The biological state hypothesis: biological state as a systems-level constraint on human adaptive capacity.

Contemporary biomedicine has largely approached the human organism in a fragmented way: each specialty addresses its own system, each biomarker its own domain. This reductionist model has produced precise knowledge of parts, yet remains poorly suited to explain the systemic nature of chronic disease and accelerated biological aging-and, in particular, why individuals exposed to comparable biological and environmental demands follow radically different adaptive trajectories, with some losing their capacity to respond long before any diagnosable pathology appears. We propose the biological state hypothesis (BSH), a theoretical framework in systems physiology. Biological state is defined as the dynamic expression of the functional order with which the organism engages its environment-an integrated physiological condition emerging from the continuous interaction among three interdependent layers: systemic regulation (the coordinated activity of the autonomic, neuroendocrine, immune-inflammatory, and circadian axes), metabolism (the capacity to generate, allocate, and utilize energy for regulatory, repair, and adaptive processes), and biological plasticity (neuroplasticity and epigenetic-genomic plasticity, through which sustained environmental pressures are inscribed into the organism's biological architecture). Environmental effects on biological plasticity occur predominantly-although not exclusively-through systemic regulation and metabolism, which together condition how the organism's architecture is remodeled. The central proposition is that adaptive capacity-the organism's capacity to maintain or restore integrated physiological coherence when confronted with environmental demands-is an emergent functional property of integrated biological state. When the three layers maintain coherence, adaptive reserve is broad; when dysregulation persists across domains, it progressively narrows. Within this framework, chronic non-communicable disease and accelerated biological aging are understood not as primary events but as downstream consequences of a sustained deterioration of biological state-one that first manifests as a narrowing of adaptive capacity and only later as overt pathology. Presented as a falsifiable framework rooted in the cross-domain integration of multiple physiological domains, the BSH generates five empirically testable predictions and provides a conceptual foundation for understanding, measuring, and restoring human adaptive capacity across the life course.