[Column] Crohn's disease was once rare in Korea. Why are more children de...

Professor Ryu In-hyeok, the Department of Pediatrics at the Catholic University of Korea Seoul St. Mary's Hospital

“Thirty years ago, Crohn’s disease was almost unheard of in Korea.”

This is something I always tell parents when explaining the diagnosis to the guardian of a child who has just been diagnosed with Crohn's disease -- or Ulcerative colitis -- in the outpatient clinic.

Before the 2000s, Crohn’s disease in Korea was considered so rare that individual cases were worthy of presentation at academic conferences. Diagnosing even a single patient could become a case report. Today, however, it is difficult to go through a pediatric gastroenterology clinic for a single day without seeing a patient with Crohn’s disease. What has changed so dramatically?

The incidence of Crohn's disease has risen rapidly over the past 20 to 30 years in countries such as Korea, Japan, China and India. While the rate of increase is not as steep as it once was, incidence continues to rise gradually compared with the more stabilized trends seen in Western countries. (Credit: Getty Images)

A disease that began in the West and moved East

Crohn’s disease was originally far more common in North America and Europe. Since the mid-20th century, incidence rates in Western countries rose rapidly before eventually reaching a plateau. Asia, including Korea, appears to be following a similar trajectory.

Over the past 20 to 30 years, countries such as Korea, Japan, China and India have all experienced a sharp increase in Crohn’s disease incidence. Although the rate of increase has slowed compared to earlier decades, the numbers are still gradually rising, unlike the more stabilized trends seen in the West.

This raises an important question. Human genes do not fundamentally change within 30 years. So what caused Crohn’s disease to become so common in such a short period?

‘Genetics loads the gun, environment pulls the trigger’

There is a phrase frequently used when discussing the causes of Crohn’s disease: “Genetics loads the gun, environment pulls the trigger.”

Certainly, some individuals possess a genetic predisposition that makes them more vulnerable to Crohn’s disease. Yet studies involving identical twins show that even when one twin develops the disease, the likelihood that the other twin will also develop it is only around 30 percent to 50 percent. Despite sharing identical genes, more than half do not become ill.

That is why environmental factors matter so much. Among them, diet has emerged as one of the most important.

Professor Ryu In-hyeok

Westernized diets are changing the gut

Countries experiencing rising Crohn’s disease rates share a common pattern: westernization of dietary habits.

In epidemiological research, the term “Western diet” refers to a diet high in animal fat and refined carbohydrates but low in dietary fiber. This does not mean fast food alone. Compared with 30 years ago, Korean diets today include significantly more meat, while traditional multigrain rice and vegetable side dishes have steadily declined.

Large prospective cohort studies have repeatedly shown that people who follow Western dietary patterns are at higher risk of developing Crohn’s disease, whereas diets rich in fruits, vegetables, fish and fiber are associated with a lower risk.

More recently, ultra-processed foods have also drawn attention as potential risk factors. A growing body of research suggests that emulsifiers, artificial sweeteners and food additives commonly found in ultra-processed foods may damage the intestinal mucosal barrier and alter the gut microbiome.

Still, the dietary risks associated with Crohn’s disease cannot be explained solely by ultra-processed foods. Diets excessively high in animal fat or lacking in fiber can negatively affect gut health even without processed ingredients. Ultimately, the issue is not a single food or nutrient, but a broader shift in dietary patterns.

How food alters the gut

Researchers have proposed several mechanisms explaining how diet may contribute to Crohn’s disease. The most extensively studied mechanism involves changes in the gut microbiome. Western diets reduce intestinal microbial diversity, particularly beneficial bacteria that ferment dietary fiber into short-chain fatty acids. Short-chain fatty acids serve as an important energy source for intestinal cells and help suppress inflammation. When their levels decline, the intestinal lining becomes weaker and more vulnerable to inflammation.

High-fat diets also increase bile acid secretion. Secondary bile acids may raise intestinal permeability, weakening the gut barrier. Once this barrier is compromised, intestinal bacteria can stimulate immune cells beneath the mucosa, potentially triggering chronic inflammation in genetically susceptible individuals.

Is there a diet that prevents Crohn’s disease?

To be honest, there is still no evidence proving that any single diet can prevent Crohn’s disease with absolute certainty. However, when epidemiological studies are viewed collectively, certain dietary patterns clearly appear to reduce risk. Diets rich in fruits, vegetables, fiber and fish are associated with lower risk, while excessive intake of animal fat, processed meat, refined carbohydrates and additive-heavy processed foods appears to increase risk.

When discussing diet with Crohn’s disease patients in the clinic, I often say: “To put it very simply, foods made at home using fresh ingredients in traditional ways are generally safe.” I believe this principle also applies to prevention.

The meals prepared by our grandparents’ generation -- multigrain rice, soybean paste stew, seasoned vegetables, grilled fish and kimchi -- were naturally rich in fiber, included fermented foods and contained very little processed food. The world has changed, and it may be unrealistic to return entirely to those eating habits. But trying to preserve at least some of that direction still matters.

Why children’s diets matter even more

Approximately 25 percent to 40 percent of Crohn’s disease cases are diagnosed during childhood or adolescence. Pediatric Crohn’s disease also tends to be more extensive and severe than adult-onset disease.

Childhood eating habits play a critical role in shaping the gut microbiome.

A child raised primarily on processed foods and fast food will inevitably develop a very different intestinal microbial environment from one raised on more traditional meals. Of course, cooking every meal from scratch is unrealistic for many families. But perfection is not necessary. Even preparing simple meals with fresh ingredients when possible and reducing the proportion of processed foods can make a meaningful difference.

The change does not have to be dramatic. Perhaps it begins with something as simple as opening the refrigerator before opening a food delivery app tonight.

Ryu In-hyeok