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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.