As many as a third of American adults with type 2 diabetes don’t even know they have the disease, according to alarming new findings from the National Institutes of Health and the CDC.

Even more disheartening, one in three adults in the U.S. either has diabetes or a prediabetes condition known as impaired glucose tolerance.

That means that 73 million American have the disease or are on their way to getting it, says Catherine Cowie, PhD, of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

“We knew that there had been an increase in diagnosed cases [of type 2 diabetes],” Cowie tells WebMD. “The hope was that this rise would be counterbalanced by a decline in undiagnosed cases. But that is not what we are seeing.”

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Blacks, Hispanics Most at Risk

Cowie and colleagues analyzed data from a national survey collected between 1999 and 2002, and compared them to data collected between 1988 and 1994. Participants were asked if they had diabetes, and they were given fasting blood tests to confirm the diagnosis, identify new cases, and identify people with the prediabetes condition.

Among the survey’s main findings:

--The prevalence of diagnosed diabetes among adults in the U.S. rose from 5.1 percent in the early survey to 6.5 percent in the most recent one.

--The percentage of adults with undiagnosed diabetes remained fairly steady.

--A total of 2.7 percent of adults in the U.S. have the disease without knowing it.

--Almost 1 in 4 U.S. adults aged 65 or over -- 22 percent of elderly Americans -- have diabetes. Diabetes was twice as common in blacks and Mexican-Americans as it was in whites.

Type 2 diabetes accounts for about 95 percent of all diabetes cases, and virtually all undiagnosed cases of the disease. Obesity is a major risk factor for type 2 diabetes; age, family history, and sedentary lifestyle also contribute to risk.

Diabetes is the most common cause of blindness, kidney failure, and amputations among adults in the U.S. It is also a major risk factor for heart disease and stroke.

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Prediabetes Not Benign

The analyzed data were taken from the National Health and Nutrition Examination Survey (NHANES), conducted by the CDC’s National Center for Health Statistics. NHANES is the only national health survey to examine both diagnosed diabetes and undiagnosed disease, confirmed by physical exams that include blood glucose testing.

Over the years studied, roughly 26 percent of adults in the U.S. had impaired fasting glucose, meaning that blood sugar levels were higher than normal after an overnight fast, but not high enough to be considered diabetes. This condition is also known as impaired glucose tolerance and prediabetes.

The prediabetes condition has no symptoms, but Cowie points out that it is far from benign.

“These people have a very high risk of developing diabetes within a decade, and even if they don’t they are still at high risk for having a heart attack or stroke,” she says.

Positive lifestyle changes can often prevent or delay the onset of diabetes in people with prediabetes. Study after study has shown that losing modest amounts of weight and getting even a moderate amount of exercise on a daily basis can make a big difference.

“Even doing something simple like walking 30 minutes a day, five days a week can lower risk,” she says.

Know Your Risk Factors for Type 2 Diabetes

But people who don’t know they are at risk may be less inclined to make diabetes-preventing lifestyle changes.

The list of potential risk factors is long, and people with any of the risks should discuss diabetes testing with their health care provider, the report concludes. The risk factors include:

--Having a family history of diabetes.

--Being overweight.

--Having an inactive lifestyle, meaning that they exercise less than three times a week.

--Being a member of a high-risk ethnic population (African-American, Hispanic/Latino American, American Indian, Alaska Native or Pacific Islander, and some Asian- Americans).

--Having high blood pressure.

--Having low HDL (good) cholesterol or high triglyceride level.

--Having a history of diseases of the blood vessels to the heart, brain, or legs.

--Having had pregnancy-related diabetes.

--Having polycystic ovary syndrome.

--Being age 45 and older.

“The important message to get out to the community is that these are the people who are at risk for developing type 2 diabetes,” says Charles M. Clark Jr., MD.

Clark is a professor of medicine at Indiana University School of Medicine and is chairman emeritus of the NIDDK’s National Diabetes Education Program.

He tells WebMD limited public health funds make both finding undiagnosed diabetes cases and taking care of people who know they have the disease a challenge.

“We only have so much to invest,” he says. “We may want to spend more on screening efforts, but the reality is that we may need to spend more taking care of the people we already know about.”

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By Salynn Boyles, reviewed by Louise Chang, MD

SOURCES: Cowie, C.C. Diabetes Care, June 2006; vol 29: pp 1263-1268. Catherine C. Cowie, PhD, director, Diabetes Epidemiology Program, NIDDK. Charles M. Clark, MD, professor of medicine, Indiana University School of Medicine; chairman emeritus, NIDDK National Diabetes Education Program.