Steadily rising levels of carbon dioxide in the atmosphere may be affecting brain chemistry and contributing to the obesity epidemic, according to a new hypothesis, which still awaits rigorous testing and debate.
The idea proposes that breathing in extra CO2 makes blood more acidic, which in turn causes neurons that regulate appetite, sleep and metabolism to fire more frequently. As a result, we might be eating more, sleeping less and gaining more weight, partly as a result of the air we breathe.
Major studies are in the works to test the hypothesis, which is still very much in the what-if stage. But if the link pans out, the research would offer yet another reason to reduce the CO2 we produce, while also potentially inspiring new obesity treatments.
"If it turns out that people are increasing their food intake due to this mechanism of global warming and greenhouse gas emissions, we would suddenly be getting a new dimension," says Arne Astrup, head of the department of obesity and nutrition at the University of Copenhagen. "This could give us an explanation for why the entire population on this planet is increasing in body weight as soon as there is available food."
"What we're doing is trying to be certain that we are not overlooking any very important factors that could be partly responsible for the obesity epidemic," he adds. "I think we should be extremely open-minded about new causes of obesity."
Calories in, calories out
Obesity and its associated health risks have escalated dramatically in the last few decades. And even though just about everyone thinks the reason is obvious - we are eating too many calories and exercising too little - research has revealed that obesity is far more complex than that, with multiple genes, metabolic pathways and even gut microbes involved, says obesity researcher David Allison, director of the Nutrition Obesity Research Center at the University of Alabama at Birmingham.
Most of the steps that governments have taken to address the problem, Allison adds, have failed to keep the kilos off, with studies showing no improvement from policies that add fruits and vegetables to menus, for example, or that give kids more time for physical education.
The balance between calories in and calories out clearly makes a difference in how much fat a person's body holds on to, he says. But it's probably not everything. And creative hypotheses are worth exploring.
"We need to be open-minded so that we don't in some obsessive fashion continue to pound our heads against the same wall over and over again under the assumption that it is the right path and the only path and that it must make sense, ignoring the fact that we keep doing it and it's not working," says Allison. "I'm not saying that it's calories or this that matters. It's that there are other factors that influence metabolism and body weight."
Weight gain across species
In a study published in 2010 in Proceedings of the Royal Society B, Allison and colleagues found that 24 populations of eight species had gained weight in the last 50 years, including lab animals that were kept in highly controlled laboratory settings and had been eating the same diet year after year.
One theory, currently under intense scrutiny, is that endocrine-disrupting chemicals in the environment or the food supply are playing a role in cross-species weight gain.
CO2 levels in the air have risen alongside obesity rates, Astrup wrote recently in the journal Nutrition and Diabetes. For two million years, atmospheric CO2 held steady between 180 parts per million and 280 ppm until about 60 years ago. Since then, levels have risen to almost 400 ppm with a projected increase to 550 pm by 2050. Indoor CO2 levels are often higher than outdoor measurements, and people spend most of their time in buildings.
Studies show that our blood becomes more acidic when we breathe in CO2-laden air for just a few weeks, Astrup wrote in his article. And it only takes a 0.1-unit drop in pH to double the firing rate of appetite and wakefulness-related neurons. One human study found that pH levels fell by 0.05 units with exposure to 7000 ppm of CO2 - more than 17 times current CO2 levels.
It's still far from clear whether the amount of CO2 we are currently exposed to is enough to make a difference, or whether exposure at specific times in development make more or less of a difference.
But a pilot study of six Danish men who spent time in respiration chambers showed that testing the idea might, at least, be possible. After seven and a half hours of breathing air with a level of 8000 ppm of CO2, three of the men ate more kilojoules than they did after breathing unaltered air - though the finding was not statistically significant and bigger studies are needed.
New path towards weight control
If, in addition to an ever-present abundance of soft drinks and fast food, air pollution turns out to be influencing our eating behaviours, it might be possible to make a small dent in the obesity epidemic by lowering ambient CO2 levels or capturing the gas in indoor environments.
At the very least, says Allison, investigating the idea could expose chemical pathways in the brain that might open opportunities for drug treatments. Scepticism is clearly warranted, but he says, the research is worth doing.
"This really interesting novel idea may very well turn out to be wrong, and I would not be shocked at all if it did," says Allison. "But some small fraction of novel ideas turns out to be right. And when we find them, they often open up incredible new vistas."