The sigmoid colon is the final S-shaped segment of the large intestine, sitting just before the rectum in the lower left side of your abdomen. It acts as a holding area where stool firms up before a bowel movement, and its curving anatomy makes it a hotspot for several common gastrointestinal problems, from diverticulitis and volvulus to polyps and the pain associated with irritable bowel syndrome. Understanding what the sigmoid colon does and what can go wrong with it helps explain a surprising number of digestive complaints.
Where Exactly Is the Sigmoid Colon
Your large intestine forms a rough frame around the contents of your abdomen. Starting at the lower right (where the appendix sits), it climbs upward as the ascending colon, crosses to the left as the transverse colon, drops down the left side as the descending colon, and then takes a distinctive S-shaped curve before connecting to the rectum. That S-curve is the sigmoid colon, named after the Greek letter sigma. In most people it sits in the lower left quadrant of the abdomen, tucked into the pelvis.
The sigmoid typically runs about 35 to 40 centimeters long, though its length varies quite a bit from person to person. Some people have a relatively short, compact sigmoid; others have a redundant or elongated one that loops more dramatically. That extra length matters clinically, because a longer, more mobile sigmoid has more room to twist on itself. The sigmoid is anchored to the back wall of the abdomen by a fold of tissue called the mesocolon, and in people with a narrow or elongated attachment point, the colon can rotate around that stalk, setting the stage for a condition called volvulus.
What the Sigmoid Colon Actually Does
By the time digested food reaches the sigmoid colon, most nutrients have already been absorbed higher up in the small intestine and the earlier parts of the large intestine. The sigmoid’s main job is to compact what remains into formed stool by absorbing leftover water and electrolytes, and then to hold that stool until you’re ready for a bowel movement. Muscular contractions in the sigmoid wall push waste downward into the rectum in coordinated waves.
But the sigmoid and the rest of the large intestine do more than just handle water. Bacteria in the colon ferment dietary fiber and resistant starch into short-chain fatty acids, and the colonic lining absorbs those fatty acids efficiently. This absorption is tied to the uptake of sodium, potassium, and water, and it also helps maintain the right acid-base balance inside the gut lumen.1PubMed. Absorption of short-chain fatty acids by the colon Short-chain fatty acids do more than just provide a modest energy source: they stimulate blood flow to the colonic wall and support the health of the cells lining the intestine.2PubMed. Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides Research measuring absorption rates in the human rectum and distal colon confirms that acetate, propionate, and butyrate are all taken up at measurable rates across the intestinal lining.3PubMed Central. Short chain fatty acid absorption by the human large intestine
The sigmoid also has a strong muscular wall that can generate powerful contractions. These contractions serve the useful purpose of propelling stool, but when they become exaggerated or poorly coordinated, they can cause cramping pain, a pattern seen in certain functional bowel disorders.
Diverticular Disease
Diverticulosis, the formation of small pouches (diverticula) that bulge outward through weak spots in the colon wall, is overwhelmingly common in Western populations, and the sigmoid colon is where it occurs most frequently. The sigmoid has the smallest diameter of any segment of the large intestine, which means pressure inside it can be higher, particularly when someone is chronically constipated or straining. Over time, that pressure pushes the inner lining through gaps in the muscular wall where blood vessels penetrate.
Most people with diverticula never know they have them. Trouble starts when one of those pouches becomes inflamed or infected, a condition called diverticulitis. Symptoms typically include pain in the lower left abdomen (matching the sigmoid’s location), fever, and changes in bowel habits. CT scanning is the go-to imaging tool for diagnosing diverticulitis, though unusual presentations can sometimes mimic other conditions, including colon cancer, which means careful interpretation is important.4PubMed Central. CT findings of misleading features of colonic diverticulitis
Mild episodes are usually treated with antibiotics and a temporary shift to easily digested foods, and most people recover without surgery. When diverticulitis keeps coming back or leads to complications like abscesses or perforations, removing the sigmoid colon (sigmoidectomy) becomes the standard recommendation. In a large multicenter study, roughly 88% of elective sigmoid surgeries for diverticulitis were performed laparoscopically, and about 84% included reconnecting the bowel ends rather than creating a permanent stoma. The rate of serious complications within 90 days was about 12%, with anastomotic leakage occurring in close to 5% of cases.5PubMed. Risk factors for severe morbidity and definitive stoma after elective surgery for sigmoid diverticulitis: a multicenter national cohort study Single-incision laparoscopic approaches have also been explored, with early results showing feasibility in the majority of patients, though this technique remains less widespread.6PubMed. Single-incision laparoscopic surgery: a promising approach to sigmoidectomy for diverticular disease
One persistent myth about diverticular disease deserves a mention: the old advice to avoid seeds, nuts, and popcorn has no solid scientific support. A high-fiber diet actually appears to reduce the risk of developing diverticulitis in the first place, and the idea that you should eat a low-fiber diet during an acute episode also lacks a strong evidence base.7PubMed Central. Long-term outcomes in recurrent diverticulitis: a systematic review of treatment strategies and recurrence rates
