Amino Acids Supplementation in Cancer: What Do We Feed, the Patient or the Tumor?

Background/Objectives: Diet and obesity contribute to approximately 50% of tumor development. Therefore, nutrition plays a key role not only in cancer prevention but also in determining prognosis. Notably, between 30% and 90% of cancer patients experience malnutrition. Furthermore, the hypercatabolic state induced by tumors leads to widespread protein degradation, clinically manifesting as sarcopenia or cachexia, and ultimately accelerating mortality. This narrative review examines the potential role of amino acids (AAs) in inhibiting tumor growth and counteracting protein–energy malnutrition—aiming to preserve muscle mass and nourish healthy cells while placing neoplastic cells in a state of metabolic stress. Methods: The analysis was conducted following the Standards for Reporting Qualitative Research guidelines. Results: Administration of targeted mixtures of essential amino acids (EAAs) has been shown to improve muscle mass, strength, and quality of life in patients with hypercatabolic conditions. Experimental in vitro and in vivo studies also suggest a potential inhibitory effect on tumor proliferation. However, increased availability of certain AAs may, in some cases, stimulate tumor growth, one reason why EAAs supplementation in cancer patients remains controversial. Conclusions: Despite prevailing concerns, emerging evidence indicates that supplementation with a complete, well-balanced EAAs formulation may be a valuable adjunct to standard cancer therapies. This approach could help correct cancer-associated protein imbalances, enhance patients’ quality of life, and create a metabolic environment unfavorable to tumor progression.