Middle paragraph
The observed metabolic scaling patterns agree with previous studies demonstrating that total basal metabolic rate increases with body size while mass-specific metabolic rate declines because larger animals are more energy efficient on a per-unit-mass basis. The findings related to lifespan can be compared with the rate-of-living theory, which proposes that organisms with higher metabolic rates experience accelerated aging due to greater energy expenditure and increased production of reactive oxygen species. A recent Mendelian randomization study further supports this hypothesis by reporting that genetically higher basal metabolic rate is associated with reduced lifespan, suggesting that elevated metabolic activity may causally contribute to aging. However, metabolism alone does not appear to fully explain lifespan variation. Research examining mitochondrial determinants of longevity has shown that mitochondrial metabolic efficiency and the stability of mitochondrial membrane proteins are stronger predictors of maximum lifespan than mass-specific basal metabolic rate alone, indicating that cellular energy production and mitochondrial function play critical roles in aging. Similarly, studies of human aging have demonstrated that age-related declines in mass-specific basal metabolic rate closely reflect mitochondrial dysfunction and may serve as biomarkers of biological aging rather than direct causes of mortality.
Although this study identified important relationships between metabolism and lifespan, several limitations should be considered. The study relied on an existing comparative database rather than controlled experimental manipulation, meaning that causal relationships cannot be established. Lifespan is influenced by numerous biological and environmental factors, including genetics, reproductive strategies, diet, ecological pressures, and disease susceptibility, which were not incorporated into the analyses. In addition, measurements obtained from different species may contain natural variation that cannot be fully accounted for in comparative analyses.
