Diabetes is a disease of the metabolism that results when the body either cannot to make insulin or cannot respond to insulin.  The characteristic feature of diabetes is high blood sugar, but there are many other very important metabolic problems that are less well know to people living with diabetes.  In order to have a better understanding of diabetes, we must first have a better understanding of what metabolism is and how insulin affects metabolism.   

What is metabolism?

Metabolism refers to the sum total all of the chemical reactions that take place in each cell of the body. Just like a machine, our cells need an uninterrupted source of chemical energy in order to live and function. Our bodies extract chemical energy from the food we eat, and use it as a metabolic fuel to power the cells.
There are 3 primary metabolic fuels: Carbohydrates, fats and proteins. As you go about your day, you go from the fed state (where metabolic fuel from food is in abundance) to the fasting state (where your body has to rely on stored fuel). Flipping between the fed and fasting state is extremely important to allowing cells to function the way they are supposed to, without interruption.  

Imagine, for example,  that you have not eaten in over a day. Every cell in your body has to adapt to this lack of food, and keep the energy flowing to each cell. Some of your cells (which specialize in storing energy) need to be told to mobilize and release energy, while other cells need to be instructed to use less energy. Hormones are what allow coordination like this to happen. Hormones are substances made by endocrine glands, which travel through the blood stream to reach all of the cells in the body and transmit messages to the cells. Hormones serve many functions, but one of their most important is helping control metabolism. Because of how important metabolism is, there are MANY hormones that help control it. I will not name all of the hormones that control metabolism, but some of the most well known include insulin, glucagon, cortisol, adrenaline, ghrelin, GLP-1, adiponectin, somatostatin, thyroxine, and growth hormone. Of these hormones, insulin is perhaps the most important in controlling metabolism.

What is insulin?

Insulin is an anabolic (growth promoting) hormone. Its function in metabolism is to promote using carbohydrates as a metabolic fuel and to promote storage of any excess consumed carbohydrate (so that it is not wasted). Insulin has several important effects on metabolism, as shown below.

  1. Insulin increases the entry of glucose into cells. This is the most well-known function of insulin, and the one most patients are already aware of. Insulin stimulates cells to form channels  in the cell’s membrane which allow glucose to enter the cell.  
  2. Insulin increases the storage of glucose in the liver, by promoting its conversion into glycogen. 
  3. Insulin increases the conversion of glucose to triglycerides in liver cells. This is another way of storing glucose as an energy source for later use.
  4. Insulin increases the conversion of glucose to triglycerides in adipose cells (adipose cells are also referred to as fat cells).
  5. Insulin increases the uptake of amino acids into cells. Amino acids are the building blocks that form proteins. Proteins are what form most of the structure of the cell, so this is an example of the anabolic effect of insulin.
Insulin stimulates cells to open channels in the cell membrane that allow glucose to enter

What happens to cells without insulin?

If insulin can no longer be made (as is the case in type 1 diabetes) or the cells are resistant to the effects of insulin (as is the case in type 2 diabetes), then metabolism of cells throughout the body will malfunction. The exact nature of the malfunction will depend on the cell type (liver cells respond differently than muscle cells or adipose cells), how severe the lack of insulin is (complete lack of insulin function versus a milder loss of insulin function), and other stressors that are going on at the same time (high blood pressure, obesity, and exposure to tobacco smoke being well-studied examples). But in most cell types, you can characterize the cells’ response as a state of chronic stress. 

There are particularly vulnerable cell types, including nerve cells, cells that make up the walls of blood vessels, and cells that make up the filtering mechanism of the kidney. Damage to these cells over time leads to the complications associated with diabetes