Diarrhea, defined as an increase in the frequency and wateriness of bowel movements, is a common issue for individuals undergoing cancer diagnosis or treatment. This side effect can lead to rapid dehydration, electrolyte imbalances, and malnutrition. Severe diarrhea may interrupt or delay cancer therapies, making prompt recognition and management a priority. Understanding whether the diarrhea stems from the malignancy itself or is a side effect of intervention is the first step toward effective control. This article explores how cancer can trigger this condition and outlines management strategies.
Diarrhea Caused Directly by the Cancer
In some cases, the presence of a tumor directly influences the digestive system, causing diarrhea independent of treatment. Cancers within the gastrointestinal tract, such as colon cancer, can cause partial physical obstruction of the bowel, leading to watery stool passing around the blockage. Malabsorption may also occur when tumors affect organs responsible for digestion, such as pancreatic cancers that block the release of digestive enzymes.
Neuroendocrine tumors (NETs) cause diarrhea by secreting high levels of bioactive substances into the bloodstream. For example, VIPomas, typically found in the pancreas, release excessive Vasoactive Intestinal Peptide (VIP). This hormone stimulates the intestinal lining to secrete large amounts of water and electrolytes, resulting in secretory diarrhea that often persists even during fasting.
Medullary Thyroid Carcinoma (MTC) produces high circulating levels of calcitonin, which accelerates the movement of contents through the colon and impairs water absorption. Similarly, carcinoid tumors release serotonin and other hormonal peptides, leading to carcinoid syndrome, characterized by flushing and diarrhea due to increased gut motility.
Diarrhea Caused by Cancer Treatments
Treatment modalities are the most frequent cause of diarrhea in oncology patients. Chemotherapy drugs target rapidly dividing cells, including the healthy epithelial cells lining the intestines, leading to mucositis. This damage compromises the gut barrier, impairing water and nutrient absorption while causing inflammation.
Specific agents like 5-fluorouracil (5-FU) and irinotecan are known culprits for inducing gastrointestinal toxicity. Irinotecan is converted in the liver to an inactive compound, which is excreted into the gut. There, gut bacteria reactivate the toxic compound, SN-38, which directly damages the intestinal wall, leading to delayed, severe diarrhea.
Radiation therapy directed at the abdomen or pelvis causes acute radiation enteritis. The radiation damages the intestinal mucosa within the treatment field, resulting in inflammation, fluid secretion, and impaired absorption. Long-term damage can scar the blood vessels supplying the bowel, leading to chronic diarrhea and malabsorption years after treatment.
Newer treatments, including immune checkpoint inhibitors (ICIs), can cause immune-mediated diarrhea and colitis (IMDC). These therapies activate the immune system to attack cancer cells. However, activated T-cells sometimes target the healthy cells of the colon, resulting in colitis. This inflammation manifests as profuse diarrhea, often accompanied by blood or mucus.
Surgical interventions can alter the digestive system, causing diarrhea. Resection of large portions of the small intestine, particularly the ileum, can lead to Short Bowel Syndrome. Loss of the ileum prevents efficient reabsorption of bile salts, which then draw water into the colon, causing chronic diarrhea and fat malabsorption. Even less extensive surgeries can cause urgency and frequent bowel movements due to faster transit time.
Immediate Home Management Strategies
The immediate priority for managing diarrhea at home is fluid and electrolyte replacement to prevent dehydration. Water alone is insufficient, as it does not replace the salts and sugars lost in the stool. Patients should use oral rehydration solutions (ORS), sports drinks, or clear broths. Drinking at least one cup of fluid after every loose bowel movement helps maintain balance.
Dietary adjustments focus on reducing the workload on the irritated bowel by temporarily switching to a low-fiber, low-fat, and bland diet. Foods that help bind the stool, such as the BRAT diet (bananas, white rice, applesauce, and white toast), are recommended for short-term relief. Other well-tolerated foods include peeled potatoes, lean protein, and white pasta.
Patients should avoid foods that stimulate the gut or are poorly digested. Beverages containing caffeine, alcohol, or high amounts of sugar or sugar substitutes, such as sorbitol, also increase gut motility and fluid secretion. Many patients temporarily restrict dairy products due to potential lactose intolerance caused by mucosal damage.
Foods to Avoid
- Raw vegetables
- Whole grains
- Nuts and seeds
- Fried or greasy foods
Frequent and acidic diarrhea irritates the skin around the anus, causing pain and breakdown. Maintaining meticulous perianal hygiene is necessary to prevent infection and discomfort. After each bowel movement, gently cleanse the area with warm water or a soft, unscented wipe, avoiding harsh soaps or vigorous rubbing. Applying a protective barrier cream, such as one containing zinc oxide, creates a shield against the irritating stool and promotes healing.
Medical Treatment and Severity Monitoring
Oncology teams use the Common Terminology Criteria for Adverse Events (CTCAE) to grade the severity of diarrhea and guide intervention. Grade 1 is an increase of fewer than four stools per day over baseline. Grade 2 involves four to six extra stools per day, limiting daily activities. Grades 3 and 4 represent severe diarrhea, defined by seven or more extra stools per day, incontinence, or life-threatening consequences.
Pharmacological treatment begins with prescription-strength anti-diarrheals, most commonly loperamide. Loperamide is a synthetic opioid that slows gut motility and increases water absorption. For severe cases, particularly chemotherapy-induced diarrhea, physicians prescribe a high-dose loperamide regimen until the diarrhea is controlled. If loperamide fails to resolve Grade 2 diarrhea within 24 to 48 hours, or if the patient presents with Grade 3 or 4 symptoms, treatment is escalated.
The next line of defense is often octreotide, a synthetic version of the hormone somatostatin, administered by subcutaneous injection. Octreotide works by decreasing intestinal fluid secretion and slowing transit time, making it effective for severe secretory diarrhea. For patients experiencing Grade 3 or 4 diarrhea, hospitalization is required to administer intravenous fluids and electrolytes, monitor for complications like neutropenia, and initiate anti-diarrheal therapy.
Patients must be aware of warning signs that necessitate an immediate call to the care team or a trip to the emergency room. The severity of diarrhea may also necessitate the temporary adjustment or pausing of the cancer treatment regimen to allow the intestinal lining to heal.
Warning Signs
- A fever of 100.4°F (38°C) or higher
- The presence of blood in the stool
- Severe abdominal pain
- Any sign of dehydration, such as dizziness or significantly decreased urination