WASHINGTON - Far too many kids are fat by preschool, and Hispanic youngsters are most at risk, says new research that's among the first to focus on children growing up in poverty.

The study couldn't explain the disparity: White, black and Hispanic youngsters alike watched a lot of TV, and researchers spotted no other huge differences among the families.

But one important predictor of a pudgy preschooler was whether the child was still using a bottle at the stunning age of 3, concluded the study being published online Thursday by the American Journal of Public Health.

"These children are already disadvantaged because their families are poor, and by age 3 they are on track for a lifetime of health problems related to obesity," said lead researcher Rachel Kimbro of the University of Wisconsin at Madison.

Some 17 percent of U.S. youngsters are obese, and millions more are overweight. Obesity can lead to diabetes, high blood pressure and cholesterol, sleep problems and other disorders - and the problem starts early. Overweight preschoolers have a five times higher risk of being fat at age 12 than do lean preschoolers, scientists reported last fall.

Kimbro focused on the poor, culling data on more than 2,000 3-year-olds from a study that tracks from birth children born to low-income families in 20 large U.S. cities.

Thirty-two percent of the white and black tots were either overweight or obese, vs. 44 percent of the Hispanics.

Why were the Hispanics at higher risk? Kimbro checked a long list of factors, from children's TV habits to whether mothers had easy access to grocery stores. Nothing could fully explain the difference. "We were surprised," she said.

Children were particularly at risk if their mothers were obese. So were those who still took a bottle to bed at age 3, as did 14 percent of the Hispanic youngsters, 6 percent of the whites and 4 percent of the blacks.

That finding supports other research that "one of the most common causes of overweight in children is overfeeding," said Dr. Philip Nader, a pediatrician and professor emeritus at the University of California at San Diego.

Pediatricians say even babies should never take a bottle to bed, and that children should start drinking from a cup at about age 1.

Kimbo now wonders what cultural differences - such as whether Hispanic mothers think chubby children are healthier - might also play a role, something the current study couldn't address.