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 lead author said that rate is worrisome among children so young, even in a population at higher risk for obesity because of other health problems and economic disadvantages.

“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.

Dr. Glenn Flores, a pediatrics and public health professor at University of Texas Southwestern Medical School in Dallas, said the research is an important contribution to studies documenting racial and ethnic disparities in children’s weight.

“The cumulative evidence is alarming because within just a few decades, America will become a ‘minority majority’ nation,” he said. Without interventions, the next generation “will be at very high risk” for heart disease, high blood pressure, cancers, joint diseases and other problems connected with obesity, said Flores, who was not involved in the new research.