Insulin is a naturally occurring hormone, made and secreted by the pancreas. The pancreas is an integral part of the digestive system and has many important functions all relating to the digestion of food. The main function of insulin relates to glucose metabolism. Glucose is the end product of carbohydrate digestion in the gut.
When Glucose is absorbed through the gut wall into the bloodstream, its baseline level becomes elevated. The response of the pancreas to this elevated glucose is to produce and secrete more insulin.
Insulin’s primary role is to push the increased amounts of glucose into the tissues that need them most in particular the vital organs and muscles. They act upon individual cells in these organs effectively through a receptor that permits entry of glucose through the cell membrane where it can be fully utilised for energy by the cell.
The other major function of insulin is to push glucose into the liver where it promotes the production of glycogen which is the storage facility for glucose, via a process call glycogenesis.
The human body is very efficient when it comes to not wasting any available energy that cannot be used immediately. This is a homeostatic mechanism that has evolved over millions of years – in primitive man, it is entirely possible that the individual might go without food for several days and it was therefore important to have a storage facility.
Glycogen, as previously discussed, is stored in the liver as the body’s primary reservoir for energy. However, this is quite limited and the body’s secondary reservoir which has far greater capacitance is adipose tissue (body fat). Insulin has a very important role in pushing any excess glucose into adipose tissue by first converting it into fatty acids through a process called lipogenesis.
The magnitude of the rise in blood insulin levels following a meal is very dependent upon the predominant macronutrient. By far the greatest rise is seen with carbohydrate which is converted to glucose by default in the gut.
A significant but attenuated rise in insulin is seen with protein and this happens via a secondary route – any protein that is ingested beyond that which is required for normal body functions, such as maintaining muscle mass and producing important hormones, is converted to glucose via a process called gluconeogenesis. This itself then produces a rise in blood insulin.
By contrast fat produces an almost negligible rise in blood insulin. It is essential to appreciate these differences when it comes to understanding what causes obesity and importantly how to treat it.
Type 1 diabetes is an autoimmune disease where by the cells in the pancreas that are responsible for manufacturing insulin are destroyed by the body’s own immune system for reasons that are poorly understood. The consequence of this however is that insufficient insulin is produced to properly regulate blood sugar. As a result, a significant proportion of the glucose that is absorbed into the body cannot be properly utilised, resulting in the blood glucose becoming excessively elevated and much of it gets excreted via the urine and therefore wasted.
Patients with type 1 diabetes tend to be very lean as they have little capability for storage. They also suffer from the chronic long term effects of an elevated blood sugar (hyperglycaemia), namely cardiovascular disease, renal failure and eye disease leading to blindness. The treatment of type 1 diabetes is to control blood glucose levels by injecting insulin.
Type 2 diabetes shares the same problem as type 1 diabetes namely uncontrolled blood sugar levels. However, by contrast the levels of insulin in the blood are greater than normal. So, in fact the body is producing too much insulin.
It does this in response to insulin resistance which develops initially as a result of the significantly elevated levels of insulin that is secondary to the almost constant ingestion of carbohydrate from the diet. This then sets up a vicious cycle in that the body’s response to developing insulin resistance is to produce even more insulin. And so, the cycle continues.
So, whilst in type 2 diabetes there is too much insulin it is in reality ineffective due to the cells of the body becoming resistant to it. This allows blood sugar levels to rise beyond the normal range and the individual then suffers exactly the same consequences as patients who have uncontrolled type 1 diabetes namely heart disease, kidney failure and eye disease.
The conventional treatment of type 2 diabetes is perversely to help the body to produce more insulin in the form of medication initially taken orally and when this is insufficient, to administer once again insulin by injection. And perversely this has exactly the opposite effect to what is desirable when it comes to reducing insulin resistance - or increasing insulin sensitivity - and in effect simply perpetuates the vicious cycle.
More information about the importance of the effects of insulin in the management of obesity can be found in the presentations relating to this topic. But overall it is vital to appreciate that the only way to break the vicious cycle of insulin further promoting insulin resistance is to invoke the body’s own biological mechanism. That is to reduce insulin secretion by the pancreas and the only way that this can be achieved is by dramatically reducing carbohydrate intake, either overall, or by fasting intermittently or ideally a combination of both.