NEW YORK (Reuters Health) - A new study provides weighty evidence that Americans are fatter than ever and suggests that the pounds are accumulating at even younger ages.

The report found that 27% of US adults aged 20 to 74 are obese by the time they reach their mid-30s, about twice the rate in the early 1960s. Overall, 61% of adults are either overweight or obese, defined as a body mass index (BMI) of at least 30. Women and ethnic minorities are at increased risk of obesity, researchers report in the June 18th issue of the Annals of Internal Medicine.

BMI is a measure of a person's weight in relation to their height and can more accurately predict the risk of weight-related medical complications than weight alone. A BMI of 30 or more, for instance, is associated with an increased risk of heart disease, some cancers, stroke and type 2 diabetes.

While previous studies have reported a rise in the number of Americans who are overweight and obese over the past four decades, the current findings illustrate how quickly obesity rates have grown, Dr. Kathleen McTigue, the study's lead author, told Reuters Health.

"We documented a shift in how fast young adults became obese, with people born in 1964 becoming obese 26% to 28% faster than (those) born in 1957," said McTigue, who is from the University of North Carolina at Chapel Hill.

The findings underscore the need for weight-control efforts that target even mildly overweight young adults. According to the report, 80% of adults who were obese in their mid-30s began to put on the excess weight in their 20s and many people began to gain weight in their teens.

"Early intervention has the potential to prevent significant morbidity and should not be overlooked," said McTigue.

Overall, 26% of men and 28% of women were obese by age 35 to 37. Obesity rates were particularly high among minorities, with black women about twice as likely as their white peers to become obese. Obesity rates were also higher among Hispanic men and women, compared to whites, the investigators found.

It is not clear why minorities are more prone to obesity than whites but the researchers suggest that ethnicity may be a marker for other factors such as dietary and exercise habits, income, education and the number of children in a family. The study did not include information on diet and exercise.

McTigue stressed that future studies will need to examine why minorities are more prone to obesity, since certain groups are also at higher risk for some of the medical complications of obesity.

Dr. Robert C. Whitaker from Cincinnati Children's Hospital in Ohio suggests that future studies explore how social class may contribute to obesity. Other factors such as neighborhood and family customs should also be investigated, he writes in an accompanying editorial.

"Obesity is not a single disorder. Individuals become obese as a result of a unique mixture of inherited genes that confers susceptibility and years of complex interaction with an environment that is increasingly more 'obesogenic,"' Whitaker writes.

The findings are based on information from more than 9,000 people living in the US, who reported their height and weight to researchers 12 times over a 17-year period. Because people tend to underestimate their weight, the results of the study may be conservative, the researchers note.

SOURCE: Annals of Internal Medicine 2002;136:857-864, 923-925.

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