Obesity is a lot more deadly than previously thought. Across recent decades, obesity accounted for 18 percent of deaths among Black and White Americans between the ages of 40 and 85, according to scientists. This finding challenges the prevailing wisdom among scientists, which puts that portion at around 5%.

"Obesity has dramatically worse health consequences than some recent reports have led us to believe," says first author Ryan Masters, PhD, who conducted the research as a Robert Wood Johnson Foundation Health & Society Scholar at Columbia University's Mailman School of Public Health. "We expect that obesity will be responsible for an increasing share of deaths in the United States and perhaps even lead to declines in U.S. life expectancy."

While there have been signs that obesity is in decline for some groups of young people, rates continue to be near historic highs. For the bulk of children and adults who are already obese, the condition will likely persist, wreaking damage over the course of their lives.

In older Americans, the rising toll of obesity is already evident. Dr. Masters and his colleagues documented its increasing effect on mortality in White men who died between the ages of 65 and 70 in the years 1986 to 2006. Grade one obesity (body mass index of 30 to less than 35) accounted for about 3.5% of deaths for those born between 1915 and 1919 -- a grouping known as a birth cohort. For those born 10 years later, it accounted for about 5% of deaths. Another 10 years later, it killed off upwards of 7%.

When the obesity epidemic hit in the 1980s, it hit across all age groups, so older Americans have lived through it for a relatively short period of time. But younger age groups will be exposed to the full brunt for much longer periods.

"A 5-year-old growing up today is living in an environment where obesity is much more the norm than was the case for a 5-year-old a generation or two ago. Drink sizes are bigger, clothes are bigger, and greater numbers of a child's peers are obese," explains co-author Bruce Link, PhD, professor of epidemiology and sociomedical sciences at Columbia University's Mailman School of Public Health. "And once someone is obese, it is very difficult to undo. So it stands to reason that we won't see the worst of the epidemic until the current generation of children grows old."

A New Way of Looking at a Growing Problem

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This study is the first to account for differences in age, birth cohort, sex, and race in analyzing Americans' risk for death from obesity. "Past research in this area lumped together all Americans, but obesity prevalence and its effect on mortality differ substantially based on your race or ethnicity, how old you are, and when you were born," says Dr. Masters. "It's important for policy-makers to understand that different groups experience obesity in different ways."

The researchers analyzed 19 waves of the National Health Interview Survey linked to individual mortality records in the National Death Index for the years 1986 to 2006, when the most recent data are available. They focused on ages 40 to 85 in order to exclude accidental deaths, homicides, and congenital conditions that are the leading causes of death for younger people. The study builds on earlier research by Dr. Masters that found, contrary to conventional wisdom, that risk for death from obesity increases with age. The new study is also influenced by previous work by co-authors Eric Reither, PhD, associate professor at Utah State University, and Claire Yang, PhD, associate professor at the University of North Carolina-Chapel Hill, which showed significant cohort differences in U.S. obesity rates.

Obesity's Varying Effects by Sex and Race

In the groups studied, Black women had the highest risk of dying from obesity or being overweight at 27 percent, followed by White women at 21 percent. Obesity in Black women is nearly twice that of White women. White men fared better at 15%, and the lowest risk for dying from being obese was 5%, for Black men. While White men and Black men have similar rates of obesity, the effect of obesity on mortality is lower in Black men because it is "crowded out" by other risk factors, from high rates of cigarette smoking to challenging socioeconomic conditions. There were insufficient data to make estimates for Asians, Hispanics, and other groups due to the highly stratified nature of the methodology.

In sum, by using a new, more rigorous approach, the new research shows that obesity is far more consequential than previously recognized, that the impact of the epidemic is only beginning to be felt, and that some population groups are affected much more powerfully than others.

Daniel Powers of the University of Texas at Austin and Andrew E. Burger of Utah State University also contributed to the paper.