Post-Diagnostic Diet and Breast Cancer Outcomes: A Comprehensive Systematic Review of Prospective Cohort Studies and Randomized Clinical Trials

Background/Objectives: Diet is increasingly recognized as a modifiable factor in cancer survivorship; however, the impact of post-diagnostic dietary habits on breast cancer (BC) outcomes remains uncertain. This systematic review aimed to evaluate the association between diet after BC diagnosis and clinical outcomes and to provide an updated synthesis of the evidence, with particular emphasis on dietary assessment methods, methodological heterogeneity, and evidence gaps. Methods: We systematically reviewed randomized controlled trials (RCTs) and prospective cohort studies that investigated post-diagnostic dietary exposures among women with BC. Outcomes of interest included all-cause mortality (ACM), breast cancer-specific mortality (CSM), recurrence (R), overall survival (OS), and disease-free survival (DFS). Sources of methodological heterogeneity were assessed, and findings were synthesized using a structured narrative approach. Results: A total of 39 studies were included (8 RCTs and 31 prospective cohort studies), comprising over 90,000 participants. Evidence from RCTs, including low-fat and multi-component interventions, showed largely null or inconsistent effects on BC-specific outcomes. Findings from cohort studies were heterogeneous. Associations for individual nutrients and food groups were generally inconsistent across studies, whereas healthier dietary patterns and higher diet quality scores were associated with reduced ACM. Associations with BC-specific outcomes were less consistent. Overall, the available evidence was characterized by substantial methodological heterogeneity, overlapping study populations, and low-to-moderate certainty. Conclusions: Current evidence suggests that post-diagnostic dietary habits may contribute more to overall survivorship than to BC-specific prognosis. Methodological heterogeneity, particularly in dietary assessment, should be taken into account when interpreting findings. The principal contribution of this review is the identification of important methodological limitations and evidence gaps that currently limit confidence in the available literature. Future research should employ more accurate dietary assessment methods and comprehensive nutritional evaluation to clarify the role of nutrition in BC outcomes.