Expanding endurance exercise physiology research: insights from animal migration

Much of our understanding of human physiological responses to exercise comes from human or rodent exercise models. In particular, there is clear evidence that endurance-trained individuals have increased fat breakdown, but only after weeks of physical training. In contrast, long-distance migratory animals such as birds, fish and insects exhibit increased activity of the molecular machinery involved in fat breakdown even before the exercise stressor occurs. Additionally, these migratory species rely more on fatty acids to meet the 85% VO2,max energy demand of flight compared with humans exercising at the same workload. This underscores the superior ability of migratory animals to use the energy-rich fuel of fatty acids. Despite long-distance migration being studied for decades, only a few studies have directly compared one or two ultra-athletic taxa. This Review examines diet selection, mobilization and transport, uptake and breakdown, and mitochondrial respiratory capacity in humans, migratory birds, fish and insects. Our goal is to provide a comprehensive comparison of these taxa, highlighting the degree to which the molecular machinery involved in fat breakdown is conserved, despite differences in fuel selection and preferences that enable them to meet the energy demands of long-distance exercise.