America's professional football players may be bigger than life on the field but one in four is also so fat they risk weight-related health problems, a doctor said on Tuesday.

Joyce Harp of the University of North Carolina at Chapel Hill said the estimate came from an analysis of heights and weights of National Football League players for the 2003-04 season as listed on the NFL's Web site.

"More than a quarter of NFL players had a body mass index that qualified as class 2 obesity," said Harp's report published in this week's Journal of the American Medical Association.

Class 2 obesity is defined as a level posing a high risk of health-related problems. It is unlikely, the study said, that players in that risk group landed there "because of a healthy increase in muscle mass alone."

"The high prevalence of obesity in this group warrants further investigation to determine the short and long-term health consequences of excessive weight in professional as well as amateur athletics," the report concluded.

Harp said the investigation came about because of the documented increase in obesity in the United States and the impression that there is a trend toward even larger players in the NFL.

"The high number of large players was not unexpected given the pressure on professional athletes to increase their mass," the study added.

One recent report, it added, found sleep-disorders and high blood pressure problems among the largest professional football players.