Lung cancer is primarily a disease of the respiratory system, yet many patients experience symptoms seemingly unrelated to the lungs, such as diarrhea. Although the tumor is located in the chest, the systemic effects of cancer and modern oncological treatments can manifest in the gastrointestinal tract. This article clarifies the direct and indirect causes of diarrhea in the context of lung cancer.
Systemic Effects of Lung Tumors
The cancer itself, independent of treatment, can cause diarrhea through a paraneoplastic syndrome. These syndromes occur when cancer cells produce and secrete hormones or hormone-like substances into the bloodstream that travel to distant organs and disrupt normal function. This is a direct, though less common, consequence of the tumor’s biological activity.
Paraneoplastic Syndromes
Small Cell Lung Cancer (SCLC), a neuroendocrine tumor, often causes these syndromes. SCLC cells can produce substances like serotonin, which regulates gut motility. Excessive serotonin production leads to carcinoid syndrome, characterized by profuse, watery diarrhea.
Other neuroendocrine peptides, such as Vasoactive Intestinal Peptide (VIP), may also be produced. VIP stimulates the secretion of water and electrolytes into the small intestine, and its overproduction can result in massive volumes of watery diarrhea. This hormonal disruption can cause significant fluid and electrolyte imbalances. Successful treatment of the underlying cancer typically leads to a resolution of the paraneoplastic symptoms.
Gastrointestinal Side Effects of Therapy
Treatment-related side effects are the most frequent cause of diarrhea in individuals with lung cancer. Modern cancer therapies, designed to halt rapidly dividing cancer cells, often inadvertently affect other fast-growing cells, particularly the healthy cells lining the gastrointestinal tract. This collateral damage is the root cause of most treatment-induced diarrhea.
Chemotherapy
Chemotherapy agents, such as irinotecan and fluoropyrimidines, are particularly notorious for causing this side effect. These drugs damage the delicate mucosal layer of the gut, causing inflammation, cell death, and a disruption of the balance between fluid absorption and secretion. The damage to the intestinal lining, known as mucositis, can impair the gut’s ability to absorb nutrients and water, leading to malabsorption and diarrhea.
Targeted Therapy
Targeted therapies, especially those that inhibit the Epidermal Growth Factor Receptor (EGFR), frequently cause diarrhea through a different, non-cytotoxic mechanism. EGFR is present on the surface of both cancer cells and healthy cells in the colon, where it helps regulate fluid and electrolyte transport. When drugs like EGFR-Tyrosine Kinase Inhibitors (TKIs) block this receptor, they inhibit its normal function in the gut lining.
This inhibition leads to an uncontrolled, excessive secretion of chloride ions into the colon, which draws water into the intestinal lumen. The result is a secretory diarrhea that can be severe.
Immunotherapy
Immunotherapy, which utilizes Immune Checkpoint Inhibitors (ICIs) like anti-PD-1 or anti-PD-L1 drugs, can also induce diarrhea, but through an immune-related inflammatory process. These therapies work by unleashing the body’s immune system to attack cancer cells, but they can sometimes trigger an autoimmune-like response against healthy tissues. When this response targets the colon, it causes inflammation known as colitis.
Immune-related colitis is a serious adverse event that manifests as persistent diarrhea, often accompanied by abdominal pain. When colitis occurs, it may require immediate intervention with corticosteroids or other immunosuppressive agents. If left unmanaged, immunotherapy-induced colitis can lead to severe complications and often necessitates a temporary or permanent stop to the cancer treatment.
Related Causes and Symptom Management
Beyond the direct effects of the tumor and its treatments, several other factors can contribute to changes in bowel habits. A weakened immune system due to cancer or chemotherapy leaves patients vulnerable to opportunistic infections, such as Clostridium difficile. Though rare, the physical spread of lung cancer to the gastrointestinal tract can also cause obstructive symptoms or bleeding.
Nutritional factors and general changes in daily life also play a role. Dietary changes, certain nutritional supplements, high-fat foods, or the use of specific antibiotics can all disrupt the delicate bacterial balance in the gut, contributing to loose stools.
Managing Diarrhea
Managing diarrhea is a critical aspect of cancer care to prevent dehydration and maintain quality of life. The most important step is to maintain adequate hydration by consuming clear fluids that contain electrolytes, such as broths, sports drinks, or oral rehydration solutions. Plain water alone is often insufficient to replace lost salts and minerals.
Dietary modifications are highly effective for managing milder cases. Patients should focus on bland, low-fiber foods, often referred to as the BRAT diet (bananas, rice, applesauce, toast). It is advisable to temporarily avoid foods that stimulate the bowel, including:
- Spicy foods
- Greasy foods
- High-fiber foods
- Dairy products
- Caffeine
- Alcohol
Patients should always communicate changes in bowel habits to their oncology team immediately. The provider will determine the underlying cause and recommend appropriate anti-diarrheal medications, such as loperamide. Diarrhea becomes a medical emergency if symptoms include:
- Fever
- Blood in the stool
- Severe abdominal cramping
- Signs of dehydration like dark urine or dizziness