Americans, at least as a group, may have reached their peak of obesity, according to data the Centers for Disease Control and Prevention released Wednesday.

The numbers indicate that obesity rates have remained constant for at least five years among men and for closer to 10 years among women and children—long enough for experts to say the percentage of very overweight people has leveled off.

But the percentages have topped out at very high numbers. Nearly 34 percent of adults are obese, more than double the percentage 30 years ago. The share of obese children tripled during that time, to 17 percent.

“;Right now we've halted the progress of the obesity epidemic,”; said Dr. William H. Dietz, director of the division of nutrition, physical activity and obesity at the disease control centers. “;The data are really promising.

“;That said, I don't think we have in place the kind of policy or environmental changes needed to reverse this epidemic just yet.”;

Dietz said the data probably reflected increased awareness of the obesity problem, especially among women, “;who buy food, prepare it and see it, and they're making changes for themselves that they're also making for their kids.”; He also cited a reduction in “;less healthful foods”; at school.

Some experts, though, were not optimistic that the leveling off was a result of improved eating and exercise habits.

“;Until we see rates improving, not just staying the same, we can't have any confidence that our lifestyle has improved,”; said Dr. David Ludwig, director of the Optimal Weight for Life Program at Children's Hospital Boston.

Ludwig said the plateau might just suggest that “;we've reached a biological limit”; to how obese people could get. When people eat more, he said, at first they gain weight; then a growing share of the calories go “;into maintaining and moving around that excess tissue,”; he continued, so that “;a population doesn't keep getting heavier and heavier indefinitely.”;

Furthermore, Ludwig said, “;it could be that most of the people who are genetically susceptible, or susceptible for psychological or behavioral reasons, have already become obese.”;

The numbers, published in The Journal of the American Medical Association, are based on national surveys that record heights and weights of a representative sample of Americans. People are considered obese if their body mass index—a ratio of height to weight—is 30 or greater. A 5-foot-6-inches person is obese at 186 pounds; a 6-foot person is obese at 221 pounds.

Even though the data show an overall plateau for obesity rates, they indicate an increase from 1999 to 2008 in the heaviest boys, ages 6 to 19, primarily whites. Experts speculated that heavy children in environments of unhealthy food and physical inactivity might simply be shifting into the top weight categories because their situation had not improved.

African-American adults have the highest obesity rates—37 percent among men and nearly 50 percent among women. For Hispanic women, the rate is 43 percent. Hispanic and black children have higher rates than non-Hispanic whites.

Federal health officials had set a goal a decade ago that no more than 15 percent of people would be obese in 2010.

“;We aren't near that, and we haven't moved in that direction,”; said Cynthia L. Ogden, an epidemiologist at the National Center for Health Statistics and an author of the reports.

In addition, 68 percent of adults and nearly one-third of children are considered at least overweight, with a body mass index of 25 or higher. For a 5-foot-8 person, that would be 164 pounds.

Dietz said he hoped the obesity data would follow what happened with smoking rates, which leveled off before declining. But he said obesity was difficult to address because while “;tobacco is a single source, obesity is both physical activity and diet.”;

Experts like Steven Gortmaker, a Harvard public health professor, said obesity would decline only with new policies, like penalties and incentives to promote healthier foods and exercise.

“;If you look at the reversal of the smoking epidemic,”; Gortmaker said, “;substantial change didn't really happen until there were bans on advertising and limits on consumption through things like taxation. We have to make some substantial changes.”;