger insulin release (1,2).
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What role does insulin secretion play in diabetes?
Type 2 diabetes is characterized by the inadequate or ineffective secretion of insulin, which could be a defect of either islet cell function or pancreatic beta-cells. Commonly, you will hear the risk of type 2 diabetes discussed alongside insulin resistance. Insulin resistance is a condition whereby cells fail to respond normally to insulin.
When you have poorly managed, or unmanaged type 2 diabetes, glucose can build up in the blood rather than being distributed to cells or stored. This leads to hyperglycemia, or high blood glucose levels. Unmanaged hyperglycemia can lead to various complications such as kidney disease, heart disease, and problems with the eyes and the feet.
Type 1 diabetes is the other main type of diabetes, although it is far less common than type 2, affecting around 10% of the adults with the condition (compared to around 90% of people with diabetes being affected by type 2). Type 1 diabetes is an autoimmune condition that usually presents in childhood.
Whilst in type 2 diabetes, one of the main characteristics is the inadequate or ineffective secretion of insulin, in type 1, no insulin is secreted at all. If you have type 1 diabetes, you will need to take insulin injections to replace the insulin your pancreas is not producing. This treatment aims to maintain your blood glucose levels in the optimum range. According to Diabetes UK, this is:
- between 5 and 7mmol/l when you wake up and before meals, and
- between 4 and 7mmol/l before meals at other times of the day.
For those with type 2 diabetes, Diabetes UK recommends the same targets when you wake up and before meals, although 2 hours after meals, they recommend blood sugar level should be:
- Less than 8.5mmol/l (4).
Note that you should always agree on individual targets with your specialist diabetes team. They will look at your recent results, including your HbA1c, and in addition, the measurement of endogenous insulin secretion may be a helpful guide to insulin therapy.
Some people with type 2 diabetes will also need to take insulin to control their blood sugar levels. However, they can manage their blood glucose with lifestyle changes and/or medications such as metformin. There is a risk of side effects from insulin, specifically hypoglycemia or low blood sugar levels.
Hypoglycemia (or a ‘hypo’) can occur in people with diabetes if their balance of medication (usually insulin) is wrong for the food they are eating. Hypos can also happen after doing exercise. However, it is important to note that not everyone with diabetes will experience hypoglycemia (5).
Concerning insulin secretion, the lower the blood glucose levels, the less insulin that is secreted. Conversely, when you have high blood glucose levels, more insulin is generally secreted. As already mentioned, this may not be the case in people who have type 2 diabetes, particularly if it is yet to be diagnosed or is poorly managed.
According to core diabetes research, including that done by Diabetes UK, the development of type 2 diabetes happens most frequently, although not always, in people who are recognized as being clinically overweight or obese.
How to manage insulin secretion
As already discussed, the carbohydrate we eat is largely responsible for the amount of secreted insulin we produce. That is not to say we should not eat carbohydrate, as foods rich in carbohydrate can be very nutritious, for example providing:
- Energy
- Fiber
- B vitamins
- Calcium
- Satiety
- Enjoyment
Yet, eating the right kind of carbohydrates can help to keep your blood glucose levels steady, thus regulating insulin secretion. Wholegrain carbohydrates such as brown or wholegrain bread, rice, and pasta are typically more nutritious than their white counterparts and provide a steadier release of glucose into the bloodstream.
Wholegrain carbohydrates can also help with weight management, with key studies demonstrating their consumption can contribute to a more calorie-controlled diet and is linked to a lower risk of disease, including diabetes and heart disease. Along with the other macronutrients, protein, and fat, carbohydrates can form a key part of a healthy diet.
Of course, for those with diabetes, taking your medication as advised and aiming to lead an active life (where possible) is also essential. Remember to always speak with your specialist diabetes team if you have any concerns about your treatment or your GP if you are concerned about diabetes risk. It is not advisable to make changes to your treatment without seeking professional advice first.
Conclusion
Glucose-induced insulin secretion occurs following a sequence of complex processes and reactions. For people with type 1 diabetes, insulin injections are necessary to control blood glucose levels, whereas, for those with type 2 diabetes, they may be able to do this using lifestyle changes and/or diabetes medications. However, the need for insulin in this group is not uncommon. Healthy lifestyle changes are recommended for most people with diabetes, although it can be particularly important for those with type 2 diabetes or insulin resistance.
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