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.
0 comments:
Post a Comment