Even though I'm an identical twin, I've outweighed my brother John from
the moment we emerged by caesarian section. I was a 6-pound little
piggy, whereas John was the 4-pound, 4-ounce victim of my embryonic
gluttony (or so the family joke/legend has forever maintained).
In
the decades since our birth, I have regularly climbed into "overweight"
BMI territory and struggled to escape. Staying lean has been as
difficult for me as it is effortless for John—this despite lifelong
eating and exercise patterns that strike our friends as spooky in their
similarity. To this day at family gatherings, we match each other bite
for bite—and, just as in boyhood, we still fight over the last pork chop
on the platter. Same behavior, same DNA: What's left to account for
such divergent weights?
My only consolation is that whatever's
going on, apparently I'm not alone. In recent years, scientists have
observed an equally mysterious increase in girth around the globe.
"Obesity seems to have risen in virtually every country where detailed
data is available," says David B. Allison, Ph.D., director of the
Nutrition Obesity Research Center at the University of Alabama at
Birmingham. This even includes areas in the world like rural China,
where stereotypically American fat factors—from junk food to video
games—have yet to infiltrate peoples' lives.
The phenomenon is
difficult for critics of the Western diet and sedentary lifestyle to
explain. Even harder to explain: mounting evidence that it's not just
humans growing heavy. In a 2011 study, Allison and his colleagues
reviewed weight changes over several decades in 24 separate mammal
populations. They analyzed 20,000 critters, from chimps at primate
research centers to pet dogs and cats across the nation to wild Norway
rats and their lab-coddled cousins. Every species they surveyed showed a
trend toward packing on the pounds. The odds of this happening by
chance alone: 1 in 1.2 million.
"I think what our study clearly
shows," says Allison, "is that there must be other factors influencing
global weight gain beyond the usual suspects of food marketing and
reductions in physical activity brought about by changes in our man-made
environments and schedules."
Allison's conviction is shared by
burgeoning ranks of researchers worldwide. These investigators are
quickly closing in on a host of unusual suspects that can prove to be
surprisingly potent adversaries in our battle of the bulge. Ubiquitous
and often operating invisibly, these emerging puppetmasters can't
explain every weight problem. But for those of us who've always felt as
if something unseen has been tilting the balance against us, it now
seems almost certain our suspicions were right.
Who—or what—are
they? Armed with an unhealthy dose of paranoia, I've set out to find the
invisible thumbs pushing down on my scale while leaving John's
untouched.
SUSPECT 1: Heavy Air
In
2012, Arne Astrup, M.D., a renowned obesity researcher at the
University of Copenhagen, reasoned that given the global weight gain
noted in humans and animals alike, a specific environmental factor must
be common to all. Diet, livelihood, access to technology, health
care—these vary dramatically across the fattening globe. Not so the
atmosphere we all breathe.
For millions of years, the planet's
concentration of carbon dioxide hovered consistently between 180 and 300
parts per million. But thanks to industrialization, this number has
spiked to unprecedented highs in less than a century.
Last May,
researchers at the National Oceanic and Atmospheric Administration
reported that the daily average CO2 level had reached 400 parts per
million, a figure not seen for at least 3 million years, long before
humans evolved. What's more, this increase has been accelerating in
virtual lockstep with the expansion of global waistlines.
To Dr.
Astrup, this is more than just a statistical coincidence. He believes a
cause-and-effect connection exists—and that the physiology backs him up.
As
Dr. Astrup explains it, the more CO2 you breathe in, the more acidic
your blood becomes. This increase, in turn, boosts the activity of your
orexin neurons, brain cells discovered in the late 1990s that are
exquisitely sensitive to changes in blood acidity. The name "orexin" is
derived from the Greek word for appetite—an apt name for nerve cells
that so powerfully drive our urge to eat. The faster our orexin neurons
fire, the more unconscious edicts we receive to stuff ourselves and then
head for the couch. "We know that these neurons are quite potent, so it
is very likely that the increase in carbon dioxide could activate them
significantly," says Dr. Astrup.
In his study, Dr. Astrup
estimated that increases in atmospheric CO2 over the past century are
enough to hasten the average firing rate of a person's orexin neurons by
1 percent. And if anything, this change may be an underestimation due
to the fact that modern human beings spend much more time indoors—where
CO2 concentrations are even higher—than our ancestors did just a century
ago.
To test the atmosphere hypothesis, Dr. Astrup and his
colleagues recruited six young male volunteers who agreed to be exposed
to higher-than-normal levels of CO2 in a special respiration chamber.
The result: a 6.1 percent average increase in calories consumed. This
may not sound like much until you consider that this change took place
in a mere seven and a half hours. Talk about inhaling your food.
"We
now plan to follow up with a larger study to substantiate and expand
the results," says Dr. Astrup. "Many would think that this is too
fantastic of a hypothesis to be true, but if it turns out to be correct,
the solution to the obesity problem could be very different from what
we believe today."
Is It Making Me Fat? Probably Not
John
lives in New Jersey, not far from the Atlantic City Expressway and its
nonstop stream of casino-bound cars, each producing, on average, 20
pounds of CO2 per gallon of gas it burns. I live in the bucolic splendor
of western Pennsylvania. There aren't nearly as many cars here, and I
suspect that a good portion of vehicle emissions are absorbed by the
forests. So in my case, carbon dioxide doesn't appear to be the culprit.
