Diabetes Pathophysiology: How High Blood Sugar Causes Symptoms of Diabetes

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What is Pathophysiology?

Pathophysiology describes the course of progression of a disease, in terms of the mechanisms that cause the disease and the consequences of it.

In terms of diabetes mellitus, pathophysiology describes the biological events that cause high blood sugar, and the various consequences of increased blood sugar levels.

Symptoms of Diabetes

Even though the cause of both Type 1 and Type 2 diabetes is high blood sugar, the symptoms of each type are not always the same. In general, both types can cause blurry vision, fatigue, increased thirst and hunger, and increased frequency of urination. However, the patterns of symptoms experienced are slightly different, and symptoms develop at different rates.

For Type 1 diabetes, symptoms include fatigue, increased thirst and urination, nausea and vomiting, and weight loss. These symptoms are usually of sudden, rapid onset.

Symptoms of Type 2 diabetes are of very gradual onset, often over a period of several years. In some people, the progress of the disease is so gradual that it may be asymptomatic. The most common symptoms of Type 2 diabetes are fatigue, blurred vision, increased thirst, increased urination, and increased appetite.

Pathophysiology of Diabetes Mellitus

In Type 1 diabetes, the disease is caused by the destruction of beta cells in the pancreas. These cells are responsible for producing the body’s supply of insulin, the hormone that controls blood sugar levels. People with Type 1 diabetes produce no insulin. This hormone is a chemical signal that stimulates cells to take up sugars from the blood. When no insulin is present, sugars remain concentrated in the blood and cells cannot use the sugar as an energy source.

Type 2 diabetes develops as the result of an abnormal metabolic process called insulin resistance. People with this type of diabetes can make their own supply of insulin, but their bodies do not respond normally to the insulin they make. Cells become insulin resistant, which means they are unable to respond to insulin by taking up sugars from the blood. Insulin resistance does not have a single cause; it is thought that there are several contributing factors, including genetics, poor diet, sedentary lifestyle, and obesity.

With sugar concentrated in the blood as a result of Type 1 or Type 2 diabetes, the body’s fluid balance is disrupted. Fluid is drawn away from the tissues and into the blood to offset the high sugar concentration. Urination increases to aid in excreting excess sugar in the urine. Hunger and fatigue increase because the body’s cells cannot utilize sugar for energy. The body’s stores of fat and protein are used as an alternative energy source, leading to weight loss. In the case of Type 1 diabetes there is a risk of death occurring if the condition is not treated promptly, as a result of kidney failure and dehydration.

If blood sugar levels are poorly managed, both Type 1 and Type 2 diabetes can increase the risk of a number of diseases and complications. These include atherosclerosis, coronary artery disease, kidney disease, nerve damage, vision loss, sexual dysfunction, high cholesterol, hypertension, and genitourinary infections.

Most of these long term complications develop because chronically high levels of blood sugar are pro-inflammatory. Over time, this inflammation becomes chronic and causes damage to blood vessels and nerves, particularly in the peripheral areas of the body such as legs and feet, and in the delicate blood vessels of the eyes. Complications such as cardiovascular disease are related to cholesterol levels and blood pressure as well as blood sugar levels.


Kenneth Patrick L Ligaray, MD and William L Isley, MD for eMedicine: Diabetes Mellitus, Type 2

National Institute of Health Medline Plus: Diabetes

The Mayo Clinic: Diabetes

William H Lamb, MBBS, MD, FRCP (Edin), FRCP for eMedicine: Diabetes Mellitus, Type 1