Tuesday, December 8, 2015

Pollution may kill you, but it won't make you fat

There's a strange phenomenon often seen in certain branches of science, especially in medicine and public health. Despite a mountain of evidence lending its support to a particular theory, government agencies, universities and individual researchers devote their resources to investigating alternative and obviously ridiculous explanations of the same data.

The archives here are filled with examples, but I saw an example just earlier today that deserves special mention.  From the BBC:

The idea that “thin air” can make you fat sounds ludicrous, yet some extremely puzzling studies appear to be showing that it’s possible...Traffic fumes and cigarette smoke are the chief concerns, with their tiny, irritating particles that trigger widespread inflammation and disrupt the body’s ability to burn energy.
This story has all the hallmarks of a classic junk science write up: dramatic claims about some threat to human health bolstered by animal studies and questionable epidemiology, quotes from medical authorities who should know better, and claims that immediate action is needed to protect vulnerable populations--typical misuse of the precautionary principle.

On its face, a causal link between air pollution and obesity isn't impossible. The BBC piece provides a plausible mechanism by which the former could cause the latter, so why is this story so ridiculous? I can think of a few reasons.

Recent data from the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite shows conclusively that the air quality over the United States continues to improve, as it has since 1980, according to estimates published by the EPA. Recent public health research, meanwhile, shows that obesity in America has exploded since the 1980s. The point should be clear. The air is getting cleaner and people are getting fatter; there really is no link between pollution and obesity. We can even reverse the numbers to make the same argument stronger. Obesity rates in China linger below 5% yet the country remains one of the world's biggest polluters.

Looking at these global numbers gives us context for the smaller studies cited by the BBC. For example, researchers at Harvard Medical School found that babies of mothers living in polluted areas put on weight faster than babies living in healthier environments. That would seem to support the obesity-pollution hypothesis without the above numbers in mind, but something in the local environment probably offers a better explanation for the rapid weight gain. Poverty is a good candidate, since poor people have fewer nutritional options. They also tend to live in polluted environments compared to wealthier populations. Indeed the data supports this view of the matter: The poorest Americans are also the fattest.

Likewise, the risk of diabetes may go up 11% for every 10 micrograms of fine particles in a cubic meter of air, as one study claims; the same may be true of insulin resistance, as reported by another paper cited by the BBC. But the researchers are chasing down the wrong association in all three cases, and they're relying on relatively small sample sizes in the process.

The BBC piece also claims that secondhand smoke enhances the obesity-promoting effects of air pollution, citing an observational study conducted in southern California as evidence, weak as it may be. But leaving aside the inherent difficulties in trying to model the effects of secondhand smoke on the BMI of 4,000 toddlers, the association runs into the same problem as before. Smoking has declined tremendously since the 1960s  in the U.S. while obesity rates have done the exact opposite. Meanwhile, smoking in developing nations like China is slightly increasing every year while obesity remains low.

Full of errors and inconsistencies as it is, the strangest thing about the BBC article was this qualifier towards the end: "The scientists stress that ...[pollution] shouldn’t be used as an excuse for obesity by itself, without considering other aspects of your lifestyle." That's true, of course. If you're obese, your morning pumpkin spice latte and doughnut are more to blame than whatever amount of smog you're exposed to.

But since we know conclusively that bad genetics, poor diet and limited exercise can make one person fat, we know that those factors can also make an entire population of people fat. So why, then, are we supposed to hyperventilate about the speculative threat posed by air pollution?

The article ends reasonably enough with a call to reduce air pollution. That's undoubtedly a good idea since it means saving thousands of lives every year and mitigating substantial damage to the environment, especially in the developing world. But blaming obesity, even partially, on air pollution gets us no closer to that goal. Besides, it's just bad science.