During the past 60 years, experts have documented a steep rise in the incidence of both ulcerative colitis and Crohn’s disease—the two medical conditions that make up most cases of inflammatory bowel disease (IBD). For decades, this rise was confined to North America, Western Europe, and other industrialized nations. While there’s some evidence that the rise in IBD has slowed down or even plateaued in those places, IBD is becoming increasingly common in newly industrialized countries in Asia and other parts of the world.
There’s no question that genetic factors play a part in a person’s risk for inflammatory bowel disease—and especially for Crohn’s disease. But the increases in IBD incidence and the disease’s pronounced geographical patterns strongly suggest that environmental factors are also at play. “Following World War II, we’ve seen a rapid rise in IBD incidence throughout the developed world,” says Dr. Gilaad Kaplan, a professor and gastroenterologist at the University of Calgary in Canada. “Something about a Western lifestyle seems to be allowing this disease to flourish.” What is that something? That’s the unsolved mystery.
There are several theories—or rather, suspects. Researchers have found associations between IBD and air pollution, food additives, early-life antibiotic exposures, and other environmental variables. Kaplan says that several of these risk factors, not just one, likely underpin the increases in IBD. And they all have one thing in common: the gut microbiome. “Most people feel that what’s driving the inflammatory response we’re seeing, where the body’s immune system attacks the bowels, lies in the intestinal microbiome,” he says.
Your gastrointestinal tract is populated by billions of microorganisms that are critical to the health and functioning of your gut. These bacteria help digest the foods you eat, and the metabolites they produce help regulate your immune system. Kaplan says that a robust and diverse microbiome is a hallmark of a healthy GI tract, while anything that disrupts or imbalances the microbiome is associated with GI dysfunction, including IBD. “A lot of the environmental risk factors that have been studied widely are now being looked at through the lens of the microbiome,” he says. This new perspective is yielding important insights, including some related to the treatment of IBD.
Here you’ll find a rundown of the environmental risk factors that researchers have linked to IBD, as well as expert guidance on limiting those risks.
Air pollution and IBD
For a first-of-its-kind study published in 2010, researchers examined the association between ambient air pollution and the incidence of IBD. They found that young people who had grown up around high concentrations of nitrogen dioxide were more than twice as likely as other kids to develop Crohn’s disease.
Since that groundbreaking study, more work has linked air pollution to higher rates of IBD. “We’ve found that early-life exposures to both nitrogen dioxide and ozone are associated with increased risks,” says Eric Lavigne, a senior epidemiologist at Health Canada (the Canadian government’s equivalent of the U.S. Department of Health and Human Services).
Both of these pollutants are associated with automobile traffic. Fuel-burning cars and trucks emit nitrogen dioxide in their exhaust. When that nitrogen dioxide mixes with heat and sunlight, it undergoes a chemical reaction that produces ozone. “In areas where there’s a lot of traffic, we might see elevated levels of this combination,” Lavigne says. “Living in close proximity to those areas may be a risk factor for IBD.”
How could air pollution affect gut health? Research has shown that after breathing in pollutants, the lungs may actually push these out into the throat so that they’re swallowed. This process is known as mucociliary clearance. Once in the gut, Lavigne says, these pollutants may cause damage to the gut’s microbiota in ways that promote inflammation.
Based on his and others’ work, he says that air pollution exposures during childhood—not in utero or in adulthood—seem to pose the greatest IBD risks. Keeping away from heavily trafficked roads, especially on hot sunny days, is one way to avoid these risks. “Levels of these pollutants are highest within 50 meters”—about 160 feet—“of busy roads,” he says.
Lavigne has also looked at the effect of parks and other urban green spaces on air-pollution risks. His research found that kids who grew up near green spaces were at reduced risk for IBD. “Particles in the air may get trapped by the leaves from trees, and so having more trees and greener environments might actually create a buffer that reduces people’s exposures,” he explains.
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Food choices and dietary exposures
The stuff you swallow can affect the composition of your microbiome, and therefore your gut health. And researchers have identified a number of diet-related variables that appear to play a role in IBD risk.
Some of the strongest work involves the very first foods a newborn ingests. “Breastfeeding seems to be very important,” Kaplan says. Research has shown that kids who are breast-fed, as opposed to formulafed, are more than 25% less likely to develop IBD. “As an infant, when you have breast milk, there seem to be tangible benefits that support the development of a robust and diverse microbiome,” he explains.
