Nearly half of California adults — including 1 of every 3 young adults — are estimated to have undiagnosed diabetes or prediabetes, a precursor to life-threatening Type 2 diabetes, according to a UCLA study released Thursday.

But most don’t even know they have it.

The researchers said the report provides the first analysis and breakdown of California prediabetes rates by county, age and ethnicity, and offers alarming insights into the future of the nation’s diabetes epidemic.

It analyzed extensive data from the National Health and Nutrition Examination Survey and the California Health Interview Survey.

“This study is a wake-up call that says it’s time to make diabetes prevention a top state priority,” said Harold Goldstein, executive director of the California Center for Public Health Advocacy, the group that commissioned the report.

The latest study found that 46 percent of California adults — 13 million people — have prediabetes or undiagnosed diabetes, while another 9 percent have already been diagnosed with diabetes.

Since diabetes is more commonly seen among older adults, the study’s finding that 33 percent of adults ages 18 to 39 have prediabetes is of particular concern, the authors said.

Prediabetes is a condition in which blood glucose levels are higher than normal — but not high enough for a diabetes diagnosis. Up to 30 percent of those identified with prediabetes will develop Type 2 diabetes within five years, and as many as 70 percent will develop the disease in their lifetime.

Diabetes is associated with dramatically increased risk of amputation, nerve damage, blindness, kidney disease, heart disease, hospitalization and premature death.

A big concern, said Dr. Allison Diamant, a faculty associate at the UCLA Center for Health Policy Research, is that many people aren’t tested for prediabetes because the test often is not covered by insurance, particularly for those under the age of 45.

“It is critically important for individuals to know if they are diabetic or prediabetic and that there is something they can do about it,” said Diamant, noting that modifying diet, being physically active and maintaining a healthy weight all reduce a person’s risk for diabetes.

But Brenda Yamashita, chronic disease program director with the Alameda County Public Health Department, said if most physicians suspect that a patient may be at risk, they will usually approve a test or treatment.

The study broke down estimated prediabetes rates by county, finding major disparities across the state, particularly among the 18-39 population. For those young adults, prediabetes rates ranged from lows of 26 percent in Lake County to 28 percent in San Francisco. The rates jumped to 32 percent in Santa Clara County, to 37 percent in Monterey County, to a high of 40 percent in rural Kings and Imperial counties.

Racial and ethnic disparities are extremely pronounced. There are statistically higher prediabetes rates among young adult Pacific Islanders (43 percent), African-Americans (38 percent), American Indians (38 percent), Latinos (36 percent) and Asian-Americans (31 percent) than whites (29 percent), pointing to the need to focus additional prevention efforts in those communities, the report said.

Nationally, diabetes rates have tripled over the past 30 years. In California, the rate has increased by 35 percent since 2001. Someone diagnosed with diabetes by age 40 will have lifetime medical spending that is $124,600 more than someone that age who does not have diabetes.

Some health experts say one way to address the diabetes epidemic is to impose a tax on sugary beverages.

Berkeley became the first city in the country to pass a soda tax in 2014, but similar efforts have repeatedly failed in the Legislature.

This week, however, two legislators — Democratic Assemblymen Jim Wood, of Healdsburg, and Richard Bloom, of Santa Monica — proposed a “health impact fee” of 2 cents per ounce on sugar-sweetened sodas and other drinks.

And last month, a Field Poll about childhood obesity-prevention policies showed more than 7 in 10 of voters polled believe there’s a close link between a child regularly drinking sugary beverages and diabetes.

The prediabetes study was conducted by the UCLA Center for Health Policy Research and commissioned by the California Center for Public Health Advocacy. The eight-month-long study cost about $50,000 and was paid for by the California HealthCare Foundation and California Endowment.

Contact Tracy Seipel at 408-920-5343. Follow her at Twitter.com/taseipel.