What Causes Crohn’s Disease? The Genes Cannot Carry It Alone

Crohn’s disease is not caused by genes alone. Seven in ten identical twins of a Crohn’s patient stay well. And risk climbs within a single generation after people move to Western countries. Something outside the genes, acting on terrain you can change, decides who crosses the line.

Crohn’s disease is usually described as an immune system that lost its mind. That story has a hole it rarely admits. Most people who share every gene with a Crohn’s patient never get the disease.

I’m Dr. Gurpreet Singh Padda, MD, MBA, MHP, coauthor with Ami Michelle Grimes of The Angry Gut. In the Chapter 16 video, Your Immune System Is Not Confused, I argue that the immune response is a witness, not the culprit. Here the question is the one people really search. What causes Crohn’s disease? What in your own terrain can be measured? And which measurements quietly mislead?

How much of the immune system is in the gut?

For years I told audiences that seventy percent of the immune system lives in the gut. I believed it. Then I went looking for the study that measured it, and there was none. The figure traces back to a commentary that prints no percentage.

When a research team counted immune cells across the whole body, the gut held about 3% of them. Bone marrow and the lymph system hold far more. What survived the correction is more useful. The gut houses roughly 70% of the body’s antibody-making plasma cells. It is a small tissue that writes a large share of your antibodies.

In people, the IgA working at the gut lining and the IgA in your blood are two separate systems. A blood antibody panel does not show what your gut is doing. The head count is not the activity.

What causes Crohn’s disease if most identical twins escape it?

A Swedish registry of 111,080 twins gives the clearest answer. When one identical twin had Crohn’s disease, the other twin had it in 0.30 of pairs. For fraternal twins it was 0.02. Genes matter. Still, seven in ten identical twins of a Crohn’s patient stay well.

The same data gave a heritability estimate of 0.78 for Crohn’s disease, and its authors stress the gene side. Both readings are true at once. A disease can be strongly inherited and still leave most gene copies untouched. Something outside the genes decides who crosses the line. And that something acts on terrain you can change.

Why does Crohn’s risk rise after moving to a Western country?

In Denmark, first-generation immigrants had lower rates than native Danes, at an incidence rate ratio of 0.80 for Crohn’s disease. Their Danish-born children reached 0.97. Risk also climbed with 20 years or more of living there.

Ontario ran the same natural experiment with different people. South Asian adults who moved there had inflammatory bowel disease at a rate ratio of 0.32 against non-immigrants. Their Ontario-born children reached 0.90. That range crosses 1, so the data fit the gap closing without proving it. The same immigrant adults had type 2 diabetes at a rate ratio of 2.59. One move, several diseases, one direction.

Neither registry recorded what anyone ate, how they slept or which antibiotics they took. Around the world, new Crohn’s cases have mostly leveled off in the West. In Brazil, they climb 11.1% a year. The disease tracks the food supply, and the food supply tracks the money. Acellular carbohydrates (starches and sugars stripped from whole plant cells) and industrial seed oils are subsidized into the cheapest calories a family can buy.

What does the fat around the bowel do in Crohn’s disease?

In Crohn’s disease, fat can creep around the inflamed intestine.

Researchers grew live bacteria from the belly fat around the gut in 9 of 11 Crohn’s patients. They also grew them in 9 of 13 people with ulcerative colitis and 4 of 4 healthy controls. That fat sits along the exit route from the gut, placed to catch whatever leaks out. In Crohn’s, the difference was which microbe showed up. One of them pushed immune cells called macrophages toward a scar-forming state. The authors read the fat as a barrier that keeps bacteria from spreading through the body.

A separate team studying 48 Crohn’s patients found a different key bacterium in the same fat. That points to a community problem, not one culprit. And a stool report that lists species cannot see any of it.

Can you have harmful visceral fat at a normal weight?

A normal weight with too much deep belly fat is not lean, and the tissue shows it. In belly fat biopsies from women having endometrial cancer surgery, crown-like structures turned up in 48% of samples. Each one is a dead fat cell ringed by macrophages. They tracked diabetes, not body mass index.

Deep belly fat predicted heart disease at a hazard ratio of 1.44 and cancer at 1.43, even after adjusting for body mass index. The authors called that gain modest.

One study tested 264 men with the gold-standard insulin clamp test and MRI. South Asians were less insulin sensitive than Chinese or Malay men, especially at lean body sizes. Where the fat sat did not explain the gap. The pathway did. In South Asians, deep belly fat worked partly through a hormone pattern built around adiponectin and leptin.

Can removing a trigger or retraining the immune system help?

If the immune system is answering what it is shown, changing what it is shown should help. It does, with limits that make the case more believable, not less.

