Health alterations in spaceflight and ground-based analogs

Long-duration human spaceflight exposes astronauts to five major hazards identified by NASA — space radiation, isolation and confinement, distance from Earth, microgravity, and hostile or closed environments, which collectively challenge systemic homeostasis. This analytical scoping review aimed to establish a systems-level framework for understanding physiological risks and protection strategies required for long-duration human space exploration. Rather than cataloging spaceflight-associated findings as isolated outcomes, this review integrates evidence from actual spaceflight and Earth-based analog studies to examine how space-related stressors converge on shared biological mechanisms across multiple physiological systems. A PubMed-based literature search identified 2,103 records, and 85 original empirical studies published between 2000 and 2026 were included in the final synthesis. The included studies were organized across nine physiological domains: microbiome, immune, muscle, renal, cardiovascular, skeletal, cognitive/neural, sleep-related, and SANS/neuro-ocular outcomes. Across these systems, space-related stressors were repeatedly associated with overlapping molecular and physiological processes, including oxidative stress, mitochondrial and metabolic dysregulation, altered mechanotransduction, immune-inflammatory activation, fluid and calcium imbalance, proteostatic remodeling, and circadian-output disruption. These findings suggest that spaceflight-related health risks arise from interconnected multisystem adaptation rather than independent organ-specific dysfunction, and that many alterations reflect environment-appropriate adaptation that becomes clinically relevant primarily when functional reserve is exceeded or upon return to gravity, in contrast to cumulative stress-associated injury such as radiation-induced damage and neuro-ocular remodeling. The overlap between spaceflight-associated alterations and terrestrial conditions such as aging, physical inactivity, chronic stress, inflammatory disorders, and neurovascular dysfunction suggests that the space environment may amplify conserved stress-response networks. Based on this framework, future countermeasure development should move beyond single-target interventions and incorporate integrated strategies addressing mechanical unloading, radiation-associated stress, fluid redistribution, immune and metabolic imbalance, sleep–circadian disruption, neuro-ocular remodeling, and microbiome instability. This review provides a mechanism-based foundation for linking environmental hazards, shared molecular pathways, and organ-system outcomes to support human health protection during future lunar and deep-space exploration missions.