Le recul de l'espérance de vie aux Etats-Unis

Il y a quelques années, un jeune sociologue (son nom m'échappe (je l'ai retrouvé : Todd), mais c'est le fils d'un écrivain et journaliste qui a beaucoup écrit pour le Nouvel Observateur) avait annoncé la chute de l'Empire soviétique en s'appuyant sur les seules données démographiques. Un pays dont la mortalité infantile augmente est un pays qui fonctionne très mal et qui est donc appelé à imploser, expliquait-il en substance. Faut-il conclure la même chose des études qui annoncent un recul de l'espérance de vie aux Etats-Unis?
En tout cas, c'est inquiétant si l'on ne juge par cet article de Jessica Ebert publié dans Nature : Americans face drop in life expectancy

For the first time in recent history, researchers are predicting that the life expectancy of Americans may begin a sustained decline.

The forecast is based on the sharp rise in obesity in today's youth. By the middle of this century, the increased risk of diabetes, heart disease and cancer that they will face could lessen the average life expectancy by two to five years, some say.

In general, longevity predictions are determined by studying historical trends in death rates. Various agencies, such as the US Social Security Administration (SSA), have used this method to predict that the life expectancy of Americans will continue to rise over the next century.

But Jay Olshansky, a biodemographer at the University of Illinois at Chicago, argues that they ignore the effect of obesity on future generations.

Future imperfect

Instead of making predictions by studying what has happened in the past, Olshansky and a team of statisticians and demographers, "looked into the future by looking at today's younger generations," he explains.

Olshansky's team compared the death rates of obese people with those of healthy weight. Extrapolating to the whole population, they found that, at the moment, obesity reduces average life expectancy by about four to nine months.

And because the prevalence of obesity among children and teenagers has risen sharply over the past 30 years, the researchers predict that the shortening effect could become as much as two to five years by mid-century.

"When I first looked at the calculation it seemed relatively small," says Olshansky, "but in reality it's not small at all." It's equivalent to the negative effect of cancer on population longevity, which is 3.5 years, he points out.

Striking a balance

Steve Goss, chief actuary with the SSA, agrees that obesity should be a significant factor in life expectancy projections. But he points out that forecasts by agencies such as his try to balance negative factors with positives, such as medical breakthroughs.

"We will have continued improvements in mortality as long as we have a strong economy and medical innovation," says Goss.

Rather than factor in the promise of new technologies that do not yet exist, Olshansky says he prefers to base his predictions on "something we can observe and measure today".

The team's conclusions may seem pessimistic, says Olshansky, but they show that the health of young people in the United States is in jeopardy. "If nothing changes for the better," he says, "today's younger generations will live shorter and less healthy lives than their parents."

Olshansky's team is now working on a detailed description of how the trend can be reversed. "We'd like nothing more than to be proven wrong," says Olshanky's colleague David Allison, a biostatistician at the University of Alabama at Birmingham. "I hope we continue to figure out what causes obesity, and how to prevent it and more successfully treat it."