There are two overarching causes of diabetes mellitus simply called type 1 and type 2. 

Insulin is a hormone which allows sugar to move from the blood into tissues to be used as energy. The diagram shows the process by which sugars moved from your mouth to your cells

1. You eat food, it goes to your stomach, its converted into glucose. 

2. This glucose enters the blood stream from the stomach

3. The pancreas produces insulin in response to the high blood sugars. 

4. Insulin is a hormone that allows sugar to be moved out of the blood into tissues where is used as energy. 

5. Glucose levels in the blood should balance out because insulin has done its job

Diabetes is a disease characterized by the build-up of sugar in the blood. In people without diabetes, sugar (or glucose) is able to move out of the blood and into the cells of the body where it can then be used for energy. This is made possible by a hormone called insulin which acts as a "key" allowing glucose into the cells to be used as energy. We can think of diabetes as a condition in which these keys are either missing or the receptors/doors for insulin to allow sugars into other cells to not work properly. In both cases, glucose is “locked out” of the cells and therefore remains in the blood causing high blood sugar (hyperglycemia). The explanation behind this initial high blood sugar is what determines whether a patient is diagnosed with type 1 diabetes or type 2.

Type 1 diabetes mellitus

A patient with type 1 diabetes is insulin-deficient. In other words, this patient is missing "keys". This deficiency is the result of autoimmune events in which white blood cells specifically target and destroy the beta-cells of the pancreas (which produce and release insulin). For this reason, type 1 diabetics will always need to take insulin because they cannot produce their own.

Type 1 diabetes refers to patients whose pancreatic beta cells have been destroyed by and autoimmune process and therefore they can no longer produce insulin. This lack of insulin leads to elevated blood sugars. LADA (Latent Autoimmune Diabetes in Adults) is an adult-onset variant of type 1.  Beta cells are damaged and stop producing both insulin and glucagon. The diagram shows how there is lots of glucose building up in the blood stream because it cannot enter the muscle to to a lack of insulin which would normally allow sugar to flow from the blood into the muscle to be used as energy.  

Type 2 diabetes mellitus

In contrast, patients with type 2 diabetes are still producing their own insulin and  but the root problem is instead "insulin-resistance", meaning that plenty of keys are available but that they are not working properly with the available locks. In fact, the pancreas might actually release more insulin than usual in an attempt to help more glucose leave the blood. Although not fully understood, insulin resistance appears to be the result of genetic risk factors often in combination with a sedentary lifestyle and poor diet. For this reason, diet and exercise in combination with oral medications may be sufficient to regulate blood sugar in some patients. However, the progression of the disease often mandates insulin injections long-term because beta-cells may begin to die out. In other words, type 2 can progress to an insulin-deficiency over time.

Type 2 diabetes is initially caused by a lack of effectiveness of a patient's own insulin which is termed "insulin resistance".  The body initially makes more insulin to compensate but many years the pancreas degrades and eventual insulin deficiency occurs. 

• Beta cells stop working
• The insulin made by the pancreas is not used effectively, thus beta cells need to make much more insulin than is made normally
• GLP-1 stops working normally
• The liver releases too much sugar at the wrong time, in between meals and overnight

There are secondary forms of type 2 diabetes mellitus that include Maturity Onset Diabetes of Youth (MODY) which are identified genetic mutations causing high blood sugars and steroid-induced diabetes mellitus. Gestational diabetes mellitus is a form of type 2 diabetes that is diagnosed during pregnancy. There are other conditions and medications that are associated with or causative of diabetes mellitus type 2: Acromegaly, Cushing's Syndrome, Glucagonoma, Pheochromocytoma, Hyperthyroidism, Somatostatinoma, Aldosteronoma, Thiazides, Diazoxide, Glucocorticoids, Niacin, Prader Willi syndrome, Turner's syndrome, etc..

The image shows the most accepted naming system for the causes of diabetes mellitus. 

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Related pages

 Overview of Diabetes Blood Sugar testing Diabetes education Insulin pumps Insulin resistance and diabetes type 2 Diabetes type 1 Long-term Complications of Diabetes Normal sugar metabolism Short-Term Complications of Diabetes Type 2 diabetes medications    Pre diabetes   Diabetes type 2 Diet   Intensive insulin management