Metabolic complexity and nutritional reductionism. Deconstructing the single active ingredient paradigm in protein and amino acid supplements.

Metabolism is an adaptive and highly interconnected system in which biological outcomes emerge from dynamic interactions among genes, proteins, metabolites, signaling pathways, microbiota, and environmental factors. Nevertheless, nutritional research has traditionally relied on a reductionist paradigm, attributing biological effects to isolated compounds acting on specific molecular targets through linear cause-effect relationships. While this approach has been instrumental in elucidating biochemical mechanisms and addressing nutrient deficiencies, its explanatory and predictive power is limited in complex biological systems and multi-component nutritional interventions. Dietary supplements, particularly protein- and amino acid-based formulations, may exert effects through synergistic or antagonistic interactions, network modulation, and context-dependent responses that cannot be adequately predicted from isolated molecular effects. Interindividual variability, metabolic plasticity, and feedback regulation further challenge deterministic models and may contribute to inconsistent clinical findings. A systems-based perspective complements reductionist approaches by considering metabolism as a dynamic network in which biological responses emerge from nonlinear interactions across multiple organizational levels. This narrative review specifically covers protein and amino acid supplement categories, including intact protein concentrates (whey, casein, collagen, soy, pea and rice), individual amino acids (Leucine, β-hydroxy-β-methyl butyrate), and amino acid mixtures (branched chain amino acids and essential amino acids mixture), to examine their role in metabolic health. Indeed, their widespread use is often supported by mechanistic evidence that may exceed available clinical data. Integrating mechanistic knowledge with systems biology may provide a more realistic framework for interpreting supplement efficacy, understanding biological variability, and improving the translational relevance of nutritional research and clinical practice.