The microbes that reside in our intestines, collectively known as the gut microbiome, have a role in our gastrointestinal health. Gut microbes also influence the immune system, the endocrine system, our mental health and the development of several health conditions – including inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), Crohn’s disease, cardiovascular disease, obesity and type 2 diabetes.
Previous studies have shown that exposure to airborne pollutants in adult humans and in animal models is associated with changes in the composition and function of the gut microbiome. There have not, however, been any investigations into the potential effects of air pollution on the development of the gut microbiome in infants. This community of microorganisms develops within the first two to three years of life and is influenced by various factors.
A new study led by researchers from the University of Colorado at Boulder has now shown that the quality of air inhaled by an infant during its first six months of life may be one of these factors. The study, published in the journal Gut Microbes, is the first to show a link between inhaled pollutants – such as those from traffic, wildfires and industry – and changes in infant microbial health during this critical window of development.
“This study adds to the growing body of literature showing that air pollution exposure, even during infancy, may alter the gut microbiome, with important implications for growth and development,” said study senior author Tanya Alderete, assistant professor of Integrative Physiology at CU Boulder.
Newborns have a relatively small population of resident gut microbes, but this changes rapidly in their first months of life. Exposure to mother’s milk, solid food, antibiotics and other environmental influences shapes the composition of the microbial community that becomes established in the gut. Within the first two to three years of life, a child’s microbiome becomes fully developed.
Those microbes, and the metabolites, or by-products, they produce when they break down food or chemicals in the gut, influence the bodily systems that affect , among other things, appetite, insulin sensitivity, immunity, mood and cognition. While many are beneficial, some microbiome compositions have been associated with diseases and chronic illnesses.
“The microbiome plays a role in nearly every physiological process in the body, and the environment that develops in those first few years of life sticks with you,” said first author Maximilian Bailey, who graduated in May with a master’s in Integrative Physiology and is now a medical student at Stanford University.
In order to obtain data on infant gut microbes, the researchers collected fecal samples from 103 healthy, six-month old, primarily breast-fed Latino infants who were enrolled in the Southern California Mother’s Milk Study. They then used genetic sequencing techniques to identify the species of bacteria that were present in the samples.
Using their street addresses and data from the U.S. Environmental Protection Agency’s Air Quality System, which records hourly data from monitoring systems, the researchers estimated exposure to air pollution for each infant. Specifically, the researchers considered levels of PM2.5 and PM10 (fine inhalable particles from things like factories, wildfires and construction sites) and Nitrogen Dioxide (NO2), a gas largely emitted in vehicle exhaust fumes.
The results of the analysis showed that increased exposure during the first six months to the air pollutants considered was associated with changes in the infant gut microbiota at multiple taxonomic levels. Different genera and species of microbes were characteristic of infants exposed to higher levels of air pollution.
Many of these infant gut bacteria associated with exposure to polluted air have previously been linked with adverse health outcomes such as systemic inflammation, gastroenteritis, multiple sclerosis, and mental health disorders. Further, several of the identified taxa are involved in the production of important gut microbial derived metabolites that play an integral role in human physiology, state the authors.
“Overall, we saw that ambient air pollution exposure was associated with a more inflammatory gut-microbial profile, which may contribute to a whole host of future adverse health outcomes,” said Alderete.
For instance, infants with the highest exposure to PM2.5 had 60 percent less Phascolarctobacterium, a beneficial bacterium known to decrease inflammation, support gastrointestinal health and aid in neurodevelopment. Those with the highest exposure to PM10 had 85 percent more of the microorganism Dialister, which is associated with inflammation.
The researchers state that inhaled pollutants have the potential to reach the gut because they first become trapped in mucus in the respiratory tract, after which ciliary action moves the mucus up to the back of the throat, where it is ingested down the esophagus and into the gut. Infants are particularly vulnerable to the health hazards of air pollution because they breathe faster and their gut microbiome is just taking shape. “This makes early life a critical window where exposure to air pollution may have disproportionately deleterious health effects.”
It is often people from racial minority groups and low-income communities who tend to work, live and attend school in areas closer to busy highways or factories, placing them at even greater risk. One 2018 Environmental Protection Agency study found that communities of color are exposed to as much as 1.5 times more airborne pollutants than their white counterparts.
“Our findings highlight the importance of addressing the impact of pollution on disadvantaged communities and point to additional steps all families can take to protect their health,” said Alderete, who hopes her research will influence policymakers to move schools and affordable housing projects away from pollution sources.
In terms of the steps families can take to protect their health, Alderete advises the following to reduce exposure to both indoor and outdoor pollutants:
“Breast milk is a fantastic way to develop a healthy microbiome and may help offset some of the adverse effects from environmental exposures,” Alderete said.
According to the researchers, their results suggest air pollution may be an important factor in the development of the gut microbiome. The findings have significant public health relevance since 99 percent of the world’s population is estimated to live in areas where air quality guidelines are not met.