[Paleopsych] Science: A Heavyweight Battle over CDC's Obesity Forecasts
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A Heavyweight Battle over CDC's Obesity Forecasts
Science, Vol 308, Issue 5723, 770-771 , 6 May 2005
How many people does obesity kill?
That question has turned into a headache for the Centers for
Disease Control and Prevention (CDC) in Atlanta, Georgia: In the past
year, its scientists have published dueling papers with conflicting
estimates on obesity-associated deaths--the first three times greater than
the second. The disagreement, some fear, is undermining the agency's
The bidding on obesity's annual death toll started at a
staggering 400,000--the number cited in a CDC paper co-authored by CDC
chief Julie Gerberding in 2004. But dissent prompted an internal inquiry,
and CDC decided this year to lower the number to 365,000. That was still
too high for some CDC analysts, who together with colleagues at the
National Cancer Institute (NCI) in Bethesda, Maryland, published a new
figure on 20 April--112,000 deaths. The low estimate is spawning other
problems, though. A food-industry interest group is touting it as evidence
that obesity is not so risky. Even researchers who favor the low number
worry that it will lead to complacency.
After trumpeting the highest estimate a year ago and warning
that obesity deaths were poised to overtake those caused by tobacco, CDC
officials now say that numbers are unimportant. The real message should be
that "obesity can be deadly," says George Mensah, acting director of CDC's
National Center for Chronic Disease Prevention and Health Promotion. "We
really add to the confusion by sticking to one number."
But some of CDC's own scientists disagree. "It's hard to argue
that death is not an important public health statistic," says David
Williamson, an epidemiologist in CDC's diabetes division and an author on
the paper with the 112,000 deaths estimate.
Calculating whether obesity leads directly to an individual's
demise is a messy proposition. To do so, researchers normally determine by
how much obesity increases the death rate and what proportion of the
population is obese. Then they apply that to the number of deaths in a
given time, revealing excess deaths due to obesity. Both studies use that
approach, but methodological differences produced big disparities between
the two papers--one by epidemiologist Ali Mokdad, Gerberding, and their
CDC colleagues, published in the Journal of the American Medical
Association (JAMA) on 10 March 2004, and the new estimate by CDC
epidemiologist Katherine Flegal and colleagues at CDC and NCI, published
in JAMA on 20 April.
Both relied on data about individuals' weight and other
measures from the National Health and Nutrition Examination Survey
(NHANES), which has monitored the U.S. population since the 1970s. The
Mokdad group used the oldest, NHANES I. Flegal's group also used two more
recent NHANES data sets from the 1980s and 1990s. Her method found fewer
obesity-associated deaths--suggesting that although obesity is rising,
some factor, such as improved health care, is reducing deaths.
Other variations in methodology proved crucial. For example,
the two groups differed in their choice of what constitutes normal weight,
which forms the baseline for comparisons. Flegal's team adopted the
definition favored by the National Institutes of Health and the World
Health Organization, a body mass index (BMI) between 18.5 and less than
25. The Mokdad team chose a BMI of 23 to less than 25; this changed the
baseline risk of death, and with it, deaths linked to obesity.
In their paper, the Mokdad authors said they selected that
narrower, heavier range because they were trying to update a landmark 1999
JAMA paper on obesity led by biostatistician David Allison of the
University of Alabama, Birmingham, and chose to follow Allison's
methodology. (CDC spokesperson John Mader said that Mokdad and his
co-authors were not available to be interviewed.) "There's no right
answer" to which BMI range should be the "normal" category, says Allison.
He felt his choice was more "realistic," and that expecting Americans to
strive for even lower BMIs might be asking too much. But that relatively
small difference in BMI had a big effect on the estimates: Had Flegal's
team gone with the 23-to-25 range, she reported, the 112,000 deaths
estimate would have jumped to 165,000.
The scientists also diverged sharply in how they tackled age.
It's known that older individuals are less at risk and may even benefit
from being heavier: A cushion of fat can keep weight from falling too low
during illness. And young obese people tend to develop more severe health
problems, says David Ludwig, director of the obesity program at Children's
Hospital in Boston.
Flegal's group took all this into account by assigning risks
from obesity to different age groups. Stratifying by age meant that when
Flegal turned to actual death data--all deaths from the year 2000--she was
less likely to count deaths in older age groups as obesity-related.
Allison concedes that in retrospect, his decision not to
stratify by age was a mistake. And it had a big impact on the estimates.
"Very minor differences in assumption lead to huge differences in the
number of obesity-induced deaths," says S. Jay Olshansky, a biodemographer
at the University of Illinois, Chicago.
Olshansky, Allison, and Ludwig published their own provocative
obesity paper in The New England Journal of Medicine in March. It argued
that U.S. life expectancy could begin decreasing as today's obese children
grow up and develop obesity-induced diseases, such as diabetes and heart
disease (Science, 18 March, p. 1716).
But Olshansky now says that in light of Flegal's recent paper
on obesity deaths and a companion paper that she, Williamson, and other
CDC scientists authored in the same issue of JAMA, his life expectancy
forecasts might be inaccurate.
The companion paper, led by CDC's Edward Gregg, examined how
much cardiovascular disease was being driven by obesity. The findings were
drawn from five surveys, most of them NHANES, beginning in 1960 and ending
in 2000, and they dovetailed with the conclusions in Flegal's 112,000
deaths paper. All heart disease risk factors except diabetes were less
likely to show up in heavy individuals in recent surveys than in older
ones. That suggests, says Allison, that "we've developed all these great
ways to treat heart disease" such as by controlling cholesterol. This
could also explain, he and others say, why NHANES I led to much higher
estimates of obesity-associated deaths than did NHANES I, II, and III
combined. Although obesity rates are rising, obesity-associated deaths are
Ludwig disagrees that this trend will necessarily continue or
that Gregg's paper disproves the one he co-authored with Olshansky. Type 2
diabetes, which is becoming more common in youngsters, "starts the clock
ticking towards life-threatening complications," he notes.
Olshansky is uncomfortable with the kind of attention Flegal's
112,000 estimate is getting. "It's being portrayed," he says, as if "it's
OK to be obese because we can treat it better." In fact, one of Flegal's
conclusions that sparked much interest--that being overweight, with a BMI
of 25 to 30, slightly reduced mortality risk--had been suggested in the
Certainly, food-industry groups are thrilled by Flegal's work.
"The singular focus on weight has been misguided," says Dan Mindus, a
senior analyst with the Center for Consumer Freedom, a Washington,
D.C.-based nonprofit supported by food companies and restaurants. Since
Flegal's paper appeared, the center has spent $600,000 on newspaper and
other ads declaring obesity to be "hype"; it plans to blanket the
Washington, D.C., subway system with its ad campaign.
Some say that CDC needs to choose one number of deaths and
stand behind it. "You don't just put random numbers into the literature,"
says antitobacco activist and heart disease expert Stanton Glantz of the
University of California, San Francisco, who disputed the Mokdad findings.
Scientists agree that Flegal's study is superior, but it may
also be distracting, suggests Beverly Rockhill, an epidemiologist at the
University of North Carolina, Chapel Hill. Even if obese individuals' risk
of death has been overplayed in the past, she says, we ought to ask: "Are
they living a sicker life?"
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