Nearly a quarter of Californians are obese, and the disease disproportionately affects low-income people and certain racial and ethnic groups, a new study reports.

The UCLA Center for Health Policy Research found that 24.8 percent of adults were obese in 2011–2012, compared to 19.3 percent a decade earlier.

“Healthy eating requires a combination of money, time and resources, which not everyone has,” Joelle Wolstein, lead author of the study, said in a release. “Obesity results from a complex web of factors. Can you get fresh vegetables nearby? If not, can you get to the store? Is there a safe place to exercise nearby?”

Nearly 18 million California adults and adolescents are considered overweight or obese, and about 7.4 million of them can be classified as obese, according to the report, published June 25 on the UCLA Center’s website.

Adults with a body mass index of 25 or greater are considered overweight; those with a BMI of 30 or more are considered obese.

Meanwhile, childhood obesity remained constant over the decade included in the study. The researchers found that 16 percent of Californians ages 12 to 17 were overweight and 17 percent were obese, similar to the numbers in 2001.

Obesity rates increased in every major region of the state since 2001, but certain counties had even higher rates among adults, the study found.

San Francisco County had the lowest rate at 11.3 percent, while Imperial County had the highest at 41.7 percent.

Obesity rates among the poorest Californians, those with household incomes below 200 percent of the federal poverty level, jumped during the past decade from 24.4 percent to 30.5 percent.

Obesity rates also varied considerably by race, although the figures for all racial and ethnic groups increased during the decade, researchers found. Groups with the highest obesity rates in 2011–12 were Pacific Islanders at 37.1 percent, American Indians at 36.2 percent, African-Americans at 36.1 percent and Latinos at 32.6 percent. The rate for whites was 21.9 percent. Asians had the lowest obesity rate at 9.7 percent.

Nationally, 28 percent of adults are obese, according to Behavioral Risk Factor Surveillance System data.

The researchers used data from the California Health Interview Survey starting in 2001.

Counties with the lowest obesity rates:

Lowest

San Francisco: 11.3

San Luis Obispo: 12.6

Marin: 13.9

San Mateo: 16.6

Yolo: 17.9

Counties with the highest obesity rates:

Imperial: 41.7

Tehama/Glenn/Colusa (combined to make a sample size): 38.2

Tulare: 38.0

Kings: 36.6

Solano: 35.9