An increasing number of people engage in endurance exercise, however, current literature lacks information regarding changes during ultramarathons of very long distances at very low intensity, i.e., walking instead of running. In addition, adverse conditions, like very cold climate, have not been investigated in such settings. Furthermore, the physiological processes linking functions like energy expenditure, metabolism, stress, and resilience, are still not well understood or lack investigation all together. The Yukon Arctic Ultra (YAU) has been coined to be the longest and the coldest ultramarathon in the world, as it challenges athletes to complete the very long distance of 690 km under the extremely cold climate conditions of North-Canadian subarctic winter. YAU athletes face the challenges of long-term endurance exercise in a very cold climate and under diminished resting conditions. Thusly, the YAU served as a model to investigate physiological changes among healthy athletes during an ultra-long endurance exercise in extremely cold climate. The presented investigation revealed considerable increases in energy expenditure among the athletes, of up to more than four times resting metabolic rate, along with a marked energy deficit, changes in body composition with reductions in fat mass while fat free mass could mostly be retained, possibly to the exercise- and cold-induced release of myokines like irisin and follistatin. Furthermore, analysis of vegetative control expressed through heart rate variability, as well as of mood through psychometric measurements, revealed that the more successful athletes, who would eventually be able to finish the race, showed better adaptation to the race demands, with less depression, anxiety and anger, but greater vigor and higher alertness. They also exhibited a faster restoration of vagal predominance during the race with a better ability to relax and restore, leading to less sleepiness and greater vigilance compared to the non-finishing athletes. Resilience, as the ability to cope with stressful events, appears to be a key element during such a race and neuropeptide Y has been discussed as a mediator in resilience reactions. The analyses revealed that during this race, neuropeptide Y was associated with less confusion and better quality of recovery among the finisher group and that overall neuropeptide Y was increasingly released among the athletes compared to the less challenged control group. Another influence was the factor sex, indicating that the female athletes were not only as successful as the male participants to complete the whole race, but that women completed an overall greater distance, when accounting for all covered distances. Women, with a moving speed of 3.7 km/h, were considerably slower than men at 4.6 km/h, which may have saved energy stores and allowed them to predominantly use fat as a long-term energy source. In addition, it was shown that ultra-long endurance exercise may lead to shedding of endothelial glycocalyx elements and that these elements appear to be differently susceptible to that shedding. Sex, age, and covered distance all appeared to have an influence on the observed changes. The analyses of physiological changes during the Yukon Arctic Ultra have revealed a multitude of endurance- and cold-exposure-related alterations. This ultramarathon has thusly proven to be an outstanding model to study human adaptation capabilities to extreme environments under real-life field conditions that could otherwise not be replicated in a laboratory setting.
