The number of new diabetes cases among US adults keeps falling, even as obesity rates climb, and health officials aren't sure why.

New federal data released Tuesday found the number of new diabetes diagnoses fell to about 1.3 million in 2017, down from 1.7 million in 2009. Earlier research had spotted a decline, and the new report shows it's been going on for close to a decade. But health officials are not celebrating.

"The bottom line is we don't know for sure what's driving these trends," said the lead author of the new report, Dr. Stephen Benoit of the Centers for Disease Control and Prevention. Among the possibilities: Changes in testing and getting people to improve their health before becoming diabetic.

The report was published by the journal BMJ Open Diabetes Research & Care. The statistics run through 2017; last year's numbers are not yet available, Benoit said.

Diabetes is a disease in which sugar builds up in the blood. The most common form is tied to obesity, and the number of diabetics ballooned as US obesity rates increased. But other factors also might have pushed up annual diabetes diagnoses from 2000 to 2010, and they may partly explain why the numbers have been going down since, some experts said.

First, the diagnostic threshold was lowered in the late 1990s. That caused more people to be counted as diabetics, but the impact of that may have played out. "We might have mined out a lot of the previously unrecognized cases" and so new diagnoses in the last several years are more likely to be actual new illnesses, said Dr. John Buse, a University of North Carolina diabetes expert.

Meanwhile, doctors have increasingly used a newer blood test to diagnose diabetes. It's much easier than tests that required patients to fast for 12 hours or to undergo repeated blood draws over 2 hours.

The American Diabetes Association recommended the new test, known as the hemoglobin A1C blood test, for routine screening in 2010. Because it's easier to do, it would be expected to lead to more diagnoses. But some experts say it may miss a large proportion of early cases in which people aren't showing symptoms. "You may be missing people that would have been diagnosed" with older tests, Benoit said.

Another possibility: Increasingly, more doctors have been diagnosing "prediabetes," a health condition in which blood sugar levels are high but not high enough to hit the diabetes threshold. Physicians typically push such patients into exercise programs and urge them to change their diet.

"Prediabetes is becoming a more accepted diagnosis" and may be causing an increasing number of patients to improve their health before becoming diabetic, said Dr. Tannaz Moin, a UCLA expert.

The new report is based on a large national survey conducted by the government every year. Participants were asked if they had been diagnosed with diabetes, and also if the diagnosis was made in the previous year.

It found the rate of new diabetes cases fell to 6 per 1,000 US adults in 2017, from 9.2 per 1,000 in 2009. That's a 35% drop, and marks the longest decline since the government started tracking the statistic nearly 40 years ago, according to the CDC. The decrease was mainly seen among white adults, the researchers said.

Meanwhile, the overall estimate of how many Americans have diabetes—whether the diagnosis is recent or not—has been holding steady at 80 per 1,000 US adults. That translates to about 21 million Americans. Diabetes has continued to become more common among young adults and people who are black and Hispanic, the report noted.