“But there is one problem: the obesity epidemic has developed quite irregularly in time and place, even in a small country such as Denmark, and only a part of the population is affected even though we all breathe the same air,” says Sørensen.

This, I think, says it all. Plus, they are making the usual thermostatic CI/CO argument for obesity.

I think its a promising idea. Here are three reasons to take it more seriously.

1. The CO2 theory works in conjunction with - and not necessarily in opposition to - other theories of obesity. I'm sure you are aware of this, but it is worth noting that in terms of evaluating the CO2 hypothesis, population-level trends on America's east coast are more informative than anyone's personal weight-loss experience after moving there.

2. Since CO2 concentrations decrease with exercise, and increase with sedentary time spent indoors, the theory complements and enhances the idea that activity levels or exercise contributes to weight control.

3. The CO2 hypothesis fits nicely with the ideas of paleo thinkers (eg, Taubes, Kruse, Guyenet) who emphasize the role of hormones and the brain in obesity. Taubes stresses the idea that obesity is a consequence of dysfunctional hormonal regulation, and that we need to focus on variables that affect hormones. Guyenet's focus on food reward ties directly into how brain signaling affects appetite - and thus consumption - levels. And Kruse's leptin reset rests entirely on a theory about rewiring brain activity in the hippocampus, which is the brain region that the CO2 researchers explicitly target in their pilot study.

If you don't feel like reading the article, there's really only a few sentences you need to read:

"This discovery made it possible to develop a precise hypothesis for how CO2 makes us fatter: We breathe more CO2, which makes our blood more acidic; this affects our brain, so we want to eat more."

“If you’re out running, you get your blood circulating and you can pump much of the CO2 out of your body, so our hypothesis is really further evidence that exercise is healthy. And exercise may be even more necessary in the future, when we can expect even higher CO2 concentrations in the atmosphere.”

Hersoug adds that fruit and vegetables also reduce the blood’s pH value, so the CO2 theory is also an argument for eating more healthily."

As far as I can tell, there's not really going to be a way to disprove the hypothesis. It's a pretty useless hypothesis, though. And I think the idea that exercising results in CO2 release through higher blood circulation is simply wrong. I think it's possible that exercising can result in greater CO2 release, but via increasing pulse rate seems ridiculous to me. For example, my pulse rate is around 50 at rest, so say I spend an hour exercising with an average pulse rate of 160 (which would be a pretty intense hour, just for the example) and with all of my time not resting (walking, etc.) I have a pulse rate of 80 for a cumulative 4 hours of the day. This would make for 85,800 beats per day, roughly. A person who does not exercise at all and doesn't even get out of bed would only need a baseline pulse of 60 bpm throughout the day to have more beats than me. I'd even guess that the typical obese person has a pulse rate between 90 and 110. I realize that there's more to oxygen delivery and CO2 removal than simple beats per minute, but the idea that blood circulating is why exercising is good for you is pretty ridiculous in my opinion.

Ultimately, though, the hypothesis is useless because it doesn't really provide any remedies that we don't already use for better reasons (exercising and eating fruits/vegetables).

I think they answer it in the last line 'They hope to be able to raise money for further, more comprehensive trials of e.g. the role different types of nutritious diets play together with CO2.'

They hope to get money for more trials. Probably from the global warming/climate change crowd. One of the arguments is that there's little anti-global warming research, and the counterargument is "that's because all the money for research is pro-global warming, and if you want grants, you've got to research things that are pro-global warming".

This seems to be one of those items "see, increased CO2 from humans is making the world hot AND fat, give us money to research more!"

Maybe I'm just cynical.

Leaving aside the bits about blood CO2 / lowered pH affecting orexins and then metabolism downstream (because I don't know if they're right about that or not), blaming obesity on atmospheric CO2 going from 200ppm to 285ppm or whatever it's done in the last 200 years is a little silly. That's a tiny amount of CO2. They've neglected a huge source of CO2 in carbonated soft drinks. Even if the hypothesis turns out to be valid, I think it will still just be one factor.

In thinking about this, I think they've got something. People who drink a lot of sodas are fat too. Sodas have a lot of CO2 in it. Ergo CO2 causes obesity! Right? Right? Yeah, correlation <> causation...

They ate a little bit more calories and had higher pulse rates which is a good indicator that metabolism increased. The scientists assume the higher calories will result in weight gain which is not proven at all and the study is being sensationalized by the media like every new theory about obesity.

I'm coming out with an ebook soon on CO2's benefits.

Nope. Mostly because CO2 seems to be beneficial and conducive to life. You can start your own investigation by looking at the CO2 levels during the Cambrian explosion. Oxygen in the atmosphere was much lower than it is now and CO2 much higher.

If CO2 levels are increasing as a percentage of what we breathe, doesn't that mean that other atmospheric gasses are declining as a percentage? Whu attribute an effect to CO2's increase, and not to other gasses declining? Just asking as a scientist.

You wrote:

"Apparently people will believe fat, protein, animal foods, and now CO2 makes people fat but not carbohydrates, inflammation, or dysbiosis ..."

Those are some sweeping generalizations. Hydroganted cottonseed and soybean oil are not obesigenic? Broccoli and root vegetables are? Please stop blaming "carbohydrates," broadly, as neolithic agents of disease. If actually do think that carbohydrates are inherently fattening, then you should wonder why hunter gatherer's become leaner when eating MORE starchy tubers (a carbohydrate) and LESS animals (protein and fat) (1), and why a 100% potato diet results in improved body composition, health markers, and weight loss (2).

1. http://onlinelibrary.wiley.com/doi/10.1002/ajpa.21040/abstract

2. http://wholehealthsource.blogspot.com/2010/12/interview-with-chris-voigt-of-20.html

Oh, this is obviously not an answer, just it was too long to put in the comments, and I was compelled to write because I did not want this ignorance persisting (down vote me, I do not care).

You should also wonder that why, since the 1860s, carbohydrates have declined as a percentage of calories, and yet, people have been getting consistently fatter since then (I don't have the source, but you can find it on one of the slides presented by Guyenet on one of his TED talks).

To simultaneously answer your question about paleohacks "losing steam," perhaps it is, and that would be because it is being overrun by people who prefer dogma, pseudo-science, and confirmation bias over objectivity, relevant science and studies, and actual real life results.

Fermented foods make you fat then