Insulin – Carbohydrate Metabolism

Insulin is necessary for your body to regulate the amount of glucose in your blood stream. Together with other hormones, it plays an important role in insuring your body has access to energy sources as needed. In regulating glucose, insulin also works with the liver and muscle cells to help metabolize carbohydrates.

In the Beginning

Immediately upon the appearance of glucose in the blood stream, insulin is secreted. The islets of Langerhans scattered across the pancreas work to provide insulin for the blood stream among other tasks. Once insulin enters the bloodstream and meets glucose, it quickly directs it to the cells that require it for various functions.[1] Those cells that demand it most and will use and/or store the glucose are the following:

  • The muscles – small glucose supply quickly used
  • Liver – glucose storage and production
  • Fat cells – larger glucose supply for longer term energy supply

The mechanisms involved and the means of metabolism for glucose storage and use are similar in these various types of cells. Variations are due to particular cell demands.

Insulin, Glucose and the Muscles

It is not easy for many substances to enter into the muscle cells. This is particularly true of glucose molecules. In fact, glucose requires insulin in order to break through the membrane (cell walls) of the muscle cells. If the body is at rest – sleeping, sitting or not in motion, there is simply not sufficient available insulin to gain entry inside the muscle cell. When this is the case, the muscles rely on readily available fatty acids to fulfill their more minimal requirements.[2] Insulin production is increased and available for use when one of two things happens:

  • You consume a meal
  • You exercise

When the resting stage involves eating, glucose intake increases the amount of insulin production. In fact, large quantities of both glucose and insulin become available for usage after a meal. As a result, insulin guides the glucose through the portals of the membrane into the muscle cells. The muscles do not need to turn to fatty acids to fulfill their demands.

A similar thing happens when exercise occurs. This is truly a potent stimulator or trigger that opens the membrane barrier to glucose absorption. In fact, glucose can and does enter the cells during exercising without the help of insulin.

Insulin, Glucose and the Liver Cells

The meal is always the source of glucose.  As food is digested, the carbohydrates are broken down into glucose. The glucose enters the blood stream from the digestive system and is swiftly carried directly to the liver. Within the liver, some of the glucose is released into the bloodstream for general distribution while excess glucose is converted into storage glycogen. This storage amount may account for as much as 6% of the weight of the liver.[3]

The storage form of glucose, called glycogen, can be converted back into its more portable form of glucose as required through an enzymatic process.

When you eat, the liver absorbs and then stores up to 60% of the glucose ingested during a meal. The glucose is used for energy in between meals. As a result, the circulation level of glucose decreases and the pancreas cuts back on the flow of insulin. Generally speaking, the process for converting glucose into glycogen is reversed. With no insulin and assisted by glucagon, the intracellular (stored) glucose undergoes conversion into a form capable of exiting the storage cells and entering into circulation.[4]

In looking at the relationship between the liver, glucose and insulin, there is one other alternative pathway for glucose. This pathway is taken as a result of more glucose entering the liver than the liver can store. In other words, there is an excess of glucose in terms of the liver’s capability to convert and/or store it as glycogen. What happens to this excess glucose?

Insulin comes to the rescue. It helps change the extra glucose into another form – fatty acids or triglycerides. Triglycerides are a neutral fat comprised of glycerol and three fatty acid molecules. In turn, the triglycerides become converted to what are called very low density lipoproteins (VLDL). The VLDL are taken from the blood stream. They are then placed in storage in the adipose (fat) tissue. Insulin also play an additional role in the process. It acts directly on the membrane of the fat cell to help the glucose to enter. Here it will combine with the fatty acids to fashion glycerol.[5]

The “Exceptional” Brain

The brain is the exception to the rule, and medical researchers don’t know why at this point. While the other body parts clamor for insulin to help them absorb and process glucose, the brain does not. It does not require any glucose transport mechanism. Yet, the brain, like other cells, does rely on the glucose to function. Moreover, unlike the muscle cells, it cannot use a substitute energy source.

While being able to absorb glucose easily is an advantage, it also places the brain at a disadvantage. When the availability of circulating glucose for the body drops, the supply to the brain drops also, and there is no alternative source. If this occurs, the brain is not able to function normally and a person can have confusion or even decreased consciousness and coma.

Conclusion

Insulin functions within the body in a variety of ways. It acts as a mechanism through which the muscle, liver and fat cells can absorb and then utilize or store glucose. It helps in the conversion of glucose to glycogen. The complex interaction of the organs, bloodstream and hormones is still being researched, but one thing is known for sure. Blood glucose levels must be regularly monitored and maintained at healthy levels to enjoy general good health.

References

[1] Warshaw, HS; and Pape, J (2009). Real-Life Guide To Diabetes. Alexandria, VA: ADA.

[2] Saltiel, AR; and Pessin, JE (eds) (2007). Mechanisms of Insulin Action. New York: Springer.

[3] Marcovitch, H (2006). Black’s Medical Dictionary 41st edition. Lantham, Maryland: Scarecrow Press.

[4] Guyton, AC; and Hall, JE (2011). Textbook of Medical Physiology, 12th ed. Philadelphia: Saunders.

[5] Meltzer, SJ; and Belton, AB (2009). Diabetes in Adults. Toronto: Key Porter Books

This article was originally published July 12, 2012 and last revision and update of it was 9/10/2015.