PHILADELPHIA (American College of Physicians-American Society of Internal Medicine) -- Americans are getting fatter at younger ages, and the percentage of adults who are actually obese doubled since the 1960s, according to a new study of 9,179 U.S. adults.

The study, "The Natural History of the Development of Obesity in a Cohort of Young U.S. Adults Between 1981 and 1998," appears in the June 18, 2002, issue of Annals of Internal Medicine.

The study of found:

• Twenty-six percent of men and 28 percent of women were obese by the time they were 35 to 37.
• Young adults are becoming obese faster. People born in 1964 became obese 26 percent to 28 percent faster than those born in 1957.
• Ethnic differences appeared in weight development. Black women and Hispanic men became obese most rapidly.
• Overweight young adults, especially women, were at significant risk of becoming obese by age 35 to 37. In women, mild overweight was linked with at least a 60 percent chance of obesity.

According to U.S. statistics, in 1960-1962, 13 percent of adults aged 20 to 74 were obese, while a total of 44 percent weighed more than is healthy. Less than four decades later, in 1999, 27 percent of Americans were actually obese, while 61 percent weighed more than is healthy.

Obesity is defined as body mass index (BMI) of 30 or above, and overweight is defined as a BMI of 25 to 29.9. BMI is a formula calculated from an individual's height and weight. To determine BMI, use the computerized calculator at http://www.doctorsforadults.com/dfa_obes.htm.

Obesity is a serious health problem. It is a risk factor for four of the six most frequent causes of death in the United States: heart disease, certain cancers, stroke, and diabetes. It is also linked to hypertension, osteoarthritis, and pregnancy complications. Losing weight is an important part of treatment for many chronic health conditions, especially diabetes, hypertension and heart disease.

"The compelling thing about these lists is that obesity puts people at risk for both common causes of death and long-term illness," says Kathleen M. McTigue, MD, MS, MPH, the lead researcher. "For example, diabetes doesn't just kill, it requires daily care and close medical follow-up and can lead to complications such as kidney failure, multiple infections and blindness."

Obesity is treatable, although losing weight is not easy, Dr. McTigue says. "One logical way of attacking the problem is to identify those at high risk of becoming obese or those who have recently become obese and target these people for intervention before the health consequences of obesity take their toll."

"Our study clarifies who is at highest risk for obesity, identifies an age when this risk is particularly high, and looks at how excess weight develops over time," Dr. McTigue said. "Our findings suggest that more focus needs to be placed on the young adult period, with particular emphasis on selected subpopulations."

"From this study we can't answer why there are ethnic differences in how people gain weight," said Dr. McTigue. "It's an important question though, because the answer could give us some clue as to how to reverse these trends."

Some of the sex, age and ethnic differences in weight development the study found include:

• Among women, black women had the highest BMI and white women the lowest average BMI at each age sampled.
• Average BMI reached overweight status in black women by 26, in Hispanic women by 28 and in white women by 35.
• Black women developed obesity 2.1 times faster and Hispanic women develop obesity 1.5 times faster than white women.
• Hispanic men showed the highest mean BMI throughout young adulthood among men.

"We could predict who would be obese at 35 to 37, based on sex, race/ethnicity, and body mass index at 20 to 22," says McTigue. "This was especially striking in women. A 21 year-old woman with just a few extra pounds was quite likely to be obese by the time she was in her mid-30s. So we recommend that doctors pay more attention to weight in young people."

Dr. McTigue, a general internist in the Robert Wood Johnson Clinical Scholars Program, and other researchers from the University of North Carolina at Chapel Hill, studied data on 9,179 people from a large, prospective study, the National Longitudinal Survey of Youth 1979. Men and women in the study were born between 1957 and 1964 and have been followed for more than two decades.