In celiac disease, just 50 mg of gluten a day for 90 days cut the villus-to-crypt ratio by 20%. That ratio measures the health of the gut lining. On placebo, it improved by 9%. On a gluten-free diet, full healing of the lining reached only 0.36 of patients overall. It reached 0.47 of those known to follow the diet strictly. Removing the trigger works. It works less well in adults. When the diet seems to fail, hidden gluten is the usual suspect.

Tolerance is far easier to build early than to rebuild later. At-risk babies fed peanut early had 1.9% peanut allergy at five years, against 13.7% with avoidance. The gap held at twelve years, 4.4% against 15.4%. In established disease the record is poor. Worm therapy in Crohn’s disease matched placebo, 40.7% against 42.9%. Fecal transplant in active ulcerative colitis stands out, at an odds ratio of 4.11 for remission. Yet pooled trials in hard-to-treat colitis came back null.

What to measure instead of guessing

• Ask which fat was measured. Weight and body mass index are not a fat store. If you have a CT or MRI for any reason, ask whether the deep belly fat was described.
• Ask for insulin and blood sugar context, not just a weight goal. You cannot out-inject a broken metabolism, and you cannot out-medicate one either.
• If you have celiac disease and still have symptoms after six months of a strict diet, ask about hidden gluten. Do that before a second diagnosis lands on your chart.
• Write down your own history. Note where you grew up, when you moved, when symptoms began and what changed the year before. No lab recovers that.

If you already take a biologic, stay on it and work every change through your physician. The terrain is what you repair while the drug holds. See the gut-joint connection and inflammaging after 50. Next in the series is what night shifts do to insulin. The Chapter 16 Deep Dive holds every study in full, limits included.

Frequently asked questions

Is Crohn’s disease genetic?

Partly. In a study of 111,080 Swedish twins, heritability for Crohn’s disease was estimated at 0.78. Yet an identical twin of a Crohn’s patient got the disease in only 0.30 of pairs. Genes raise the odds. They do not settle the outcome. Risk also shifts within one generation after moving, which genes cannot do. Why the same treatment works in one terrain and fails in another explains what that means in practice.

Is 70% of the immune system really in the gut?

No. That figure was never measured. A whole-body count found the gut holds about 3% of immune cells. Bone marrow and the lymph system hold far more. The gut does hold roughly 70% of the antibody-making plasma cells. That is why it matters so much for tolerance. How the gut barrier decides what the immune system meets covers the next layer.

Why do Crohn’s rates rise after people move to Western countries?

Registries in Denmark and Ontario show immigrant rates starting below native rates. They rise toward native rates in children born after the move, and with longer stays. Genes cannot change that fast, so something in the setting is doing it. Neither registry recorded diet, sleep or antibiotics. So the exact driver is still open, and the food supply is our top suspect. What the modern food supply does to the gut covers that side.

Can a normal-weight person have inflamed visceral fat?

Yes. In belly fat biopsies, crown-like structures tracked diabetes, not body mass index. These are dead fat cells surrounded by macrophages. Deep belly fat on scans predicted heart disease and cancer, even after adjusting for body mass index. South Asians carry more deep belly fat for their body size. A normal scale weight is not a clean bill of metabolic health. The metabolic audit shows what a fuller check looks at.

Does fecal transplant work for inflammatory bowel disease?

It depends on the disease and its stage. In active ulcerative colitis, randomized trials pooled to an odds ratio of 4.11 for combined remission on symptoms and scope exams. In hard-to-treat ulcerative colitis, pooled trials showed no benefit. It changes what the lining is exposed to, not how the immune system answers. That makes it the odd one out among tolerance approaches. What a microbiome test can and cannot tell you is the natural next question.

What triggers Crohn’s disease to start?

Genes load the odds, but something outside the genes decides who gets sick. Most identical twins of a Crohn’s patient stay well. Risk rises in the children of immigrants and with longer stays in Western countries. The registries did not record diet, sleep or antibiotics, so the exact trigger is not pinned down. The changing food supply is our top suspect.

Is the immune system attacking the body by mistake in Crohn’s disease?

We see it differently. In Crohn’s disease the immune response is a witness, not the culprit. It answers what the bowel shows it. That is why changing what it is shown can help, as removing gluten does in celiac disease. It is also why we measure the terrain: deep belly fat, insulin and your history. We do not treat the immune system as simply confused.

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Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, board certified in anesthesiology, pain medicine, interventional pain management, addiction medicine and obesity medicine. Adapted from Chapter 16 of The Angry Gut by Dr. Padda and Ami Michelle Grimes. Last reviewed September 2026.