Beyond infancy, there’s evidence that consuming sugary beverages—especially soft drinks—increases a person’s risks for ulcerative colitis. The more soda someone consumes, the more their risk goes up. On the other hand, eating vegetables is associated with lower rates of ulcerative colitis, while eating whole fruits or other fiber-rich foods seems to reduce a person’s risks for Crohn’s disease.
“There’s also some really interesting research on preservatives that prolong a food’s shelf life,” Kaplan says. A 2021 study in the journal BMJ found that higher intakes of ultra-processed foods—soft drinks, but also salty snack foods, processed meats, and other packaged goods—were associated with a steep rise in IBD. Compared with people who ate less than one serving of these foods per day, those who ate five or more servings were at nearly double the risk of IBD.
“Things like emulsifiers and additives and heavily processed food particles could actually lead to alterations to the microbiome that may be associated with risk for IBD,” Kaplan says. “Choosing whole foods and staying away from things that are processed or packaged may reduce your risks.”
Early-life hygiene and antibiotics
Antibiotics can save lives when someone has a bacterial infection. But these medicines kill indiscriminately—meaning they take out good bacteria as well as bad. And there’s evidence that when taken early in life while a child’s microbiome is still forming, antibiotics may cause imbalances that promote IBD.
“Antibiotics can alter the composition of the human gut microbiota by decreasing taxonomic richness and diversity,” wrote the authors of a 2019 research review in the journal Gastroenterology. They cited work linking early-life use of broad-spectrum antibiotics—basically anything but penicillin—to a more than 50% increase in IBD risk.
“If you have a bacterial infection, you need antibiotics,” Kaplan says. But too often, these drugs are prescribed when they’re not really needed—for example, when a child has a respiratory-tract infection that would likely resolve on its own without antibiotics. Doctors are increasingly aware of the risks posed by antibiotic overuse. But parents still need to be wary, he says.
Meanwhile, while hygiene is usually considered a good thing—and not just a good thing but a safety measure that has saved countless lives—there’s strong evidence that being too clean, especially during infancy and childhood, may actually weaken the gut microbiome. The “hygiene hypothesis,” as it’s called, argues that kids who interact with siblings, farm animals, pets, dirt, and other sources of germs tend to have healthier and more resilient gut microbial ecosystems, and research has linked all of these factors to lower rates of IBD (as well as allergies and autoimmune diseases).
“Early-life exposure [to germs] has an important programming role on the microbiome and immune system,” says Dr. Emeran Mayer, founding director of the Microbiome Center at the University of California, Los Angeles. The theory is that when the developing microbiome encounters germs and bacteria, this exposure trains its sensitivity and reactivity in ways that lower a person’s risks for IBD. And so raising kids in squeaky-clean environments and apart from other children, animals, or sources of germs could imperil their gut health as well as their immune competency. (Some experts have even speculated that COVID-19 safety measures, like the heavy use of hand sanitizers, may inadvertently lead to an increase in IBD among young people.)
A complicated puzzle
While researchers have made a lot of progress in the study of IBD’s environmental risk factors, they say the relationship between a person’s gut health and these variables is immensely complicated. “Someone’s risks may be completely different when in utero or in childhood or in adulthood,” Kaplan points out. He offers cigarette smoking as an example. It may be that smoking during adolescence, more so than in adulthood, is a greater risk factor for gut disorders. Or vice versa. A person’s risk may also depend on the amount they smoke, as well as on their genetic predisposition for GI disease. “There are so many variables that create so much heterogeneity,” he says. “Saying that this is a risk factor and this is not is very difficult to do.”
With that caveat in mind, Kaplan says there are steps everyone can take to reduce their IBD risks. “These are often things that promote healthy living in general,” he says. “Eating more whole foods, getting regular physical activity, and trying to reduce stress in your life are all on the checklist I go through with patients.” For those who live in parts of the country where sunlight is sparse, he says that taking a vitamin D supplement may be helpful. “If you look at people who have IBD, you often see a vitamin D deficiency,” he explains. This may just be a by-product of the condition—not its cause. Still, he says that taking a 1,000-IU daily supplement is a low-risk bulwark against gut issues that may be related to a deficiency.
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The role of external factors like diet, drugs, and pollution in IBD is complex. But medical science is making big strides in considering the impacts of environmental factors. “It’s pretty revolutionary the way the field has opened up,” Mayer says.