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

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 actually search: what causes Crohn’s disease, what in your own terrain can be measured, and which measurements quietly mislead.

The seventy percent I repeated from podiums

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 built a whole-body census, the gastrointestinal tract held about 3% of the body’s immune cells. Bone marrow and the lymphatic system hold far more. What survived the correction is more useful. The gut houses roughly 70% of the body’s antibody-producing plasma cells. It is a small tissue that writes a large share of your antibodies.

In humans, the IgA working at the gut lining and the IgA in your bloodstream are two separate systems. A serum antibody panel is not a readout of what your gut is doing. The census is not the activity.

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

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

The same data produced a heritability estimate of 0.78 for Crohn’s disease, and its authors emphasize the genetic side. Both readings are true at once. A disease can be strongly heritable and still leave most genetic copies untouched. Something outside the genome decides who crosses the line, and that something acts on terrain you can influence.

Risk that changes with a passport

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 residence.

Ontario ran the same natural experiment with different people. South Asian adults who immigrated carried inflammatory bowel disease at a rate ratio of 0.32 against non-immigrants. Their Ontario-born children reached 0.90, although that interval crosses 1, so convergence fits the data without being proven by it. The same immigrant adults showed 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. Worldwide, new Crohn’s cases have largely leveled off in the West, while incidence in Brazil climbs 11.1% a year. The disease tracks the food supply, and the food supply tracks the money: acellular carbohydrates and industrial seed oils are subsidized into the cheapest calories a family can buy.

The fat around the bowel is a checkpoint

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

Researchers grew live bacteria from the mesenteric fat of 9 of 11 Crohn’s patients, 9 of 13 people with ulcerative colitis and 4 of 4 healthy controls. The fat lies along the exit route from the gut, placed to intercept whatever leaks out. In Crohn’s, the difference was which microbe showed up. One of them pushed macrophages toward a scar-forming profile, and 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 signature bacterium in the same fat. That points to a community problem, not a single culprit, and a stool report that lists species cannot see any of it.

Why body mass index is the wrong instrument

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

Visceral fat predicted cardiovascular disease at a hazard ratio of 1.44 and cancer at 1.43 after adjusting for body mass index. The authors called that gain modest.

In 264 men studied with the reference-method insulin clamp and MRI, South Asians were less insulin sensitive than Chinese or Malay men, especially at lean body sizes. Fat distribution did not explain the gap. The pathway did: in South Asians, visceral fat worked partly through a hormone pattern built around adiponectin and leptin.

Removing a trigger versus retraining the immune system

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 credible, not less.

In celiac disease, just 50 mg of gluten a day for 90 days cut the villus-to-crypt ratio by 20%, while placebo improved it by 9%. On a gluten-free diet, complete healing of the lining reached only 0.36 of patients overall and 0.47 of those documented to adhere strictly. Trigger removal works. It works less well in adults, and 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 infants fed peanut early had 1.9% peanut allergy at five years, against 13.7% with avoidance, and 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 refractory colitis came back null.

What to measure instead of guessing

• Ask which fat was measured. Weight and body mass index are not a fat depot. If a CT or MRI exists for any reason, ask whether the visceral compartment was described.
• Ask for insulin and glucose context, not just a weight target. 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 before a second diagnosis lands on the chart.
• Write down your own history: 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, inflammaging after 50, and next in the series, 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 developed the disease in only 0.30 of pairs. Genes raise the odds; they do not settle the outcome. Risk also shifts within a single generation after migration, 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 census found the gastrointestinal tract holds about 3% of immune cells, while bone marrow and the lymphatic system hold far more. The gut does hold roughly 70% of the antibody-producing plasma cells, which 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 and rising toward them in children born after the move, and with longer residence. Genes cannot change that fast, so something in the environment is doing it. Neither registry recorded diet, sleep or antibiotics, so the exact driver is still open, and the food supply is our leading suspect. What the modern food supply does to the gut covers that side.

Can a normal-weight person have inflamed visceral fat?

Yes. In omental biopsies, crown-like structures, dead fat cells surrounded by macrophages, tracked diabetes rather than body mass index. Imaged visceral fat predicted heart disease and cancer even after adjusting for body mass index. South Asians carry more visceral fat for their body size. A normal scale weight is not a clean bill of metabolic health. The metabolic audit shows what a fuller assessment 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 clinical and endoscopic remission. In refractory ulcerative colitis, pooled trials showed no benefit. It changes what the lining is exposed to rather than how the immune system answers, which makes it the outlier among tolerance approaches. What a microbiome test can and cannot tell you is the natural next question.

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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.