Preclinical evidences suggest that reducing the concentration of blood metabolites and growth factors reduces the in vivo growth of several tumor models, while protecting normal tissues from the cytotoxic effects of chemotherapeutical treatments. In recent years, a plant-based, calorie-restricted, low-carbohydrate, low-protein diet, also known as Fasting Mimicking Diet (FMD), has been proposed as a potential anticancer dietary intervention. The FMD is safe when administered cyclically (every 21-28 days) to healthy volunteers, and is capable of significantly reducing the concentration of plasma glucose, serum insulin and IGF-1, while increasing levels of plasma IGFBPs and ketone bodies. The FMD has been shown to inhibit the in vivo growth of several tumor models, including breast cancer and melanoma mice models. The anticancer effects of the FMD are likely mediated by two concomitant mechanisms: 1) one direct anticancer effect that is mediated by the inhibition of energy production and anabolic pathways, such as protein and fatty acid synthesis, in cancer cells; 2) one indirect effect that is mediated by the activation of antitumor immunity, with the result of enhanced tumor infiltration by cytotoxic CD8+ T-lymphocytes and reduced infiltration by immunosuppressive populations. According to the currently accepted model, the anticancer and immunomodulatory effects of the FMD mostly derive from the reduction of circulating glucose, insulin and IGF-1 levels, and a parallel increase of ketone body and IGF-1 binding protein concentration. However, recent observations in healthy volunteers and cancer patients, suggest that FMD-mediated changes in many other metabolites, such as specific amino acids or fatty acids, could contribute to the cell-autonomous or immune-mediated anticancer effects of the FMD. While the study of the effects of the FMD in combination with standard treatments (e.g. chemotherapy, molecular targeted therapy) in advanced cancers represents the final objective of the ongoing studies, fully uncovering the metabolic and immunological effects of the FMD alone is essential to design future combination studies. From this perspective, the pre- and post-operative clinical settings in cancer patients who are not candidate to other medical treatments represent an ideal context to assess the effects of the FMD without other confounding factors. This trial primarily aims to assess the immunological and metabolic changes induced by the FMD in the pre-operative and post-operative setting in breast cancer and melanoma patients. Three cohorts of patients will be enrolled: 1) Cohort A: patients with resectable breast cancer (cT1N0M0 stage or cT1cN1M0-cT2cN0M0 stages not requiring pre-operative systemic treatment at the judgment of the investigator) who are candidate to curative surgery; 2) Cohort B: patients with malignant melanoma patients candidate to dissection of the lymph node basin because of a positive sentinel lymph node (stage IIIA-IIIB-IIIC); 3) Cohort C: patients with resected malignant melanoma (including radicalization and, in case, lymph node dissection) who are not candidate to any adjuvant treatment, but only to clinical and radiological follow-up (stage IIB-IIC). Patients in cohorts A and B will undergo one 5-days FMD cycle about 13-15 days before surgical removal of primary tumor (breast) or lymph nodes (breast, melanoma). Patients in cohort C will undergo 4 consecutive FMD cycles every 28 days, starting one month after surgery.