ALWAYS GO ON?
Ultra marathons over many weeks and with impressive distances and similar events in this category seem to push the limits of human performance ever further upwards. The participants cover long distances, sometimes over weeks, often under extreme climatic conditions. Always higher, faster, further seems to be the motto here.
But is there actually a physiological limit to endurance beyond which it stops at some point? This is suggested by a study published in Science Advances magazine. In this blog post you will find out exactly how this happened, what results were shown and what insights you can take away from it.
THE STUDY
The partner study between Duke University and the University of Aberdeen examined a group of competitive athletes who took part in the Race Across the USA in 2015. The aim was to run a distance of 252km per week over a period of five months. Initially, the BMR (Basal Metabolic Rate), i.e. the body's energy consumption to maintain vital functions, was determined and the variation in total metabolic rate over the duration of the race was measured using spirometry. The researchers then compared the data with other races. Their duration ranged from 0.5 to over 250 days.
THE RESULTS
A clear trend emerged in cumulative terms. Graphically, the curve initially rose rapidly and sharply, but then fell again until it stabilized at a continuous level. While the daily turnover was still very high at the beginning of longer races, it gradually decreased in negative proportion to the duration until it leveled off just below three times the BMR. The subjects' bodies continued to reduce their overall energy consumption despite the same distance.
In the next step, the research group used data from various overfeeding studies in which subjects were specifically given higher amounts of energy above their basal metabolic rate. The conclusion could be drawn from this that the body is only able to effectively convert a maximum of 2.5 times the BMR into energy. If the daily turnover is higher, he is forced to use his own body mass to provide energy. He falls into a catabolic state in which, in addition to breaking down fatty tissue and intramuscular glycogen stores, he also begins to break down muscle tissue.
THE KNOWLEDGE
The results allow for various theories. The most likely, according to the study group, is the following:
The plateau that emerged in the study could represent a protective mechanism for the body to prevent it from literally “using itself up”. By reducing the basal metabolic rate and maintaining an overall more economical energy balance, he can carry out the workload for longer. At the same time, this probably also represents the absolute biological threshold for such long-term events that the body can achieve. If you forcefully try to exceed this limit, it will inevitably end in exhaustion in order to protect your own resources. However, as mentioned, this effect only occurs in competitions of exceptional duration. This was not observed in shorter races.
THE PROBLEM
Not only do the results allow for several possible interpretations, the design of the study also raises inconsistencies. First, the very small group of test subjects should be listed here, which included five men and one woman. In addition, not everyone in this group finished the race. One participant dropped out of the race due to exhaustion, while the other chose an alternative, longer route. Likewise, one participant marched with a backpack instead of actually running the route. The preliminary determination of the BMR was estimated using a formula without testing it for actual accuracy, which can result in large deviations from the actual value. Finally, spirometry, as a method for determining BMR, has often been criticized in the past due to its inaccurate results.
Accordingly, the study results no longer appear in such a clear light.
Take home message
While the study can certainly provide a clue for further investigation, it cannot be used to make a clear, if any, conclusion on its own. Further research with better design in this direction is definitely needed to gain more solid insights.
Link to study
Extreme events reveal an alimentary limit on sustained maximum human energy expenditure
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