Ultra-marathons over many weeks and with impressive track lengths and similar events of this division seem to push the limits of human performance ever higher. Sometimes for weeks, the participants cover long distances, often under extreme climatic conditions. Always higher, faster, further than motto seems to find its realization here.
But is there actually a physiological limit of endurance from which at some point is over? This suggests a study published by Science Advances magazine. You can find out exactly how this went, what results were shown and what you can take with you in this blog post.
The partner study between Duke University and the University of Aberdeen examined a group of competitive athletes who participated in the Race Across the USA in 2015. The ame was to run a distance of 252km over a period of five months per week. At the beginning, the BMR (Basal Metabolic Rate), i.e. the energy consumption of the body for maintaining the vital functions, was determined and the variation of the total turnover over the duration of the race was measured by means of spirometry. Subsequently, the researchers compared the data with other races. Their duration ranged from 0.5 to over 250 days.
In cumulation showed a clear trend. In the graph, the curve initially rose quickly and strongly, but then declined again until it stabilized at a continuous level. Thus, while the daily turnover was still very high at the beginning of longer races, it gradually lowered itself in negative proportionality of the duration until it settled just below the triple BMR. The body of the subjects continued to reduce the total energy consumption despite the same length of the route.
In the next step, the study group used data from various overfeeding studies in which subjects were specifically supplied with higher amounts of energy above the basic turnover. From this the conclusion could be drawn that the body is only able to effectively convert 2.5 times the BMR into energy. If the daily turnover is above that, he is forced to attack his own body mass for energy supply. It falls into a catabolic state in which, in addition to the breakdown of adipose tissue and intramuscular glycogen stores, it also begins to break down muscle tissue.
The results allow for various theories. The most likely, even in the study group's view, is that
the plateau found in the study could be a protective mechanism of the body so as not to literally "consume" itself. By reducing basic turnover and an overall more economical energy budget, it can provide the burden for longer. At the same time, however, this is probably also the absolute biological threshold for such long-term events, which the body can afford. If this is forced to exceed, it inevitably ends in a depletion to protect one's own resources. However, as mentioned above, this effect only occurs in competitions with an extraordinary duration. This was not observed in shorter races.
Not only does the results allow for several possibilities of interpretation, but the design of the study also raises inconsistencies. First of all, the very small group of subjects, which included five men and one woman, is to be listed here. In addition, not all of this group finished the race. One participant canceled the race for exhaustion, while the other chose an alternative, longer route. Likewise, a participant marched with a backpack instead of actually walking the route. The pre-determination of the BMR was estimated on the basis of a formula without testing it for actual avaluation, which may cause significant deviations in the actual actual value. Finally, spirometry, as a method of determining The BMR, has often been criticised in the past for its inaccurate results.
Accordingly, the study results no longer appear in such a clear light.
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While the study can certainly provide an indication for further investigation, it is not possible to make a clear, if any, statement on the sole basis. Here, further research with better design in this direction is definitely needed in order to gain more in-depth insights.
Link to study
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