Divergent and Convergent Protein‑Sparing Strategies During Fasting Adaptation Among Selected Bilaterian Taxa and the Effects of Oxidative Stress

Abstract

This paper takes the protein-sparing effect as its center and argues that this metabolic strategy, already well studied in human fasting adaptation, is, in fact, a general logic of energy allocation widely shared within Bilateria (bilaterian animals). The effect can be taken apart into two levels. One level is a set of highly conserved upstream mechanisms. The mammalian‑derived three‑phase starvation model “glycogen‑gluconeogenesis‑protein‑sparing” serves as a comparative heuristic for cross‑species analysis, and this tri‑phasic pattern has been empirically documented in a limited number of taxa. Additional conserved components include the glucose‑sparing fuel pathway mediated by ketone bodies and the AMPK/mTOR nutrient‑sensing axis, which is nearly universally conserved across eukaryotes. The other level is the downstream execution, which clearly diverges. The paper discusses how far sparing goes depending on the size of non-protein energy reserves, provides evidence of tissue specificity and timing, and examines several extreme phenotypes. Recent studies on a few particular lineages are brought together, and a picture emerges in which different molecular routes converge on the same end. The paper also analyses the roles of reactive oxygen species (ROS) and oxidative stress within this system from a relatively novel perspective. This proposed coupling remains to be validated by systematic experimental testing. The paper ends with what the strategy may suggest, in a biomimetic sense, for alternative protein production systems such as cultured meat and microbial protein fermentation.