The Award for the Most Unusual Explanation for the Obesity Epidemic Goes to...

A proposed potential role for increasing atmospheric CO2 as a promoter of weight gain and obesity

Human obesity has evolved into a global epidemic. Interestingly, a similar trend has been observed in many animal species, although diet composition, food availability and physical activity have essentially remained unchanged. This suggests a common factor—potentially an environmental factor affecting all species. Coinciding with the increase in obesity, atmospheric CO2 concentration has increased more than 40%. Furthermore, in modern societies, we spend more time indoors, where CO2 often reaches even higher concentrations. Increased CO2 concentration in inhaled air decreases the pH of blood, which in turn spills over to cerebrospinal fluids. Nerve cells in the hypothalamus that regulate appetite and wakefulness have been shown to be extremely sensitive to pH, doubling their activity if pH decreases by 0.1 units. We hypothesize that an increased acidic load from atmospheric CO2 may potentially lead to increased appetite and energy intake, and decreased energy expenditure, and thereby contribute to the current obesity epidemic.

Stranger things have happened. I particularly thought that the following bit was good:

Coinciding with the sharp increase in the prevalence of human obesity over the last 50 years, weight gain has also been observed in animals. A recent study found that 24 populations of 8 different species, including laboratory animals that had been fed the same diets for decades, all displayed significant weight gain.3 This suggests that a shared environmental factor, favouring weight gain, may be involved in the regulation of energy balance. Such a factor has yet to be identified.

As I asserted previously, decent explanations for the obesity epidemic should take into account the obesity epidemic in pets, or as the authors above note, in lab animals which have apparently been fed the exact same food for decades.

Another thing that a decent explanation must take into account is the global nature of the epidemic. While the U.S. could long claim to be No. 1 in obesity, many countries are catching up, even countries such as China and India. One explanation has been that the U.S. government's low-fat dietary advice has caused the epidemic. (I've used that explanation myself.) But have the Chinese or the Indians been following our government's dietary advice? Highly doubtful.

The CO2 hypothesis above makes some, at least preliminary, sense. But I shudder to think what the IPCC will do with this.