Almost half of Americans have diabetes or are well on their way to getting it, yet of those with the disease, around a third are going around undiagnosed, a new study suggests.

Twenty years ago, around one in 10 adults had diabetes; the number is now closer to one in seven or eight, researchers say.

A further 38 percent of the U.S. population has prediabetes, in which blood sugar levels are abnormally high, putting them on the cusp of a diagnosis of diabetes, the study authors say.

"We need to better educate people on the risk factors for diabetes — including older age, family history and obesity — and improve screening for those at high risk," says lead study author Andy Menke, an epidemiologist in Silver Spring, Maryland.

Of those whose diabetes was undiagnosed, Asian Americans were predominant, the researchers found; around 51 percent of them were unaware they had the condition, the study found.

This is compared with around a third of U.S. whites who did not know they had the condition, and to 37 percent of blacks and 49 percent of Hispanics.

"Ethnic minority groups are at high risk for complications of diabetes, so the racial disparities and burden of diabetes and prediabetes are particularly concerning," says Elizabeth Selvin of the Johns Hopkins Bloomberg School of Public Health.

The study was based on an analysis of data from more than 26,000 U.S. adults participating in surveys from 1988 to 2012, who were asked if they had ever been diagnosed with diabetes.

They also gave blood samples for the researchers to analyze.

During the 24 years of the study, the incidence of diabetes grew from 10 percent to more than 12 percent, paralleling increases in the rate of obesity, considered the greatest risk factor for the disease, the researchers reported.

The data analyzed for the study was from the U.S. government's long-standing National Health and Nutrition Examination Survey.

There is some good news amid the numbers, experts say.

New cases of diabetes are starting to drop for the first time in decades as people are made aware of the lifestyle choices that can make someone susceptible to diabetes, says Dr. David Nathan at Massachusetts General Hospital in Boston.

"Behavioral changes, including healthy eating and more activity can prevent, or at least ameliorate, the diabetes epidemic," says Nathan, director of the hospital's diabetes center.

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