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  What Is Type 2 Diabetes?

By Amy Adams, MS

Reviewed By Jeremy Walston, MD

Type 2 diabetes is the most common form of diabetes, accounting for roughly 90 percent of all cases. In the United States, 15 million people have Type 2 diabetes, of which five million cases are undiagnosed because of the slow onset and mild symptoms during early stages of the disease.


What Is Type 2 Diabetes?

Type 2 diabetes generally occurs after the age of 45. People with Type 2 diabetes often produce insulin, but their muscle and fat cells are unable to use it properly (referred to as insulin resistance). To make matters worse, their pancreas often cannot increase insulin production to make up for the insulin resistance.



Who Gets Type 2 Diabetes?

Type 2 diabetes tends to occur in people who are obese and do not exercise regularly. Type 2 diabetes is also much more common in some racial groups. These include people of African, Native American, Hispanic, and Pacific Island descent. In fact, although the overall prevalence of Type 2 diabetes in the United States is six percent, one Native American tribe in Arizona (the Pima Indians) has a prevalence of nearly 50 percent. (For more information about increasing rates of diabetes, see Related News below.)



Family Medical History

One strong risk factor for developing Type 2 diabetes is a family history of Type 1 or Type 2 diabetes, or a personal history of gestational diabetes (diabetes during pregnancy). Although the risk for the general population is about five percent, people who have a parent or sibling with the disease have an increased risk of about 10 to 15 percent. If that sibling is an identical twin, then the risk is almost 100 percent. However, this risk may not be entirely genetic. Because Type 2 diabetes is also triggered by a lifestyle that lead to obesity, twins may share a strong lifestyle similarity that puts them at similar risk.

Despite the fact that Type 2 diabetes clearly runs in families, researchers have not found one mutation that specifically causes the disease. Instead, there are many different genes that appear to be involved, each of which contributes to a person's risk. Some of these genes have been identified, but researchers are actively looking for new genes that can be used to predict who will get diabetes.

One Disease, So Many Genes

One reason that so many genes are involved in Type 2 diabetes is the shear number of genes that are involved in the body's mechanisms for controlling fuel intake and regulation, such as glucose metabolism, insulin secretion, insulin resistance, and energy usage. Slight variations in each of these genes changes how the body metabolizes glucose. A mutation in any one or two genes won't cause diabetes. However, if a person inherits many risk factors their chance of developing diabetes increases.

In some, very rare, cases of Type 2 diabetes, people develop diabetes in their 20s rather than in their 40s or 50s. This condition is called maturity onset diabetes of the young (MODY). MODY is inherited in an autosomal dominant fashion. This means that a person only has to inherit one mutated copy of the gene in order to develop diabetes. Because of how we inherit genes, a person whose parent has MODY has a 50 percent chance of developing MODY themselves.

Another rare cause of Type 2 diabetes is maternally inherited diabetes and deafness (MIDD). In MIDD, the risk for developing diabetes is inherited only from the mother.


Environment Factors That Affect Risk

Although there is a strong genetic component to Type 2 diabetes, not all people who are genetically at risk develop the disease. This means that a person's environment plays an important role in whether they develop diabetes. The strongest environmental risks for developing Type 2 diabetes are obesity and inactivity. In fact, many people can eliminate or decrease diabetes symptoms simply by losing weight and becoming more active. Because of these risk factors, people living Western lifestyles — with a high-fat diet and little exercise — have a higher risk of developing diabetes. (For recent news about how obesity affects Type 2 diabetes risk, see Related News below.)

Diabetes in the Pima Indians

One example of the influence of environment on diabetes is the Pima Indians. The Pima tribe lives in the Gila River valley near Phoenix. For 2,000 years the Pima Indians irrigated land for agriculture. But when their water supply was diverted, the Pima Indians turned to a more Western lifestyle. Not only did obesity increase in the tribe, but the diabetes incidence went from nearly 0 percent to about 50 percent. The genetics of the tribe did not change, but the environment did.

Scientists think that many populations inherited genes that helped them survive in times of famine. People with these so-called thrifty genes stored food more efficiently and were more likely to survive starvation and pass their thrifty genes on to offspring. Although these genes helped native populations handle famine when living a traditional lifestyle, those same genes put them at risk for diabetes, obesity, and obesity-related diseases when living a Western lifestyle.




Type 2 diabetes develops slowly over time and may go undetected for many years. Because of the slow onset and minimal symptoms early in the disease, as many as 33 percent of the people who have the disease are not yet diagnosed. Untreated diabetes can lead to serious health risks, because high blood sugar levels can cause damage long before a person knows that they have diabetes. As a result, the American Diabetes Association recommends that doctors screen at-risk individuals over the age of 45 every three years using either a fasting glucose test or glucose tolerance test.


Related News
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Increasing Rates of Type 2 Diabetes
Diabetes rises sharply among Native Americans
CDC to study type 2 diabetes in children
New diabetes guidelines target prevention
Experts fear type 2 diabetes epidemic in US children
Environmental Factors
Hormone may be key link between obesity and diabetes
Diabetics benefit more from 5 to 10 pound weight loss
Diet and exercise prevent diabetes
Obese kids seen at risk for pre-diabetic syndrome


Li, H., Isomaa, B., et al. (2000). Consequences of a family history of type 1 and type 2 diabetes on the phenotype of patients with type 2 diabetes. Diabetes Care, 23, 589-594.

Barnett, A. H., et al. (1981). Diabetes in identical twins. A study of 200 pairs. Diabetologia, 20, 87-93.

Harris, M., (1999). Classification, Diagnostic Criteria, and Screening for Diabetes. In Diabetes in America. Bethesda, MD: National Institute of Diabetes and Digestion and Kidney Diseases, National Institute of Health.


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