Lee Seung-hoon, a professor in the Department of Neurology at Seoul National University Hospital, recently shared critical insights on extending healthy life expectancy by focusing on a surprising factor: the frequency of bowel movements.
Appearing on the YouTube channel ‘Sseol Doc,’ Professor Lee discussed colorectal cancer prevention in detail. "The aggressive factor that causes colorectal cancer is stool," he explained. "Colorectal cancer occurs most frequently where stool tends to get trapped—the rectum, located just above the anus. When highly concentrated stool cannot be expelled for a long time, its components attack the colon lining, triggering the disease."
Time, Not Just Content, Is the Risk Factor
While long-term studies identify red and processed meats as major carcinogens, Professor Lee noted that the duration of exposure is what truly matters. "Someone who eats processed meats like sausages but has regular, effective bowel movements is at a much lower risk," he said. "The cause is not just eating something unhealthy, but stimulating the colon for an extended period."
He further clarified that common risk factors like obesity and a lack of exercise ultimately increase cancer risk by slowing bowel transit speed. When stool remains in the colon for too long, the risk of malignancy rises.
Anatomical Vulnerability of the Rectum and Sigmoid Colon
Colorectal cancer is most prevalent in the "end portions" of the digestive tract, where stool is stored before expulsion. According to data from the U.S. National Center for Biotechnology Information (NCBI), rectal cancer accounts for roughly 49.7% of cases. Within the colon itself, the sigmoid colon is the most common site, representing 55% of occurrences.
The biological plausibility of this is supported by Asan Medical Center in Seoul, which notes that dietary fiber prevents cancer primarily by reducing colonic transit time, thereby minimizing the duration that potentially carcinogenic substances remain in contact with the intestinal mucosa.
Combatting Constipation Through Lifestyle
To maintain a healthy transit speed, medical institutions such as the U.S. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) and Mayo Clinic recommend a combination of high dietary fiber, adequate hydration, and regular physical activity.
In Korea, Seoul National University Bundang Hospital advises increasing the intake of fiber-rich vegetables, fruits, and seaweed, alongside exercising enough to break a sweat at least three times a week. Experts also emphasize the importance of establishing a consistent time for bowel movements to train the digestive system.
However, medical professionals warn that if constipation persists for more than three weeks—or is accompanied by alarming symptoms such as blood in the stool, sudden weight loss, anemia, or abdominal pain—it should not be dismissed as a simple lifestyle issue. In such cases, seeking medical care is essential, as persistent constipation can sometimes be a masked sign of inflammatory bowel disease or colorectal cancer.
