Crohn’s disease and ulcerative colitis are the two main forms of inflammatory bowel disease (IBD), a group of conditions where the immune system mistakenly attacks healthy cells in the digestive tract. Together, they affect an estimated 2.4 to 3.1 million people in the United States, and that number is rising. While the two diseases share symptoms like chronic diarrhea, abdominal pain, and fatigue, they differ in where they strike and how deeply they damage tissue.
How the Two Conditions Differ
Ulcerative colitis is limited to the large intestine (the colon). It typically starts in the rectum and spreads upward in a continuous line with no gaps between inflamed areas. The inflammation stays shallow, affecting only the innermost lining of the colon wall.
Crohn’s disease can show up anywhere along the digestive tract, from the mouth to the anus, though it most commonly affects the end of the small intestine and the beginning of the colon. Unlike colitis, Crohn’s often skips areas, leaving patches of healthy tissue between inflamed spots. It also digs deeper, potentially penetrating through all layers of the intestinal wall. That deeper damage is what makes Crohn’s more prone to certain complications like fistulas (abnormal tunnels between the intestine and other organs or skin) and strictures (narrowed sections of bowel caused by scar tissue).
What Causes IBD
Neither Crohn’s nor colitis has a single cause. Both develop from a collision of genetics, immune system behavior, gut bacteria, and environmental triggers. In a healthy gut, the immune system tolerates the trillions of bacteria living there. In IBD, that tolerance breaks down. The immune system treats normal gut tissue as a threat, creating inflammation that doesn’t resolve on its own.
Genetics play a significant role. Researchers have identified several genes tied to IBD, many of which govern how the immune system interacts with gut bacteria. One of the most studied, NOD2, helps the body recognize and respond to bacterial signals. Mutations in this gene are linked to shifts in the gut’s bacterial makeup, including a drop in certain protective bacteria and an increase in potentially harmful ones. Other genes involved in IBD similarly sit at the intersection of immune function and the microbiome.
Animal studies reinforce the connection between gut bacteria and IBD. In germ-free environments, colitis doesn’t develop. But transferring bacteria from diseased subjects into healthy ones can trigger inflammation, suggesting that the specific mix of microbes in your gut is a key piece of the puzzle.
Common Symptoms
Both conditions share a core set of symptoms: persistent diarrhea (sometimes bloody), cramping abdominal pain, urgency to use the bathroom, fatigue, and unintended weight loss. Bloody stool is especially common in ulcerative colitis because the inflammation sits right at the surface of the colon lining. Crohn’s disease, depending on its location, may present with more pain and less visible bleeding, along with nausea or mouth sores.
Both diseases tend to cycle between flares, when symptoms are active, and remission, when they quiet down. The frequency and severity of flares varies widely from person to person. Some people go years between episodes, while others struggle to achieve lasting remission.
Effects Beyond the Gut
IBD is not just an intestinal disease. Between 25% and 40% of people with Crohn’s or colitis experience problems in other parts of the body. Joint inflammation is the most common, affecting up to 30% of patients, typically in the knees, ankles, elbows, or lower spine.
Up to 20% of people with IBD develop skin complications, ranging from tender red bumps on the shins to canker sores in the mouth. Eye inflammation, liver problems (including a condition called primary sclerosing cholangitis that scars the bile ducts), kidney stones, and anemia all appear at higher rates in people with IBD. Some of these issues flare alongside gut symptoms, while others follow their own timeline.
For Crohn’s patients specifically, fistulas are a notable complication. About one-third of people with Crohn’s develop perianal fistulas, which are abnormal channels between the intestines and the skin near the anus that can drain blood, pus, or stool.
How IBD Is Diagnosed
Diagnosis usually starts with blood tests that check for general markers of inflammation. But a stool test measuring a protein called calprotectin is more accurate at detecting inflammation specifically in the intestines, which helps distinguish IBD from conditions like irritable bowel syndrome that don’t involve visible tissue damage.
If stool and blood tests point toward IBD, the next step is typically a colonoscopy. A small camera is guided through the colon and, if needed, the end of the small intestine, allowing doctors to see the inflammation directly and take tissue samples. The pattern of inflammation, whether it’s continuous or patchy, superficial or deep, is what determines whether you have ulcerative colitis or Crohn’s disease.
Treatment Options
Treatment for both conditions aims to calm the immune system’s overreaction, reduce inflammation, and keep symptoms in remission as long as possible. The approach depends on how severe the disease is and where it’s located.
Milder cases of ulcerative colitis often respond to medications that target inflammation directly in the colon lining. More moderate or severe disease in either condition typically requires drugs that suppress the broader immune response. Biologic therapies, which are given by injection or infusion, target specific proteins involved in the inflammatory process and have changed the landscape of IBD treatment over the past two decades. A newer class of oral medications works by blocking signaling pathways inside immune cells, offering another option for people who don’t respond to other treatments.
Despite advances in medication, surgery remains part of the picture. Roughly 20% of people with ulcerative colitis and up to 80% of people with Crohn’s disease will need surgery at some point. For ulcerative colitis, removing the entire colon can effectively cure the disease, since there’s no other tissue it can affect. Crohn’s surgery is different: because the disease can appear anywhere in the digestive tract, surgery addresses specific complications like strictures, fistulas, or abscesses, but it doesn’t cure the underlying condition. Recurrence after Crohn’s surgery is common.
Diet and Symptom Management
No single diet causes or cures IBD, but what you eat can meaningfully affect how you feel day to day. Two dietary approaches have the most evidence behind them.
The low-FODMAP diet, originally developed for irritable bowel syndrome, reduces certain fermentable carbohydrates that can trigger bloating, gas, and diarrhea. It has been shown to reduce symptoms in people with IBD who have overlapping IBS-type issues, but it does not reduce the underlying inflammation. It’s meant for short-term use, usually to identify specific food triggers, not as a permanent eating plan.
The Specific Carbohydrate Diet (SCD) has shown more promise for Crohn’s disease specifically. A study funded by the Crohn’s & Colitis Foundation found that the SCD and a Mediterranean-style diet similarly improved symptoms, inflammation, and quality of life over 12 weeks in people with mild to moderate Crohn’s. Since the Mediterranean diet is less restrictive and equally effective, it’s often the more practical long-term choice. Working with a dietitian who understands IBD can help you figure out which foods worsen your symptoms without unnecessarily limiting your nutrition.