The team of researchers found that exposure to air pollution caused mice to experience changes in the normal composition of gut bacteria. This produced a cascade of negative health effects: changes in gut bacteria promoted the circulation of cholesterol in the blood stream, and the increased presence of cholesterol in the blood stream promoted the formation of plaque in the arteries (known as atherosclerosis).
Prior epidemiological studies have associated air pollution with gastrointestinal disorders and with cardiovascular diseases.
This new research in mice is the first to determine the mechanism by which this may happen and the link between the two. The findings suggest that the gastrointestinal and cardiovascular issues are not separate but rather are intertwined; damage to the arteries may be due to changes in gut bacteria.
The gut biome has become a subject of more research in recent years as scientists learn more about its vital connection to other organ systems in the body, including the brain. Studies suggest that a diverse array of microbiota in the gut is linked to a variety of health benefits, and that changes in the gut can have effects on other organ systems.
For the study, the researchers tracked mice on a high-fat diet who ingested ultrafine particles of air pollution three days per week for 10 weeks. They found that changes in the gut due to air pollution while on a high-fat diet produced changes in the vascular system, including plaque buildup in the arteries. In humans, such buildup can impede blood flow and potentially lead to blood clots – or stroke.
The team of researchers led by Dr. Tzung Hsiai, professor of medicine and bioengineering at the David Geffen School of Medicine, comprised faculty from a variety of departments including Dr. Zhaoping Li, chief of the Division of Clinical Nutrition; Dr. Jonathan Jacobs in the Vatche & Tamar Manoukian Division of Digestive Diseases; Dr. Joseph Pisegna, chief of gastroenterology and hepatology at the VA Greater Los Angeles Healthcare System; and Rongsong Li and Mohamad Navab in the Division of Cardiology.
While a mouse model cannot provide a definitive look at how air pollution impacts humans, the study points to some avenues for future research.
"This is a clear example of how changes in gut bacteria by environmental factors could account for the increasing prevalence of inflammatory bowel diseases, primarily in large metropolitan areas," said Pisegna, who is a professor of medicine at the David Geffen School of Medicine. "With more research, we may be able to reduce inflammatory disease by improving air quality."
Next, the researchers will be taking a closer look at how different compositions of gut bacteria prior to air pollution exposure may impact results. Such research would shine light on the effects of antibiotics, which wipe out certain bacteria in the gut.
In any case, research showing a causal relationship in humans could have serious implications for public health.
"This research tells us that we need to be even more cautious of our daily exposure to air pollution," said Hsiai. "It also tells us we have a lot more to learn."