Chronic exposure to air pollution particles increases the risk of obesity and metabolic syndrome.

This time I am going to look at a recently published and widely publicised study that has experimentally shown an association between exposure to particulate air pollution and increase in weight: “Chronic exposure to air pollution particles increases the risk of obesity and metabolic syndrome: findings from a natural experiment in Beijing” by Wei Yongjie et al (1). This article does get pretty technical at times, I have tried to use as simple language and explain the findings and their relevance. Science Daily have also summarised this article (2).

This study used animal experimentation; I know this can be a difficult area for some people. When used to improve scientific knowledge which may benefit health, and where there are not alternatives to using live models; and most importantly where ethical standards for the treatment of animals are adhered to, I believe that this type of study is a valuable resource. In this study all animal experimentations were conducted in compliance with the guidelines of ethical animal research.

Before I look at the paper in detail I will just cover the basics of metabolic syndrome, obesity and heart disease and how these conditions are thought to overlap. Metabolic syndrome is the group of risk factors that raises your risk for heart disease and other health problems, such as diabetes and stroke. These risk factors include obesity, particularly in a central distribution, a high triglyceride and low HDL cholesterol levels, high blood pressure and raised blood sugar. A combination of three of these would be defined as metabolic syndrome. The term “metabolic” refers to the biochemical processes involved in the body’s normal functioning. Heart disease can be the result of metabolic syndrome, in this instance “heart disease” refers to coronary heart disease, a condition in which plaques build up in the inside of inside the coronary (heart) arteries and can lead to angina or a heart attack.

So how does any of this link to air pollution?

The study was performed because epidemiological evidence has shown that air pollution is a risk factor for childhood obesity (3). For example in a study from the US a significant positive association with attained body mass index (BMI) at age 18 and traffic density within 150m was observed. This study accounted for a variety of confounding factors which might also cause increased levels of childhood obesity for example social class, race and parental education level. They also separated the effects of the traffic density from other causes of obesity for example poor diet and sedentary lifestyle. However what this study and others like it did not indicate was whether the BMI increases observed were via increased susceptibility to diet-induced weight gain, for example increased hunger or other drivers which increase dietary intake, or via metabolic and inflammatory changes caused by the air pollutions effects on the body. The study that I am looking at today tried to answer that question in an experimental setting.

As we are all aware concentrations of particulate matter (PM2.5) in Beijing’s air are high. In this study they used Beijing’s ambient air to examine whether living in this environment can lead to weight gain and what processes led to any weight gain seen. As a control they used the same air but with the particulate matter removed, using a HEPA (high-efficiency particulate air) filter, this filter did not remove any gaseous pollutants contained in the air. By using the ambient air the animals used in the experiment were exposed to the same daily fluctuations in air pollutants as someone living in the city. The experiment was done about 2km from the northwestern fourth ring road, a major artery for the city which carries nearly 220 000 vehicles per day.

I won’t go into great detail about how the experiment was set up as it does not alter the conclusions I will draw. However here is a brief summary: the animals were split into two groups, one group housed in a room with the air inlet coming directly from outside, the experimental group; the other group housed in a room with a HEPA filter at the air inlet. The animals were weighed on entering the room and then every 3 days afterwards until the experiment finished on day 14. Additionally blood was taken and analysed for markers of inflammation and immune response. These tests included serum triglyceride and cholesterol levels which are well known for their association with cardiovascular health.

Results

Over the 2 week period the PM2.5 concentrations were averaged to approximately 73.5microgram/m3 in the experimental room and 19.8 microgram/m3 in the filtered room. There were large daily and day-to-day fluctuations in the PM2.5 measurements. There was a statistically significant increased weight gain in the experimental group. The difference in weight gain occurred over a relatively short period of time, with a 7% difference after just two weeks. The experimental group also showed signs of inflammation and immune response not seen in the control group. For example there were signs of inflammation in the linings of the lungs. The experimental group also showed a significantly worsened plasma lipid profile compared to the control. LDL cholesterol was 50% higher in the experimental group, triglycerides were 46% higher and total cholesterol was 97% higher. The level of insulin resistance, a precursor to type 2 diabetes, was also higher than the control group.

The results indicate that exposure to Beijing’s unfiltered air causes an inflammatory stress response and this response seems to be linked to changes associated with metabolic syndrome. There seemed to be a tendency for the adipose (fat) tissue to move towards an inflammatory status that may be linked to metabolic dysfunction.

Discussion

The way in which airborne particles may cause metabolic dysfunction is important in understanding the role of air pollution in cardio-metabolic diseases. This study attempted to elucidate whether and how inhaled particles could lead to non-diet induced weight gain and how any weight gain relates to metabolic changes. It makes sense that breathing in chemical particles would cause an inflammatory response as they will irritate the lining of the lungs. However the spillover to the whole body is less intuitive. The findings of the study are consistent with previous studies including a thorough review of available evidence produced by the American Heart Association in 2010 (4). These studies report that PM2.5 exposure induces oxidative stress and inflammation in organ tissues and the circulatory system. Their summary concludes that exposure to PM2.5 has a causal relationship with cardiovascular morbidity and mortality.

In conclusion what we seem to be seeing here is an inflammatory response to toxins in the air, that does not just stay at the site of exposure (the lungs) but spills into other body systems. It seems that this inflammation is closely linked to factors which have been considered to be metabolic, and hence cause weight gain. The study provides mechanistic evidence that chronic exposure to air pollution increases the risk of developing obesity and metabolic syndrome. It seems to add another mechanism by which air pollution can disrupt our general, cardiovascular and metabolic health. Adds weight to the argument for an inflammatory process being at least in someway responsible for the changes seen in metabolic syndrome, therefore metabolic syndrome can not only be put down to the traditional lifestyle choices of poor diet and lack of exercise. But that other environmental factors such as the air we breathe are likely to have a role.

If these findings are translated to and verified in humans, they add support to the urgent need to reduce air pollution exposure, given the growing burden of obesity and related metabolic abnormalities in today’s highly polluted world.

What does this study mean for people living in high pollution environments?

This study indicates that it is at least likely that people exposed to high levels of particulate air pollution are exposed to another health danger in terms of their risk of developing metabolic syndrome, obesity, diabetes, stroke and heart disease. So not only should we be protecting ourselves by eating healthily, exercising, stopping smoking, modifying risk factors such as high blood pressure and lipid profile we should also be considering the air we are breathing. The use of a HEPA filter in the home and air-pollution masks in polluted cities should reduce some of these risks.

Glossary

Epidemiological — epidemiology — the study of populations

Confounding factors —

Body mass index —

statistically significant

oxidative stress

morbidity and mortality

aetiology

References

1. Chronic exposure to air pollution particles increases the

risk of obesity and metabolic syndrome: findings from a

natural experiment in Beijing

2. Science Daily. Exposure to air pollution increases risk of obesity. If findings of rat studies are verified in humans, urgent action needed. (2016) Accessed from URL: https://www.sciencedaily.com/releases/2016/02/160219111219.htm. Accessed 3rd March 2016.

3. Automobile Traffic around the Home and Attained Body Mass Index: A Longitudinal Cohort Study of Children aged 10–18 Years

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4334364/

4. Brook R, Rajagopalan S Pope III C et al. Particulate matter air pollution and cardiovascular disease: an update to the scientific statement from the American Heart Association. 2010. Circulation 121, 2331-78. Accessed from URL: http://circ.ahajournals.org/content/121/21/2331.full.pdf+html. Date accessed 9th March 2016.