NEW YORK — The number of new diabetes cases among U.S. adults keeps falling, even as obesity rates climb, and health officials aren’t sure why.

Federal data released Tuesday found that the number of new diabetes diagnoses fell to about 1.3 million in 2017, down from 1.7 million in 2009.

The report shows that the decline has been going on for close to a decade. But 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.

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 U.S. obesity rates increased.

But other factors also might have pushed up annual diabetes diagnoses from 2000 to 2010.

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.

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 two hours.

But some experts say it may miss a large proportion of early cases in which people aren’t showing symptoms.

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 to exercise and change their diet.

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

Researchers said the decrease in the report was mainly seen in white adults.

Meanwhile, the overall estimate of how many Americans have diabetes has been holding steady at 80 per 1,000 U.S. adults, or about 21 million Americans.

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