Adaptations to breath-hold diving: from traditional divers to elite athletes.
Breath-hold diving exposes humans to repeated episodes of profound hypoxia and hypercapnia, eliciting physiological adaptations that enable prolonged underwater performance. This article summarises current knowledge on chronic adaptations in elite breath-hold athletes and traditional diving populations, including the Bajau sea nomads of Southeast Asia and the Korean Haenyeo divers. Evidence indicates that repeated apnoea induces adaptations across multiple physiological systems. Haematological changes include increased spleen size and enhanced splenic contraction, augmenting circulating haemoglobin and oxygen stores during apnoea. In elite divers, structured training can increase resting spleen volume, whereas the Bajau exhibit genetically associated splenic enlargement linked to variants near the PDE10A gene. Cardiopulmonary adaptations include modified pulmonary vascular responses to hypoxia, improved oxygen conservation, and metabolic shifts favoring efficient mitochondrial energy production. Molecular adaptations involve enhanced antioxidant defenses and activation of hypoxia-responsive pathways that may mitigate oxidative stress associated with repeated hypoxia-reoxygenation cycles. Emerging evidence also suggests neural plasticity and possible structural brain adaptations, although the long-term neurological consequences of chronic intermittent hypoxia exposure remain uncertain. Studies of traditional diving populations indicate that both phenotypic plasticity and genetic selection contribute to diving capacity, highlighting interactions between training and evolution. Despite these benefits, breath-hold diving also carries risks, including hypoxic blackout, decompression sickness, and potential neurological injury. Understanding the mechanisms underlying human tolerance to extreme hypoxia may have implications beyond diving physiology, including applications in cardiovascular medicine, hypoxic diseases, and rehabilitation. Further longitudinal, genomic, and mechanistic studies are needed to clarify the limits, benefits, and clinical relevance of these adaptations.