Doubling previous figures, the government estimates that 41 million Americans have pre-diabetes - blood sugar high enough to dramatically increase their risk of getting the full-blown disease.
The figures released Wednesday are significantly higher than previous estimates because doctors have changed the criteria for diagnosing the condition after research showed they were missing too many at-risk patients.
"These latest numbers show how urgent the problem really is," said Health and Human Services Secretary Tommy Thompson, who was announcing the new estimates at a federal health meeting Thursday in Baltimore.
"We need to help Americans take steps to prevent diabetes or we will risk being overwhelmed by the health and economic consequences of an ever-growing diabetes epidemic."
The good news is that modest diet and exercise can delay, if not prevent, the onset of diabetes in many pre-diabetics.
But "most of these people have no idea" they're at risk, said Dr. Francine Kaufman, past president of the American Diabetes Association.
Some 18 million Americans have full-blown diabetes, a leading cause of blindness, kidney failure, amputations and heart disease that claims 180,000 U.S. lives a year.
Some people are born with it, but the vast majority have Type 2 diabetes, an illness that develops, often in middle age, when their bodies lose the ability to turn blood sugar into energy. Obesity, an increasing problem in the United States, is associated with diabetes.
The loss in ability to turn blood sugar into energy is very gradual, and it can be measured by blood tests. Glucose levels that are above normal but not yet in the diabetic range signal pre-diabetes - and a change in what one test considers normal prompted the government's new increased estimates.
Doctors once thought blood sugar levels below 110 milligrams per deciliter as measured by the "impaired fasting glucose" test - given before eating anything in the morning - were normal. But the American Diabetes Association in November changed the definition of normal to below 100 milligrams - meaning anyone with a fasting glucose between 100 and 125 milligrams is now classified pre-diabetic.
That seems like a small change. But a lot of people are in that 100 to 110 range, data that conclude about 40 percent of people ages 40 to 74 are pre-diabetic, explained Dr. Frank Vinicor, diabetes chief for the Centers for Disease Control and Prevention.
Changing the pre-diabetes cut-off "isn't an arbitrary decision," Vinicor said. "It's based on emerging science from the last two to three years," that found the risk of glucose-spurred heart disease began rising at lower levels than once thought.
Cut-offs for a second test - where blood sugar levels are measured two hours after a glucose-rich drink - remain unchanged. Levels between 140 and 199 milligrams are considered pre-diabetic in that test.
So who needs to seek one of these tests? The ADA says:
Doctors typically repeat the test every three years if results are normal, but may test people with multiple risk factors more often.
If the test diagnoses pre-diabetes, there are proven ways to lower the risk of full-blown illness, Vinicor stressed, such as walking 30 minutes a day, five days a week, and losing 5 percent to 7 percent of body weight.
By Lauran Neergaard