Metabolic Reprogramming as a Therapeutic Target in Cancer: A Qualitative Systematic Review (QualSR) of Natural Compounds Modulating Glucose and Glutamine Pathways

Background: Despite advances in gene-targeted and immunotherapies, many aggressive cancers—including glioblastoma and triple-negative breast cancer—remain refractory to treatment. Mounting evidence implicates metabolic reprogramming, especially dysregulation of glucose and glutamine metabolism, as a core hallmark of tumor progression. Natural compounds with metabolic-modulatory effects have emerged as promising adjuncts in oncology. Research Question and Objectives: This review investigates the following question: How can metabolic-targeted therapies—particularly those modulating the Warburg effect and glutamine metabolism—improve cancer treatment outcomes, and what role do natural compounds play in this strategy? The objectives were to (1) evaluate the therapeutic potential of metabolic interventions targeting glucose and glutamine metabolism, (2) assess natural compounds with metabolic regulatory activity, (3) examine integration of metabolic-targeted therapies with conventional treatments, and (4) identify metabolic vulnerabilities in resistant malignancies. Methods: A qualitative systematic review (QualSR) was conducted following PRISMA guidelines. A total of 87 peer-reviewed studies published between 2000 and 2024 were included. Inclusion criteria required clearly defined mechanistic or clinical endpoints and, for clinical trials, sample sizes ≥ 30. Data extraction focused on tumor response, survival, metabolic modulation, and safety profiles. Results: Curcumin significantly reduced serum TNF-α and IL-6 (both p = 0.001) and improved antioxidant capacity (p = 0.001). EGCG downregulated ERα (p = 0.002) and upregulated tumor suppressors p53 and p21 (p = 0.001, p = 0.02). High-dose intravenous vitamin C combined with chemoradiotherapy yielded a 44.4% pathologic complete response rate in rectal cancer. Berberine suppressed Akt/mTOR signaling and glutamine transporter SLC1A5 across tumor types (q < 10−10). However, poor bioavailability (e.g., EGCG t½ = 3.4 ± 0.3 h) and systemic toxicity limit their standalone clinical application. Conclusions: Metabolic-targeted therapies—particularly natural compounds acting on glucose and glutamine pathways—offer a viable adjunct to standard cancer therapies. Clinical translation will require biomarker-driven patient stratification, improved delivery systems, and combination trials to optimize the therapeutic impact in treatment-resistant cancers.