How Insulin Works in the Body

Computer artwork of molecules of insulin circulating in the blood stream
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Insulin has a hand in several processes in your body: Not only does it assist with metabolizing carbohydrates and storing glucose for energy in cells, but it also helps utilize the fat, protein, and certain minerals you eat. Because this hormone is so important in helping your body use the foods you ingest, a problem with insulin can have widespread effects on all of your body's systems, tissues, and organs -- either directly or indirectly.

If you have type 2 diabetes, learning how insulin works can help you understand why so many other medical conditions are associated with diabetes, why certain lifestyle practices are beneficial, and how your body reacts to food.

Where Insulin is Produced in the Body

Insulin is a hormone made up of a small polypeptide protein that is secreted by the pancreas, which acts as both an endocrine and exocrine gland. Endocrine glands are the system of glands that secrete hormones to regulate body functions. Exocrine glands aid in digestion.

The pancreas sits behind the stomach, nestled in the curve of the duodenum (the first part of the small intestine), and contains clusters of cells called islets of Langerhans. Islets are made up of beta cells, which produce and release insulin into the bloodstream.

Insulin is Part of a Balancing Act

Insulin affects carbohydrate, protein, and fat metabolism. Your body breaks these nutrients down into sugar molecules, amino acid molecules, and lipid molecules, respectively.

The body can also store and reassemble these molecules into more complex forms. Insulin causes the storage of these nutrients, while another pancreatic hormone called glucagon releases them from storage.

Insulin is involved in your body's careful balancing act to keep your blood sugar levels within a normal range.

In simple terms:

  • If your blood sugar is high: The pancreas releases insulin to help cells absorb glucose from the bloodstream to lower blood sugar levels.
  • If your blood sugar is low: The pancreas releases glucagon to help the liver release stored glucose into the bloodstream to raise blood sugar levels.

Blood sugar levels rise when most foods are consumed, but they rise more rapidly and drastically with carbohydrates. The digestive system releases glucose from foods and the glucose molecules are absorbed into the bloodstream. The rising glucose levels signal the pancreas to secrete insulin to clear out glucose from the bloodstream. Insulin binds with insulin receptors on cell surfaces and acts as a key to open up the cells to receive glucose. Insulin receptors are on almost all tissues, including muscle cells and fat cells.

Insulin receptors have two main components -- the exterior and interior portions. The exterior portion extends outside the cell and binds with insulin. When this happens, the interior part of the receptor sends out a signal inside the cell for glucose transporters to mobilize to the surface and receive glucose.

As blood sugar and insulin levels decrease, the receptors empty and the glucose transporters go back into the cell.

Insulin and Type 2 Diabetes

In a perfect situation, glucose from carbohydrates gets cleared rapidly. However, when there is insulin resistance, this does not happen, and sustained high glucose levels become a problem. Insulin resistance can be due to a problem with the shape of the insulin (preventing receptor binding), not having enough insulin receptors, signaling problems, or glucose transporters not working properly. Whatever the specific cause, the function of insulin is impaired.

Insulin resistance develops before type 2 diabetes is diagnosed. To make up for less effective insulin, the pancreas works overtime to increase insulin output. Eventually, some of the insulin works and blood sugar levels remain normal for a while. As insulin resistance worsens and the pancreas cannot keep up with the demand, glucose levels begin to rise and diabetes is diagnosed when levels get too high.

How Insulin Affects Fat Metabolism

Carbohydrate and fat metabolism are closely connected and both influenced by insulin. If insulin is not working properly, problems can occur. For example, high levels of insulin can send the wrong signals to the brain. These signals tell the brain that there is excess insulin and that your cells are starving for glucose. So in response, your brain creates cravings for carbohydrates, signals your body to store fat, and orders carbs to be burned for energy rather than body fat. This is why weight loss can be difficult when you have type 2 diabetes.

Insulin also plays a key role in the development of high triglyceride levels:

  • In the Liver: Insulin stimulates the creation and storage of glycogen from glucose. High insulin levels cause the liver to get saturated with glycogen. When this happens, the liver resists further storage. Glucose is used instead to create fatty acids that are converted into lipoproteins and released into the bloodstream. These break down into free fatty acids and are used in other tissues. Some tissues use these to create triglycerides.
  • In Fat Cells: Insulin stops the breakdown of fat and prevents the breakdown of triglycerides into fatty acids. When glucose enters these cells, it can be used to create a compound called glycerol. Glycerol can be used along with the excess free fatty acids from the liver to make triglycerides. This can cause triglycerides to build up in the fat cells.

How Insulin Affects Protein and Minerals

Insulin helps amino acids from protein to enter cells. When this process is hindered, it can make it difficult to build muscle mass.

Insulin also makes cells more receptive to potassium, magnesium, and phosphate. These substances are also known as electrolytes, which help conduct electricity within the body. They influence muscle function, blood pH, and the amount of water in your body.

An electrolyte imbalance can be worsened by high blood sugar levels as this can cause excessive urination with water and electrolyte loss.

Normal and Impaired Insulin Function

To break things down in an easy-to-remember format, this list illustrates what happens with normal and impaired insulin function.

Normal function: Insulin transports glucose into cells to clear glucose from the bloodstream.
Impaired function: Glucose is not cleared from the bloodstream.

Normal function: Insulin helps convert glucose to glycogen (the storage form of glucose) in liver and muscle cells.
Impaired function: Glucose is not easily stored as glycogen. Cells "starve," which can cause the body to behave as if there are starvation conditions and even cause cravings for sweet carbohydrates, although there are high levels of glucose in the body.

Normal function: Insulin slows down the creation of glucose from non-carbohydrate sources, like protein in the liver.

An example of this is when the liver releases glucose into the bloodstream when blood sugar levels are too low.
Impaired function: The liver releases glucose into the bloodstream inappropriately even when blood sugar levels are high.

Normal function: Insulin stops the breakdown of fat and stimulates the creation of fat under the right circumstances.


Impaired function: High insulin levels can signal for less fat loss and more fat storage.

Normal function: Insulin regulates the formation of fat from simple sugars, which can eventually become triglycerides.
Impaired function Triglycerides levels increase inappropriately.

Normal function: Insulin stimulates protein synthesis (creation), which affects muscle growth.
Impaired function: You may have difficulty building muscle.

How to Help Insulin Work Better

These strategies may help you increase insulin sensitivity and reduce insulin resistance:

Sources:

Diabetes: What is Insulin? Endocrineweb. Accessed: February 5, 2012. http://www.endocrineweb.com/conditions/diabetes/diabetes-what-insulin

Endocrine Pancreas. University of Berkley, California. Accessed: Feburary 10, 2012. http://mcb.berkeley.edu/courses/mcb135e/pancreas.html

Pathophysiology of the Endocrine System, Physiologic Effects of Insulin. Colorado State University. Accessed: February 9, 2012. http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/pancreas/insulin_phys.html

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