More people in the United States are overweight than ever before. U.S. government researchers have found that Americans of all ages and races have gained weight in the last decade, but some ethnic populations have seen the biggest increases.

Two U.S. Centers for Disease Control studies show that the proportion of the American population that is overweight or obese has been increasing. The leader of both studies, Cynthia Ogden, says one shows the fraction of overweight adults has risen to 30 percent from 23 percent a decade ago. The second reveals that children and adolescents are part of the trend, with 15 percent too heavy for their height.

"The problem of obesity is getting worse in the United States," he said. "Now we know that one-third of adults are obese, and in the last two decades, overweight among children has tripled."

Ms. Ogden and her colleagues compared heights and weights of more than 4,000 adults across the United States and an equal number of children and teenagers. The figures, published in The Journal of the American Medical Association, reveal that the growing trend toward obesity affects Americans of all ages and races. But Ms. Ogden says the biggest increases were in four groups, people over age 60, African-American women and teenagers, and Mexican-American teens.

"The increase among African-American women has occurred among all ages, but now for women, African-American women 40 and above, half are obese," she said. "Among adolescent African-American girls, over 25 percent are overweight. The same is true among Mexican-American adolescent boys, and that's a 10 percent increase in the last decade."

Although the studies examine only U.S. obesity, the researchers say they may be viewed as part of a long term trend toward heavier bodies in all affluent and well nourished societies. What is causing waistlines to expand?

"There have been many suggestions as to why it's happening, including increasing portion sizes, eating out more, and the fact that we're very inactive. We don't get enough exercise," she said.

Dr. Ogden says this is a public health crisis, because obesity can lead to a number of serious health problems, such as diabetes, heart disease and stroke.

She points out that overweight children are building toward these conditions. "Adolescents who are overweight are more likely to be overweight adults and so that can lead into problems in adulthood," she said. "But also, overweight kids are more likely to have high blood pressure, high cholesterol, and the precursors to diabetes."

Dr. Ogden calls the prevalence of overweight and obesity alarming, adding that individuals, families, and communities must work together to solve this problem. She concedes, however, that reversing the trend is likely to be difficult.