Most of us expect to live longer, healthier lives than our parents, and we expect that our children will live longer, healthier lives than we will.

Improved living conditions, better sanitation, enhanced nutrition, infectious disease control, medical technologies, and advances in public health in the developed world have all played a part in increasing life expectancy. In fact, life expectancy has increased slowly and steadily over the past two thousand years.

But will the past predict the future? Not in this case, according a special report in the March 17, 2005 New England Journal of Medicine . Here, scientists predict that life expectancy will actually decline in the United States later this century.

The scientists based their prediction on the rise in obesity in this country, especially among children and minorities. They report that obesity will reduce the average life expectancy of Americans by 4–9 months unless something is done to reverse the obesity trend. And they caution that life expectancies could decline even further if the obesity epidemic continues to spread.

About the Study

In order to determine the impact of obesity on life expectancy, the researchers calculated how much longer people would live if everyone who is currently obese (body mass index, or BMI, of 30 or higher) lost enough weight to attain an optimal BMI.

First, the researchers determined the current prevalence of obesity in men and women of various races and ages in the United States. Then they linked this data to death rates associated with race, gender, and BMI to calculate how likely an average person in each of these categories was to die at a given age.

Next, they assigned everyone who was obese a BMI of either 30 or 35, and calculated their likelihood of death at a given age. To estimate the effect of eliminating obesity from the population, they then reassigned everyone who was obese an optimal BMI of 24, recalculated their age-specific risk of death, and compared the two estimates.

The Findings

Overall, if the death rates associated with obesity remain constant in the next century, obesity will decrease the average American life expectancy by 4–9 months.

The researchers found that eliminating obesity would increase the life expectancy of white males by up to 11 months, of while females by up to almost 10 months, of black males by up to 13 months, and of black females by up to almost 9 months.

As the researchers themselves note, their methods provide estimates, rather than precise calculations of effects on life expectancy. Their approach most likely overestimates the effect of obesity on life expectancy on those with a BMI of 30, for example, and underestimates its effect on those with a BMI of 35.

How Does This Affect You?

This study suggests that, contrary to many forecasts, life expectancy is likely to decline over the next century. Accurate predictions of life expectancy are important for many reasons, including the development of health and economic policy. The current drive to “save Social Security,” for example, is based on the prediction that life expectancy will increase dramatically in the coming century.

In arriving at their calculations, the study authors assumed that the prevalence of obesity and its associated death rates will remain constant. Unfortunately, however, the prevalence of obesity in this country has been rising, and there is no reason to expect a change in this trend in the future. Already in the United States, 28% of men, 34% of women, and 50% of non-Hispanic black women are obese. Overall two-thirds of adults in the United States are obese or overweight.

The greatest increases in obesity are occurring among children and minorities. Obesity increases the risk of death, but it is also associated with an increased risk of type 2 diabetes , hypertension , coronary heart disease , and other conditions that have their own negative effects on the quality and length of life.

Of course, advances in medical treatments are likely to reduce the mortality associated with these illnesses in the coming century. Nevertheless, if this research is correct, the current epidemic of obesity in the United States threatens to reverse the clock on life expectancy—something that has presumably never happened in the course of human history.

Steps must be taken now to reduce obesity in our society. Adults can and should lead by example—making healthy eating and regular exercise a part of their lives, and the lives of their children.