Scientists say it provides new reasons

Breathing polluted air may be doing more than simply harming your lungs, it could also be making you overweight.

Rats that were left to breathe the smog which envelops Beijing for three weeks gained more weight than those who were given clean air, even though they were fed the same diet.

The findings suggest pollution in the air could be altering the body's metabolism so that it stores fat in a different way.

Scroll down for videos

Beijing is one of the most polluted cities in the world and the authorities there regularly issue smog alerts (pollution in Beijing pictured). A new study has shown breathing the air in Beijing can cause rats to gain weight and develop other symptoms such as insulin resistance and high cholesterol levels that are related to obesity

It may also help to explain why developing nations, which have undergone rapid industrialisation, are finding themselves struggling with an obesity crisis.

Most research on obesity has focused on the role that diet and a lack of exercise can have on the body, but this new study suggests other aspects in the environment may also play a role.

HOW POLLUTION CAUSES OBESITY Air pollution has been linked to increased inflammation in the human body and in animals. It is thought tiny particles of soot and dirt called PM2.5s may be able to cross into the bloodstream through the lungs. As a foreign body, it is thought these then trigger a low level immune response through out the body that causes inflammation. Other aspects of pollution, including nitrogen dioxide, have also been linked to increased inflammation. Scientists believe this increase in inflammation may be then leading to a range of diseases including harm to the cardiovascular system, stiffening of blood vessels. This could also be altering the way the body's metabolism works. Chronic inflammation has been linked to obesity in other studies and is thought to play a role in the disease. Advertisement

Professor Jungfeng Zhang, an expert on global and environmental health at Duke University who led the research, said pollution is known to increase levels of inflammation in the body.

He said: 'Since chronic inflammation is recognised as a factor contributing to obesity and since metabolic diseases such as diabetes and obesity are closely related, our findings provide clear evidence that chronic exposure to air pollution increases the risk for developing obesity.

'If translated and verified in humans, these findings will support the urgent need to reduce air pollution, given the growing burden of obesity in today's highly polluted world.'

The researchers, whose study is published in the Journal of the Federation of American Societies for Experimental Biology, placed pregnant rats and their offspring in one of two chambers.

One was exposed to outdoor air in Beijing and the other was fitted with an air filter to remove most of the pollution particles.

After 19 days the lungs and livers of the pregnant rats exposed to the pollution were found to be heavier and showed increased inflammation than those who had clean air.

The relationship between diet, exercise and obesity is well known, but breathing in polluted air could also be playing a role by disrupting the body's metabolism, according to the study in rats (stock image pictured)

Rapid industrialisation in many developed countries has lead to massive increases in air pollution (stock picture of factories in China). The new study, if translated to humans, could help explain why these countries are struggling with obesity too

The scientists report the rats who had polluted air also had 97 per cent higher total cholesterol, with LDL cholesterol 50 per cent higher than in the animals who had clean air.

Their insulin resistance was also higher than those who had breathed clean air, suggesting they were at risk of developing Type 2 diabetes.

The pollution exposed rats were also 'significantly' heavier at the end of their pregnancy even though they were given the same diet.

Similar results were shown in the offspring, which were kept in the same chambers as their mothers.

Professor Zhang and his colleagues said the findings show air pollution results in 'metabolic dysfunction' which can lead to obesity.

However, the researchers said the negative effects of air pollution were less pronounced after three weeks than after eight weeks.

Air pollution (pictured drifting over the Yellow Sea and east coast of China) is known to play a role in lung diseases and is thought to increase the risk of cardiovascular disease. The new study provides new evidence that it could also be increasing the risk of obesity too

At eight weeks old, female and male rats exposed to the pollution were 10 percent and 18 percent heavier, respectively, than those exposed to clean air.

This suggests long-term exposure may be generating continuous inflammation and metabolic changes.

The researchers said: 'In a rodent model, we found that breathing Beijing's highly polluted air resulted in weight gain and cardiorespiratory and metabolic dysfunction.

'Compared to those exposed to filtered air, pregnant rats exposed to unfiltered Beijing air were significantly heavier at the end of pregnancy.'

The findings come amid growing concern among the authorities in China at the levels of air pollution in its major cities.

China regularly issues smog warnings in its capital Beijing as the city becomes choked with pollution. It is now considered to be one of the most heavily polluted cities in the world.

The city was heavily criticised in the run up to the Beijing Olympics in 2008 over fears of the impact the pollution would have on athletes and spectators.

Tiny particles in air pollution known as PM2.5s, which are made up of soot and dirt from burning of fossil fuels and wood, have been linked to a wide range of lung and heart conditions.

They have also been linked to increased inflammation in the body.