Background: Excess adiposity is linked to an increased risk of worse outcome among EBC pts. Pts undergoing EBC treatment are susceptible to change in nutrition status. However, implementation and assessment of the adherence to lifestyle interventions have been limited. This prospective trial aimed to evaluate the impact of an evidence-based nutrition intervention, according to the ADG, in terms of body composition changes in EBC pts. Methods: Entry criteria: EBC pts candidate to neoadjuvant/adjuvant therapy. At study entry, pts received a nutrition evidence-based tailored intervention. Dietary and anthropometric assessments were evaluated at baseline and after 12-months nutritional intervention. Waist circumference (WC) was assessed as a surrogate measure of fat distribution. ADG was estimated by Med-Diet 14-item questionnaire. Health-Related Quality of Life was analysed with EORTC QLQ-C30. Descriptive statistics was adopted. Associations between variables and groups according to nutritional variables were analysed (Chi-square test). Results: From February 2016 to December 2019, 243 pts were enrolled (median age 49 years): 27.6%/48.6% neoadjuvant/adjuvant treatment. At baseline, 38.3% of pts were overweight and 23.9% were obese. Notably, tumor size was significantly correlated with WC in the whole population (p = 0.003). Moreover, pts with central obesity were more likely to present HER2-negative tumors (57.4% vs. 42.5%, p = 0.03). Most pts reported relevant nutrition impact symptoms and symptoms affected QoL. Particularly, dyspepsia and constipation were more prevalent in overweight and obese pts (p < 0.0001 and p = 0.009, respectively), as well as in pts who gained ≥5% of weight (p = 0.04 and p = 0.02, respectively). At baseline, there was low ADG. After the 12-months intervention, ADG significantly increased (median Med-Diet score: 6 vs.12, p < 0.0001). A high ADG (defines as a Med-Diet score ≥10) significantly correlated with: 1) loss of weight ≥5% from the baseline weight (p = 0.003); 2) change in terms of BMI; 3) prevalence of central obesity. Conclusions: A tailored evidence-based nutritional intervention for EBC pts represents a tool to improve their ADG, weight management and, thus, to potentially influence the disease outcome.