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.
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 Education 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.
Children were considered obese if their body-mass index, a height-weight ratio, was in the 95th percentile or higher based on government BMI growth charts. For 4-year-olds, that would be a BMI of about 18.
For example, a girl who is 4½ years old, 40 inches tall and 42 pounds would have a BMI of about 18, weighing 4 pounds more than the government's upper limit for that age, height and gender.
Some previous studies of young children did not distinguish between kids who were merely overweight versus obese, or they examined fewer racial groups.
The current study looked only at obesity and a specific age group. Anderson called it the first analysis of national obesity rates in preschool kids in the five ethnic or racial groups.
The researchers did not examine reasons for the disparities, but others offered several theories.
Flores 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 pastas, rice and other high-carbohydrate foods that contribute to what Burger said is often called a "commod bod."
"When that's the predominant dietary base in a household without access to fresh fruits and vegetables, that really creates a better chance of a person becoming obese," she said.
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.
To address the problem, her clinic has created activities for young Indian children, including summer camps and a winter break "outdoor day" that had kids braving 8-degree temperatures to play games including "snowsnake." That's a traditional American Indian contest in which players throw long, carved wooden "snakes" along a snow or ice trail to see whose lands the farthest.
The hope is that giving kids used to modern sedentary ways a taste of a more active traditional American Indian lifestyle will help them adopt healthier habits, she said.