Intermittent Fasting and an Ultra Processed Food Diet Differentially Influence Tumor Progression and Gut Microbiome Composition in a Triple Negative Breast Cancer Mouse Model

Breast cancer is the most common malignancy in women, and triple‑negative breast cancer (TNBC) represents its most aggressive subtype due to high metastatic potential and limited therapeutic options. Dietary patterns are increasingly recognized as modulators of cancer progression. Ultra‑processed foods (UPF), which are energy‑dense and nutrient‑poor, are associated with obesity and chronic inflammation, whereas intermittent fasting (IF) has been linked to improved metabolic profiles and reduced inflammatory responses. The intestinal microbiome is highly responsive to dietary interventions and may reflect physiological changes relevant to tumor development. In this study, female BALB/c mice were assigned to a standard diet (SD), an UPF diet, or an IF regimen, with or without TNBC induction using 4T1 cells. Tumor progression was evaluated through primary tumor measurements and liver metastasis, and gut microbiome composition, diversity, and predicted metabolic pathways were analyzed using 16S rRNA sequencing. UPF‑fed mice developed larger tumors and exhibited increased metastatic burden. Their microbiome showed reduced diversity, enrichment of taxa associated with inflammatory profiles, and decreased predicted metabolic flexibility. IF was associated with smaller tumors and fewer metastases, accompanied by a more diverse microbial community enriched in taxa with potential immunoregulatory or antitumor characteristics. Importantly, breast cancer had a detrimental impact on gut microbiome parameters in both dietary contexts, as TNBC‑bearing mice displayed reduced diversity, compositional shifts, and lower predicted metabolic adaptability compared to their respective non‑tumor controls. Overall, UPF and IF produced distinct effects on gut microbiome structure and metabolic potential, as well as on TNBC progression. These findings highlight how contrasting dietary patterns differentially influence microbiome features and tumor outcomes.