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 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.

“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 of preventive medicine at the Icahn School of Medicine at Mount Sinai in New York, who was not involved in the study.

These results, surprising to many experts, arose from a rare study tracking children’s body weights for years, from kindergarten through eighth grade. Experts say they may reshape approaches to combating the nation’s obesity epidemic.

The findings, published 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 as well as environments encouraging 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 already were 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. While valuable in documenting the extent of childhood obesity, they gave an incomplete picture of how the condition developed, researchers said.

The study involved 7,738 children from a nationally representative sample. Researchers measured the children’s heights and weights seven times between kindergarten and eighth grade.

When the children entered kindergarten, 12.4 percent were obese – defined as having a body mass index at or above the 95th percentile – and 14.9 percent were overweight, with a BMI at or above the 85th percentile. By eighth grade, 20.8 percent were obese, and 17 percent were overweight.

Half of the obese kindergartners were obese when they were in eighth grade, and nearly three-quarters of the very obese kindergartners were obese in eighth grade. The risk that fat kindergartners would be obese in eighth grade was four to five times that of their thinner classmates, the study found.