The study plans to evaluate the impact of the Modified Mediterranean Diet on the biodiversity of the intestinal microbiota (the set of physiological bacteria present at the intestinal level) in patients suffering from metastatic colorectal cancer undergoing chemotherapy treatment +/- biological agent, comparing the microbiota with that of patients following a Western or standard diet.

Specifically, as well demonstrated in clinical studies, there are populations of bacteria that play a role in protecting the intestinal barrier and whose proliferation could be promoted by the Modified Mediterranean Diet.

The study also intends to evaluate the influence of the microbiome on the clinical progress of the disease in terms of side effects and quality of life.

The study will be divided into:

• Screening visit (V0) and Baseline visit (V1): during a control visit, the patient will be asked to participate in the study. After signing the written informed consent for participation in the MIC-1 study, the patient will be screened for his nutritional status. During the visit, a folder will be used to assess nutritional status. Routine blood tests will be carried out, a serum sample will be taken to measure nutritional markers and lifestyle will be assessed. Furthermore, the detection of anthropometric measurements (weight, height, BMI), bioimpedance analysis, estimation of calorie-protein needs and Malnutrition Universal Screening Tool (MUST) nutritional screening is envisaged. Once the study inclusion criteria are met, the patient will return for a visit and randomization into the study arms will take place. Arm A will then be given the modified Mediterranean diet (DMM), while arm B will continue to follow its usual diet. The patient will deliver the fecal sample. Subsequent patient visits will take place every 15 or 21 days depending on the ongoing therapy.
• Visit V2: this visit will be carried out approximately 45 days after the baseline visit. The patient will deliver the stool sample.
• Visit V3: this visit will be carried out approximately 90 days after the baseline visit. During this visit, as per clinical practice, the radiological re-evaluation will be carried out.

Inclusion Criteria:

• Eastern Cooperative Oncology Group performance status (ECOG): 0 or 1;
• Age ≥ 18;
• Diagnosis of metastatic colorectal cancer, documented histologically and radiologically according to RECIST 1:1 criteria;
• 1st line chemotherapy treatment +/- biological agent (anti-EGFR, anti-VEGF);
• MUST score 0/1;
• Written informed consent.

Exclusion Criteria:

• Previous oncological medical therapies;
• Indication for parenteral and/or enteral nutrition;
• MUST score > 2.