CHICAGO — A striking new study says almost 1 in 5 American 4-year-olds is obese, and the rate is alarmingly higher among American Indian children, with nearly a third of them obese.

Researchers were surprised to see differences by race at so early an age.

Overall, more than half a million 4-year-olds are obese, the study suggests. Obesity is more common in Hispanic and black youngsters, too, but the disparity is most startling in American Indians, whose rate is almost double that of whites.

“The magnitude of these differences was larger than we expected, and it is surprising to see differences by racial groups present so early in childhood,” said Sarah Anderson, an Ohio State University public health researcher. She conducted the research with Temple University’s Dr. Robert Whitaker.

The study is an analysis of nationally representative height and weight data on 8,550 preschoolers born in 2001. Children were measured in their homes and were part of a study conducted by the government’s National Center for Educational Statistics. The results appear in Monday’s Archives of Pediatrics &Adolescent Medicine.

Almost 13 percent of Asian children were obese, along with 16 percent of whites, almost 21 percent of blacks, 22 percent of Hispanics, and 31 percent of American Indians.

The researchers did not examine reasons for the disparities, but others offered several theories.

Dr. Glenn Flores, a pediatrics and public health professor at University of Texas Southwestern Medical School in Dallas, cited higher rates of diabetes in American Indians, and also Hispanics, which scientists believe may be due to genetic differences.

Also, other factors that can increase obesity risks tend to be more common among minorities, including poverty, less-educated parents, and diets high in fat and calories, Flores said.

Jessica Burger, a member of the Little River Ottawa tribe and health director of a tribal clinic in Manistee, Mich., said many children at her clinic are overweight or obese, including preschoolers.

Burger, a nurse, said one culprit is gestational diabetes, which occurs during a mother’s pregnancy. That increases children’s chances of becoming overweight and is almost twice as common in American Indian women, compared with whites.

She also blamed the federal commodity program for low-income people that many American Indian families receive. The offerings include lots of pasta, rice and other high-carbohydrate food that contribute to what Burger said is often called a “commod bod.”

Also, Burger noted that exercise is not a priority in many American Indian families struggling to make ends meet, with parents feeling stressed just to provide basic necessities.