Introduction: In the 2023-2025 period, colorectal cancer (CRC) will be the third most common type of cancer in Brazil. The most commonly used therapeutic approaches are chemotherapy (QTx) and radiotherapy (RTx). QTx agents, such as capecitabine, affect both malignant cells and normal cells such as gastrointestinal, capillary and immune cells. Cellular damage in the gastrointestinal tract (GIT) results in various symptoms, such as mucositis and diarrhea. Diarrhea is linked to mucosal damage and can result in dehydration, malnutrition and hospitalization, leading to cardiovascular complications and death. Mucositis is inflammation that affects the GIT. This condition makes treatment difficult, leading to reductions, delays or interruption of QTx. These scenarios harm the patient's prognosis and quality of life, resulting in high costs for symptom control, nutritional assistance, management of secondary infections and hospitalization. The Mucositis Study Group (MASCC/ISOO) guidelines recommend the use of probiotics as a preventive measure against diarrhea in cancer patients undergoing QTx and/or RTx. However, the safety of using probiotics in immunosuppressed patients is still controversial and hypotheses are based on epidemiological and experimental studies. This makes it necessary to evaluate whether supplementation with pro- or synbiotics before chemotherapy would have the same beneficial results.

Objective: To evaluate the effect of synbiotic supplementation on the prevention, incidence and severity of mucositis in cancer patients undergoing QTx. Method: This is a single-center parallel double-masked randomized clinical trial to be carried out at the Borges da Costa/UFMG Outpatient Clinic at Hospital das Clínicas in Belo Horizonte - Minas Gerais (HC-BH/MG). The inclusion criteria are patients diagnosed with CRC eligible for first-line treatment with Capecitabine, aged ≥ 18 years, both sexes, signed the informed consent form. The study was approved by CEP-UFMG. Expected results: It is expected that supplementation with synbiotics in the pre-QTx period will promote modulation of the microbiota and strengthening of the intestinal barrier, resulting in a lower incidence and severity of mucositis and diarrhea, improving the quality of life of these patients.