From: University of Colorado, Boulder
By Lisa Marshall
February 23, 2023 – Excess
weight or obesity boosts risk of death by anywhere from 22% to
91%—significantly more than previously believed—while the mortality risk of
being slightly underweight has likely been overestimated, according to new CU
Boulder research.
The findings, published
Feb. 9 in the journal Population
Studies, counter prevailing wisdom that excess weight boosts mortality risk
only in extreme cases.
The statistical
analysis of nearly 18,000 people also shines a light on the pitfalls of using
body mass index (BMI) to study health outcomes, providing evidence that the
go-to metric can potentially bias findings. After accounting for those biases,
it estimates that about 1 in 6 U.S. deaths are related to excess weight or
obesity.
“Existing studies have
likely underestimated the mortality consequences of living in a country where
cheap, unhealthy food has grown increasingly accessible, and sedentary
lifestyles have become the norm,” said author Ryan Masters, associate professor
of sociology at CU Boulder.
“This study and others
are beginning to expose the true toll of this public health crisis.”
Challenging the
‘obesity paradox’
While numerous studies
show that heart disease, high blood pressure and diabetes (which are often
associated with being overweight) elevate mortality risk, very few have shown
that groups with higher BMIs have higher mortality rates.
Instead, in what some
call the “obesity paradox,” most studies show a U-shaped curve: Those in
the “overweight” category (BMI 25–30) surprisingly have the lowest mortality
risk. Those in the “obese” category (30–35) have little or no increased risk over
the so-called “healthy” category (18.5–25). And both the “underweight” (less
than 18.5) and extremely obese (35 and higher) are at increased risk of death.
“The conventional
wisdom is that elevated BMI generally does not raise mortality risk until you get
to very high levels, and that there are actually some survival benefits to
being overweight,” said Masters, a social demographer who has spent his career
studying mortality trends. “I have been suspicious of these claims.”
He noted that BMI,
which doctors and scientists often use as a health measure, is based on weight
and height only and doesn’t account for differences in body composition or how
long a person has been overweight.
“It’s a reflection of
stature at a point in time. That’s it,” said Masters, noting that Tom Cruise
(at 5 feet 7 inches and an extremely muscular 201 pounds at one point), had a
BMI of 31.5, famously putting him in the category of “obese.” “It isn’t fully
capturing all of the nuances and different sizes and shapes the body comes in.”
Duration matters
To see what happened
when those nuances were considered, Masters mined the National Health and
Nutrition Examination Survey (NHANES) from 1988 to 2015, looking at data from
17,784 people, including 4,468 deaths.
He discovered that a full
20% of the sample characterized as “healthy” weight had been in the overweight
or obese category in the decade prior. When set apart, this group had a
substantially worse health profile than those in the category whose weight had
been stable.
Masters pointed out
that a lifetime carrying excess weight can lead to illnesses that,
paradoxically, lead to rapid weight loss. If BMI data is captured during this
time, it can skew study results.
“I would argue that we
have been artificially inflating the mortality risk in the low-BMI category by
including those who had been high BMI and had just lost weight recently,” he
said.
Meanwhile, 37% of those
characterized as overweight and 60% of those with obese BMI had been at lower
BMIs in the decade prior. Notably, those who had only recently gained weight
had better health profiles.
“The health and
mortality consequences of high BMI are not like a light switch,” said Masters.
“There’s an expanding body of work suggesting that the consequences are
duration-dependent.”
By including people who
had spent most of their life at low-BMI weight in the high-BMI categories,
previous studies have inadvertently made high BMI look less risky than it is,
he said.
When he looked at
differences in fat distribution within BMI categories, he also found that
variations made a huge difference in reported health outcomes.
Exposing a public
health problem
Collectively, the
findings confirm that studies have been “significantly affected” by BMI-related
bias.
When re-crunching the
numbers without these biases, he found not a U-shape but a straight upward
line, with those with low BMI (18.5–22.5) having the lowest mortality risk.
Contrary to previous
research, the study found no significant mortality risk increases for the
“underweight” category.
While previous research
estimated 2 to 3% of U.S. adult deaths were due to high BMI, his study pegs the
toll at eight times that.
Masters said he hopes
the research will alert scientists to be “extremely cautious” when making
conclusions based on BMI. But he also hopes the work will draw attention to
what he sees not as a problem for individuals alone to solve but
rather a public health crisis fueled by an unhealthy or “obesogenic”
environment in the U.S.
“For groups born in the
1970s or 1980s who have lived their whole lives in this obesogenic environment,
the prospects of healthy aging into older adulthood does not look good right
now,” he said. “I hope this work can influence higher-level discussions about
what we as a society can do about it.”
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