For many obese adults, the die was cast by the time they were 5 years old. A major new study of more than 7,000 children has found that a third of children who were overweight in kindergarten were obese by eighth grade. And almost every child who was very obese remained that way.

Some obese or overweight kindergartners lost their excess weight, and some children of normal weight got fat over the years. But every year, the chances that a child would slide into or out of being overweight or obese diminished. By age 11, there were few additional changes: Those who were obese or overweight stayed that way, and those whose weight was normal did not become fat.

“The main message is that obesity is established very early in life, and that it basically tracks through adolescence to adulthood,” said Ruth Loos, a professor at the Icahn School of Medicine at Mount Sinai in New York, who was not involved in the study.

These results arose from a rare study that tracked children’s body weight for years, from kindergarten through eighth grade. Experts say they may reshape approaches to combating the nation’s obesity epidemic, suggesting that efforts must start much earlier and focus more on the children at greatest risk.

The findings, to be published Thursday in The New England Journal of Medicine, do not explain why the effect occurs. Researchers say it may be a combination of genetic predispositions to being heavy and environments that encourage overeating in those prone to it. But the results do provide a possible explanation for why efforts to help children lose weight have often had disappointing results. The steps may have aimed too broadly at all schoolchildren, rather than starting before children enrolled in kindergarten and concentrating on those who were already fat at very young ages.

Previous studies established how many children were fat at each age but not whether their weight changed as they grew up.

“It is almost as if, if you can make it to kindergarten without the weight, your chances are immensely better” said Dr. Jeffrey P. Koplan, the vice president of the Emory Global Health Institute in Atlanta, who was not associated with the new study.

Steven L. Gortmaker, a professor at the Harvard School of Public Health, said he saw a bright side to the findings. Young children, he said, can cross a line between being fat or normal weight by gaining or losing just a few pounds. For adults, it can be 20 to 30 pounds, or even 40 to 50 pounds. “It can take a long time to turn that around,” said Gortmaker, who wrote an editorial accompanying the new study.