A narrative review published in Frontiers in Nutrition argues that protein and amino acid supplements are widely sold on a model of biology that does not match how metabolism actually works. The authors, based at the Università di Brescia and Ruaha Catholic University, call that model nutritional reductionism: the assumption that one isolated ingredient acts on one molecular target in a straight line.

The products under discussion are the familiar ones. Intact protein concentrates such as whey, casein, collagen, soy, pea and rice; single amino acids including leucine and beta-hydroxy-beta-methylbutyrate; and mixtures such as branched chain amino acids and essential amino acid blends.

Why one ingredient is the wrong unit

The review's core argument is that metabolism behaves as an interconnected network of genes, proteins, metabolites, signaling pathways, microbiota and environment, and that outcomes emerge from nonlinear interactions rather than from any single component. On that reading, synergistic and antagonistic effects, feedback regulation and interindividual variability are not experimental noise to be controlled away but intrinsic properties of the system.

The authors say this helps explain something the field has lived with for years: the inconsistent and discordant findings that keep turning up in clinical studies of supplements. Their phrasing on the commercial side is unusually direct, noting that widespread use is often supported by mechanistic evidence that may exceed available clinical data.

Where supplementation is actually justified

The review is not a blanket dismissal. It cites current European guidance putting average daily protein requirement for healthy adults at roughly 0.83 g/kg of body weight, an intake the authors say is readily achievable through a conventional diet that often supplies more than the minimum.

The case changes when demand outruns intake. In older adults, the authors point to anabolic resistance and a higher target of 1.0 to 1.2 g/kg per day, and say isolated proteins or essential amino acid blends are clinically justified when appetite loss or chewing difficulty blocks that level. They also report that supplementation improved strength and physical performance more when combined with resistance exercise than either supplementation or exercise alone.

Limitations

This is a narrative review, not a systematic one: the authors state plainly that it did not follow PRISMA guidelines and should not be read as one. Studies were selected for conceptual relevance rather than by protocol, animal work was included alongside human trials, and generative AI was used for part of one figure and for grammar checking.

The underlying literature carries its own problem. The authors flag high methodological heterogeneity across recent meta-analyses in supplement type, dosage and duration, and urge caution in generalizing from them.

Bottom line

The argument here is epistemological before it is practical, and it lands: if effects emerge from a network, a label naming one active ingredient is telling you very little about what happens in a given person. The authors propose integrating mechanistic knowledge with systems biology rather than discarding either, which leaves the harder question unanswered for now, namely who responds and why.