High Altitude Increases Energy Expenditure at Rest and During Physical Activity in Healthy Subjects

Introduction: Survival at high altitudes depends on efficient energy resource management, where hypobaric hypoxia acts as a metabolic accelerator, altering thermodynamic efficiency and increasing basal caloric cost. This study compared variations in resting energy expenditure (REE) and physical activity energy expenditure (PAEE) in permanent residents of an altitudinal gradient that includes the cities of Lima (154 m), Arequipa (2,335 m), Puno (3,827 m), and La Rinconada (5,100 m). Methodology: One hundred and forty-one healthy subjects aged 18 to 38 years were evaluated using photoplethysmography (PPG) to estimate REE and PAEE, the latter after a 6-minute walk test (6MWT). Hemoglobin (Hb), hematocrit (Hct), and oxygen saturation (SpO2) levels were also analyzed as indicators of physiological status and acclimatization. Results: A progressive and significant increase in REE and PAEE was observed proportional to altitude, with the highest values ​​recorded in La Rinconada. It was determined that for every 1% decrease in SpO2, REE increased by approximately 1,286 kcal. Despite the high metabolic cost at altitude, the distance covered in the 6MWT did not vary significantly between cities, demonstrating a greater biological effort for the same mechanical workload. At extreme altitudes, men exhibited a significantly higher PAEE than women (50.60 ± 10.17 kcal vs. 40.78 ± 5.21 kcal). Furthermore, hemoglobin levels above 18 g/dL were associated with an exponential increase in caloric expenditure due to blood hyperviscosity. Conclusions: Living at critical altitudes induces a state of systemic hypermetabolism primarily regulated by SpO2 deficit. The findings suggest a metabolic threshold near 2,500 m, above which energy efficiency declines sharply. The observed sexual dimorphism suggests a possible hormonal effect on total energy expenditure (TEE) behavior.