Updated on Friday, March 16, at 7:46 a.m. ET
Gaining weight? Blame the air.
That, in a nutshell, is the controversial argument from a group of Danish researchers who say the world's rising carbon dioxide levels are contributing to the obesity epidemic.
The hypothesis goes like this: increased carbon dioxide, or CO2, levels in the air make our blood more acidic (reducing pH levels). When this happens, brain cells called orexin neurons, which regulate appetite and are sensitive to blood acidity, increase their activity and cause us to eat more.
The hypothesis would explain why many animals, not just humans, have experienced an increase in obesity over the last half century, the Danish researchers say. They pointed to a study published last year showing that eight animal species, including lab animals that had been fed the same diet for years, experienced weight gain over the last 50 years, a time when CO2 levels have also risen.
Experts we spoke with were skeptical of the idea.
"Data do not support such an outrageous hypothesis," said Devanjan Sikder, an assistant professor at the Sanford-Burnham Medical Research Institute in Orlando. "Clinicians and physicians have not reported any changes in pH in humans over time," Sikder said. The body needs to maintain a blood pH within a range of 7.35 to 7.45 pH units in order to deliver the right about of oxygen to the tissues, Sikder said.
A very small change, say, from a blood pH of 7.4 to 7.38 is not impossible, said Dr. David Katz, director of the Prevention Research Center at Yale University School of Medicine. However, the Danish researchers suggest a change of around 0.1 (i.e., from 7.4 to 7.3), and this would not happen in someone with "a reasonably normal set of lungs and kidneys", Katz said.
For the most part, the obesity epidemic can be explained by people eating too much of the wrong foods and exercising too little, Katz said. While new, unconventional theories may explain a small portion of the epidemic, "they become a huge distraction," he said.
The hypothesis has problems
The Danish researchers, from the University of Copenhagen, conducted a small study to test their idea. Six men went into special chambers in which the CO2 levels could be controlled. They were randomly assigned to be exposed to ambient air or to air with high CO2 levels. After seven and a half hours, the participants were allowed to eat as much food as they wanted.
The researchers found that men who were exposed to the high CO2 levels ate 6 percent more calories than the men who were exposed to ambient air.
But Katz said the fact that the men in the experiment ate more when exposed to high CO2 levels is not necessarily surprising. When people are exposed to high CO2 levels, they increase their breathing rate to get rid of the excess CO2.
"If you are breathing faster to eliminate CO2, you're doing more physical work," he said. The men in the study exposed to high CO2 levels might be expected to eat more to make up for their extra exertion, Katz said.
The whole hypothesis may be a case of what researchers call the ecological fallacy, which is when "two things happen to be true and really have nothing to do with one another," Katz said. For instance, one could hypothesize that high-speed Internet access protects against tuberculosis because there is a lot more high-speed Internet access in areas of the world where TB rates are low, Katz said.
In a similar manner, both CO2 levels and obesity prevalence are increasing, but that does not mean they are related. Katz pointed out CO2 levels are increasing everywhere around the globe, but waistlines are not.
Contrary to the Danish researchers' hypothesis, Sikder said increased activity of the orexin neurons would not lead to obesity. The hormone orexin, produced by cells in the brain, increases appetite, but also increases how many calories you burn, Sikder said. Orexin gives you the best of both worlds in that it "lets you eat more, and lose more at the same time," Sikder said.
Not so simple
The study highlights the fact that the obesity epidemic is a complex issue and has many contributing factors, said Dr. Minisha Sood, an endocrinologist at Lenox Hill Hospital in N.Y.
"As far as obesity goes, I think it's toeing a dangerous line to put blame on the atmosphere," Sood said. "It's also toeing a dangerous line to put the blame on willpower," Sood noted.
However, we should not lose sight of the basics of healthy living, Sood said.
"This should not encourage complacency," Sood said. "Humans need to maintain a healthy, active lifestyle in order to optimize their chances of remaining at a healthy weight."
The hypothesis was described in an article published online March 5 in the journal Nutrition and Diabetes.
Pass it on: A new hypothesis proposes that rising CO2 levels are contributing to the obesity epidemic, but experts remain skeptical of the idea.
Editor's Note: This story has been updated to reflect that the researchers are Danish, not Dutch as we had stated.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.