Government statistics indicate that growing numbers of Americans are seriously overweight, and new research published Wednesday (March 16) in the New England Journal of Medicine suggests that what some have called an "epidemic" of obesity could reverse a decades-long trend of improved life expectancy.

The problem, says researcher David Allison, is that the extra weight that obese people carry has medical consequences. "The more obese you get, the more risk you have for earlier mortality, for heart disease, for gall bladder disease, for certain forms of cancer, for diabetes, for high cholesterol, for atherosclerosis, and so on and so on," he said.

In their study, Dr. Allison and his colleagues calculated that obesity currently reduces life expectancy by three to nine months for Americans as a whole. That may not sound like a lot, but it's comparable to the reduced life expectancy from accidents, suicide and homicide combined.

Later this century, the effects of obesity could actually reduce life expectancy, reversing the long-term trend. Co-author Doug Passaro of the University of Illinois at Chicago says it's because today's young people are considerably fatter than their parents. "Our real concern is if you look at adolescents and young adults now," he explained. "Their rates of obesity are so high that we could easily lose five to ten years of life expectancy if their rates of obesity are unchecked by the time they reach older adulthood."

The authors of this study are projecting current trends, and Dr. Passaro said that their grim forecast is as much warning as prediction, and he hopes they are wrong. "What we are hoping is that this is a call to action [which] will make people realize that we can't treat overweight persons in a vacuum, but we need to think about population strategies, that is community strategies and societal strategies."

Another co-author on the study, David Ludwig of Boston's Children's Hospital, said that unless obesity is reduced, it's possible that heart attacks could become common among young adults.

Although this study was done in the United States, obesity expert David Allison at the University of Alabama - Birmingham said it's a global problem. "Obesity's increasing in almost every segment of the world's population," he said. "It is increasing in China. It is increasing in Africa. It is increasing in the East. It is increasing in the West. It is increasing in Europe. It is increasing in Latin America. It is increasing in almost every segment of the world."

The causes of obesity are varied, and controversial. Increasing prosperity brings with it more food and, often, a more sedentary lifestyle. But many social critics blame other causes, such as increased consumption of fast food and heavy advertising by companies selling convenient but less-healthy processed foods.

We've been talking about obesity, but who is obese? The generally-accepted definition is a person with a body mass index, or BMI, of 30 or higher. To calculate your BMI, take your height in meters and square it - in other words, multiply it by itself. Then divide that into your weight in kilos to get your BMI. A BMI under 25 is considered normal, 25 to 30 is overweight, and higher than 30 is considered obese.