The Ketogenic Diet as Adjunctive Strategy in Cancer Treatment—Therapeutic Benefits and Metabolic Risk Using Breast Cancer as an Example

Nutritional therapy is an essential component of comprehensive care for cancer patients, playing a vital role in preventing malnutrition, improving treatment tolerance, and maintaining quality of life. In recent years, there has been growing interest in the ketogenic diet as a potential strategy to support cancer therapy. This diet, based on a significant reduction in carbohydrate intake and an increase in fat intake, leads to a state of ketosis that mimics the metabolic changes that occur during starvation. Consequently, the body utilizes ketone bodies as an alternative source of energy. The potential application of the ketogenic diet in oncology stems from the distinct metabolism of cancer cells, which often exhibit an increased demand for glucose. Although the results of preclinical studies suggest a possible anticancer effect of this dietary intervention, clinical evidence remains limited, and its routine use is not currently recommended as a standard of care in oncology. The aim of this study was to analyze the current literature on the use of the ketogenic diet in oncology, with a particular focus on breast cancer. The study assessed the impact of this intervention on metabolic parameters, body composition, and patients’ mental well-being, and analyzed the potential metabolic risks associated with its use. The study discusses the molecular basis of oncogenesis and metabolic adaptation in cancer cells, the mechanisms of action of the ketogenic diet, and the results of preclinical and clinical studies as well as meta-analyses. Importantly, the current evidence base should be interpreted with caution. Most mechanistic evidence supporting ketogenic diet in breast cancer derives from preclinical models, whereas clinical studies remain limited by small sample sizes, short intervention periods, heterogeneous ketogenic diet protocols, substantial attrition, and differences in concomitant anticancer treatment. Therefore, mechanistic plausibility and promising preclinical findings should not be interpreted as evidence of clinical anticancer efficacy.