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The air that makes you fat

2015-12-08 13:38:45

Some puzzling studies appear to show that tiny airborne particles may

contribute to obesity, diabetes and heart disease. Should we be concerned?

8 December 2015

Take a deep breath, and exhale. Depending on where you live, that life-giving

lungful of air might just be pushing you towards diabetes and obesity.

Two people can eat the same foods, and do the same exercise, but one may put on

more weight thanks to the air around their home

The idea that thin air can make you fat sounds ludicrous, yet some extremely

puzzling studies appear to be showing that it s possible. Two people can eat

the same foods, and do the same exercise, but over the course of a few years,

one may put on more weight and develop a faulty metabolism thanks to the

atmosphere around their home.

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. While the short-term effects are minimal, over a

lifetime it could be enough to contribute to serious disease besides the

respiratory illnesses more commonly associated with smog. We are starting to

understand that the uptake and circulation of air pollution in the body can

affect more than just the lungs, says Hong Chen at the University of Toronto,

Canada.

How strong is the evidence from these studies, and should you be concerned?

Without any other change in your lifestyle or diet, polluted air may be causing

you to pile on the pounds (Credit: Getty Images)

Laboratory mice offered some of the earliest concrete clues that the effects of

air pollution may penetrate far beyond the lungs. Their breeder at the Ohio

State University, Qinghua Sun, had been interested in studying why

city-dwellers seem to be at a particularly high risk of heart disease compared

to country folk. Lifestyle, of course, could be one reason: in most major

cities a fast food chain is rarely more than a block away, for instance, which

might encourage unhealthy eating. Nevertheless, he wondered if another answer

may be hanging, invisibly, in the air we breathe.

To find out more, he started to raise laboratory mice in the kinds of

conditions you might find across various cities. Some breathed filtered, clean,

air, while others were funnelled the kinds of fumes you might find next to a

motorway or busy city centre. Along the way, his team weighed the mice and

performed various tests to study how their metabolism was functioning.

After just 10 weeks, the effects were already visible. The mice exposed to the

air pollution showed greater volumes of body fat, both around the belly and

around the internal organs; at the microscopic level, the fat cells themselves

were around 20% larger in the mice inhaling a fine mist of pollutants. What s

more, they seemed to have become less sensitive to insulin, the hormone that

signals to cells to convert blood sugar into energy: the first step towards

diabetes.

Tiny particles irritating the lungs may set off a cascade of reactions

throughout the body, disrupting the hormones that control appetite (Credit:

Science Photo Library)

The exact mechanism is still debated, but subsequent animal experiments suggest

the air pollution triggers a cascade of reactions in the body. Small particles,

less than 2.5 micrometres wide, are thought to be primarily to blame the same

minuscule motes of pollutant that give city air its gauzy haze. When we breathe

in, the pollutants irritate the tiny, moist air sacs that normally allow the

oxygen to pass into the blood stream. As a result, the lungs lining mounts a

stress response, sending our nervous system into overdrive. This includes the

release of hormones that reduce insulin s potency and draws blood away from the

insulin-sensitive muscle tissue, preventing the body from tightly controlling

its blood sugar levels.

Pollution may trigger inflammation that interferes with the hormones and the

brain processing that govern appetite

The tiny irritating particles may also unleash a flood of inflammatory

molecules called cytokines to wash through the blood, a response that also

triggers immune cells to invade otherwise healthy tissue. Not only does that

too interfere with the tissue s ability to respond to insulin; the subsequent

inflammation may also interfere with the hormones and the brain processing that

govern our appetite, says Michael Jerrett at the University of California,

Berkeley.

All of which knocks the body s energy balance off-kilter, leading to a

constellation of metabolic disorders, including diabetes and obesity, and

cardiovascular problems such as hypertension.

By disrupting insulin sensitivity, air pollution may contribute to diabetes and

other serious (and life-shortening) cardiometabolic disorders (Credit: Getty

Images)

Large studies from cities across the world suggest that humans might be

suffering the same consequences. Chen, for instance, examined the medical

records of 62,000 people in Ontario, Canada over a 14-year period. He found

that the risk of developing diabetes rose by about 11% for every 10 micrograms

of fine particles in a cubic metre of air a troubling statistic, considering

that the pollution in some Asian cities can reach at least 500 micrograms per

cubic metre of air. Across the Atlantic, a Swiss study saw a similar signs of

increased insulin resistance, hypertension, and waist-circumference in a sample

of nearly 4,000 people living among dense pollution.

Children growing up in the polluted areas were twice as likely to be considered

obese

The scientists have been particularly concerned about the effects on young

children, with some concern that a mother's exposure to these pollutants may

alter the baby's metabolism so they are more prone to obesity. Consider the

work of Andrew Rundle at Columbia University, who studied children growing up

in the Bronx. During pregnancy, the children s mothers had worn a small

backpack that measured the air quality as they went about their daily business,

and over the next seven years the children s health was monitored at regular

intervals. Controlling for other factors (such as wealth and diet), the

children born in the most polluted areas were 2.3 times more likely to be

considered obese, compared to those living in cleaner neighbourhoods.

Jerrett, meanwhile, has found that the risk can come from inside as well as

outside the home: parental smoking, he showed, also led to faster weight gain

among Californian children and teens. It interacts synergistically with the

effect of the air pollution, he says in other words, the combined risk was

far greater than the sum of individual risks.

Living in a highly polluted city doubled the chance that a child would grow up

obese, according to one study (Credit: Getty Images)

Despite these troubling findings, we should be cautious about reading too much

into them. They only draw a link between exposure and outcome, but can t prove

that one factor causes another, says Abby Fleisch at Harvard Medical School.

Even so, her own findings would seem to agree with the general trend she has

shown that even in the first six months, babies of mothers living in polluted

areas appear to put on weight more rapidly than those in cleaner areas but

she stresses that we still can t be sure we haven t neglected some other

factor, besides pollution, that could explain the apparent link.

As the smog descended, signs of insulin resistance and hypertension peaked

Fortunately, a few teams are already searching for the missing pieces to fill

those gaps in our knowledge with more detailed studies. Robert Brook at the

University of Michigan and colleagues in China, for instance, recently tested a

small group of subjects in Beijing over a two-year period. They found that

whenever the city s infamous smog descended, giveaway signs of developing

problems like insulin resistance and hypertension peaked providing more

concrete evidence that the air quality was indeed driving changes to the

metabolism.

Cleaning up these exhaust fumes could save millions of people from a lifetime

of illness (Credit: Getty Images)

If the link is proven, how concerned should we be? The scientists stress that

the individual, short-term risk to any one person is relatively small, and

certainly shouldn t be used as an excuse for obesity by itself, without

considering other aspects of your lifestyle. But given the sheer number of

people living in cities with high pollution, over the long term the total

number of casualties could be enormous. Everyone is affected by pollution to

some degree, says Brook. It s continuous, involuntary exposure, across

billions of people so the overall impact becomes much greater.

The solutions are familiar, if difficult to implement: restrict traffic

pollution by promoting electric and hybrid vehicles, for instance. Jerrett

suggests streets could also be redesigned to reduce the exposure to pedestrians

and cyclists. In the short term, he points out that air purifiers could be

added to more homes, schools and offices to filter out some of the harmful

particles.

Brook agrees that action needs to be taken internationally, both in the

developing world and in cities like Paris and London that superficially, might

seem to have their pollution under control. In North America and Europe the

pollution levels have been trending in right direction but we shouldn t rest

on our laurels, he says. From the standpoint of improving health across the

world, it should be one of our top 10 worries.