Sigmoid Volvulus
Sigmoid volvulus happens when the sigmoid colon twists around the base of its mesentery, cutting off blood flow and creating a bowel obstruction. It is one of the more dramatic surgical emergencies involving the sigmoid. The condition results from rotation of a redundant sigmoid loop around its narrow attachment point, and it tends to occur in the setting of chronic constipation, an elongated colon, or other anatomical predispositions, though the exact precipitating event is not fully understood.8PubMed. The twisted colon: a review of sigmoid volvulus
Symptoms come on fast: severe abdominal bloating, pain, inability to pass gas or stool, and sometimes vomiting. An abdominal X-ray often shows a massively dilated loop of bowel with a characteristic “coffee bean” shape. Emergency treatment usually involves passing a flexible tube through the rectum to untwist the colon (endoscopic derotation), which relieves the obstruction without immediate surgery. The problem is that volvulus tends to recur once the anatomy allows it, so surgery to remove the redundant sigmoid is often recommended after the acute episode is managed.9British Journal of Surgery. O4 Rapid Review: Management of Recurrent Sigmoid Volvulus
Sigmoid volvulus is relatively rare in younger adults in high-income countries but is much more common in parts of Africa, South America, and Central Asia, where high-fiber diets and elongated colons are more prevalent. In those regions it can be one of the leading causes of bowel obstruction. Age, psychiatric illness, and nursing home residence are additional risk factors in Western settings, likely because all of these are linked to reduced mobility and chronic constipation.
Polyps and Colorectal Cancer Risk
The sigmoid colon is one of the most common sites for colorectal polyps, which are small growths on the inner lining that can, over many years, progress into cancer. Risk factors for adenomatous polyps (the type most likely to become cancerous) include smoking, obesity, and family history.10PubMed Central. Factors that increase risk of colon polyps The sigmoid’s role as a frequent polyp site is one of the reasons that both colonoscopy and the older screening tool, flexible sigmoidoscopy, have historically been effective at reducing colorectal cancer deaths.
Flexible sigmoidoscopy examines only the sigmoid and rectum, so it catches problems in the distal colon but misses anything higher up. Studies have found that screening sigmoidoscopy is associated with a roughly 55% reduction in distal colorectal cancer deaths but offers no meaningful reduction in cancer deaths from the proximal colon. Colonoscopy, which examines the entire large intestine, is associated with a much larger overall mortality reduction and catches disease in both the distal and proximal segments.11PubMed Central. Screening flexible sigmoidoscopy versus colonoscopy for reduction of colorectal cancer mortality A more recent large analysis estimated that colonoscopy screening prevents roughly 50 colorectal cancer cases per 100,000 person-years, while the additional benefit over sigmoidoscopy alone accounts for about 12 of those prevented cases.12JAMA Network Open. Effectiveness of Colonoscopy Screening vs Sigmoidoscopy Screening in Colorectal Cancer
Capsule endoscopy, where you swallow a tiny camera that photographs the colon as it passes through, has been tested as a less invasive alternative. However, its sensitivity for detecting polyps is considerably lower than optical colonoscopy. A meta-analysis found that capsule endoscopy detected any polyp with about 73% sensitivity and significant polyps with about 69% sensitivity, compared to colonoscopy as the reference standard.13PubMed. A meta-analysis evaluating the accuracy of colon capsule endoscopy in detecting colon polyps For the moment, colonoscopy remains the gold standard when the goal is both finding and removing polyps in a single session.14PubMed. Capsule endoscopy versus colonoscopy for the detection of polyps and cancer
The Sigmoid in Irritable Bowel Syndrome
If you’ve ever had cramping pain in your lower left abdomen and been told it might be IBS, the sigmoid colon is likely the culprit. Research dating back decades showed that strong circular contractions of the sigmoid wall correlated with the characteristic pain of irritable bowel syndrome in the vast majority of patients studied. High-pressure spasms in the sigmoid appear to be a key mechanical cause of the cramping.15PubMed. Painful irritable bowel syndrome and sigmoid contractions
More recent work has moved beyond the mechanics to look at what’s happening at the cellular level. People with IBS show altered epithelial function in the sigmoid colon, meaning the lining itself behaves differently in terms of permeability, and this finding appears to hold regardless of whether the patient is male or female.16PubMed Central. Sex-dependent alterations of colonic epithelial permeability: relevance to irritable bowel syndrome Separately, researchers have found that a pain-sensing receptor called TRPA1 is expressed at significantly higher levels in sigmoid tissue from IBS patients compared to healthy controls, and that this overexpression is more pronounced in the sigmoid than in other parts of the colon.17PubMed. Colonic mucosal TRPA1 expression profiles in irritable bowel syndrome and its correlation to symptom severity These findings help explain why the sigmoid seems to be ground zero for IBS-related discomfort: it has both the muscular capacity to spasm powerfully and, in susceptible people, a heightened molecular sensitivity to pain signals.
