CHICAGO (AP) -- Being obese at age 20 can cut up to 20 years off a person's life, with the biggest impact on black men, according to yet another study that underscores the long-term dangers of being overweight.

The research appears in Wednesday's Journal of the American Medical Association and was released a day after another study that said that being fat at 40 shortens a person's life by at least three years.

The JAMA study, led by University of Alabama at Birmingham biostatistician David Allison, found that life expectancy for 20-year-olds with a body-mass index of at least 45 is 13 years lower for white men and 20 years lower for black men, compared with people of normal weight.

Body-mass index is a height-to-weight ratio; 30 and above is considered obese. A person who is 5-foot-4 and 262 pounds would have a BMI of 45 -- and look like a sumo wrestler. But millions of Americans are that fat, Allison said.

The life-shortening effects were found to be lower for 20-year-old severely obese white women (eight years of life lost) and black women (five years lost).

Obesity increases the risk for several life-threatening conditions, including heart disease, diabetes and some types of cancer. Allison said younger people are especially vulnerable, in part because they have more years to live and more time for the obesity to take its toll.

Dr. JoAnn Manson of Harvard's Brigham and Women's Hospital said the study helps emphasize that obesity is far worse than just "a cosmetic problem."

Until this week, data attempting to quantify the effects of obesity on life span were scarce.

In Tuesday's Annals of Internal Medicine, Dutch researchers presented data on about 3,400 mostly white, middle-aged Americans. The researchers found that being overweight at 40 is likely to reduce life expectancy by at least three years -- as much, they said, as smoking cigarettes. Obese, or severely overweight people, lost even more years -- about six or seven.

The JAMA study was based on an analysis of nationally representative surveys of more than 14,000 Americans.

Life-shortening effects were less dramatic in people who were less obese. And there were startling racial differences in how fat people had to be before life expectancy started to drop.

In blacks, life expectancy was not shortened in obese men with BMIs under 31 and in obese women under 37. But in whites, reductions of about one year occurred in young people who were merely overweight -- in men with a BMI of about 25.5 and in women with a BMI of about 27.5.

BMIs between 25 and 30 are considered overweight; the ideal is between 18 and 25.

Allison said the reasons for the racial differences are unclear. But some researchers have speculated that blacks may have relatively more lean mass, or muscle, than fat. A JAMA editorial said the differences may be due to limitations in the study.

"It would be a great disservice to blacks if these results were used to promulgate the concept that excess weight is not harmful to them," said Manson and Shari Bassuk of Brigham and Women's Hospital.

Copyright 2003 Associated Press. All rights reserved.