Americans face the highest risk of weight gain during their 20s and 30s, when they put on an average of seven pounds a decade, a study shows.

However, by age 55, people tend to stop putting on weight and begin losing it.Researchers said their study, based on a nationwide survey of almost 10,000 adults, is the first to calculate the risk of getting fat.

"Younger adults are at greatest risk of gaining substantial amounts of weight during adulthood," said Dr. David F. Williamson. "A greater effort has to be applied to preventing obesity early in adulthood, possibly before age 25."

The study found that men and women between ages 25 and 34 are likely to gain an average of seven pounds over the next 10 years. However, about 6 percent of the people in this age group will gain nearly 30 pounds in the next decade.

The tendency to gain weight does not affect the sexes equally. An average American man puts on an extra six pounds between his 25th and 35th birthdays, but a woman gains almost eight pounds.