Inflammatory Bowel Disease and the Sigmoid
Ulcerative colitis, one of the two main forms of inflammatory bowel disease, almost always begins in the rectum and extends upward in a continuous pattern. The sigmoid colon is therefore one of the first segments affected when the disease spreads beyond the rectum. In long-standing or severe cases, imaging may reveal a thickened, tortuous sigmoid wall with a narrowed lumen and increased blood vessel visibility along the outer surface.18PubMed. Chinese dragon sign of ulcerative colitis Because ulcerative colitis involves the rectum (while ischemic colitis typically does not), clinicians can usually distinguish between the two conditions even when imaging looks ambiguous.
Crohn’s disease, the other major form of IBD, can affect any part of the gastrointestinal tract but involves the colon in a substantial fraction of patients. When Crohn’s does hit the sigmoid, it can cause strictures, fistulas, or abscesses that sometimes require surgical resection. The differing microbial environments of each intestinal segment appear to play a role in how and where IBD manifests, though the research mapping microbial communities across specific bowel segments is still in early stages.19PubMed Central. Diversity of the microbiota communities found in the various regions of the intestinal tract in healthy individuals and inflammatory bowel diseases
Endometriosis Involving the Sigmoid
One problem that surprises many people is that endometriosis, a condition in which tissue similar to the uterine lining grows outside the uterus, can infiltrate the sigmoid colon and rectum. Deep endometriosis affecting the bowel tends to cause cyclical pain that worsens around menstruation, along with symptoms like painful bowel movements, bloating, and sometimes rectal bleeding. Because these symptoms overlap with IBS and other gastrointestinal conditions, bowel endometriosis is often diagnosed late.
Hormonal treatments, including combined oral contraceptives, progestins, and other agents, can control symptoms in many women with sigmoid endometriosis, particularly when the narrowing of the bowel lumen is less than about 60%. Patients whose primary symptoms are cramping and irritative-type complaints tend to respond better than those mainly experiencing constipation. Across published studies, roughly two-thirds of women reported satisfaction with hormonal treatment regardless of which specific drug was used.20PubMed. Medical treatment in the management of deep endometriosis infiltrating the proximal rectum and sigmoid colon Surgery to remove affected bowel segments is reserved for cases where hormonal therapy fails or where the degree of stenosis is severe.
Why Transit Time Varies So Much
How quickly waste moves through the colon, and particularly through the sigmoid, varies enormously between individuals. This variability affects everything from the consistency of stool to the severity of symptoms in conditions like IBS and diverticulosis. Diet, physical activity, hydration, and the composition of gut bacteria all influence motility. Ethnic and cultural dietary patterns also play a role, which is why researchers have argued for population-specific approaches when studying or diagnosing disorders related to slow transit.21PubMed Central. Unlocking the colon clock: Bridging the gap in colonic transit time studies for optimal management of chronic constipation
From a practical standpoint, this means two people eating the same diet might have very different experiences with their sigmoid colon. Someone with naturally faster transit may never develop the high intraluminal pressures that promote diverticula, while someone with slow transit and a tendency toward constipation accumulates more pressure over time. Regular physical activity, adequate fluid intake, and a fiber-rich diet are the simplest levers for keeping transit on the healthier end of the spectrum, though genetics and the microbiome set baseline limits that lifestyle alone cannot always override.
The Sigmoid’s Microbial Landscape
The bacterial communities living in the sigmoid colon are not identical to those found higher up in the intestinal tract. The colon’s microbial makeup shifts along its length, influenced by local pH, oxygen levels, available nutrients, and the speed at which contents move through each section. The sigmoid and rectum, being the final stations before waste exits, harbor communities shaped by the end products of fermentation that began further upstream. These microbes continue to produce short-chain fatty acids and other metabolites that interact with the immune system embedded in the gut wall.
Disruptions in these microbial communities have been linked to inflammatory bowel disease, and research is increasingly focused on understanding whether the microbial differences between intestinal segments help explain why certain diseases preferentially affect one part of the colon over another.19PubMed Central. Diversity of the microbiota communities found in the various regions of the intestinal tract in healthy individuals and inflammatory bowel diseases The science here is still more map-making than map-reading. Researchers know the communities differ by location, but translating that into treatments or diagnostics remains a work in progress. What is clear is that the sigmoid colon is not just a passive conduit for waste. It is a metabolically and immunologically active segment, and the balance of microbes living there plays a real role in local and systemic health.