I have recently stumbled upon an article on prolonged endurance events and the limits on energy expenditure. (C. Thurber et al., Extreme events reveal an alimentary limit on sustained maximal human energy expenditure, Science Advances, vol. 5, no. 6, (2019) eaaw0341). I found it particularly interesting and I decided to give here a short summary of its findings.
The champions of prolonged effort are migrating birds. Several studies have been devoted to the measure of the energy expenditure during their long treks, the consensus being that migrating birds can sustain energy budgets of the order of 5 times their basal metabolic rate. So the natural question is where do humans stand in this.
Clearly humans can produce energy at rates much high than the basal metabolic one, provided the efforts are of short duration. There are however events which require weeks' or even months' long efforts. The article that inspired this post was based on a study of the energy expenditure of a group of athletes who ran six marathons a week for five months as part of the Race Across the USA, from west to east coast, a race of almost 5000 kms. The findings are summarised in the figure below.
While for short-duration events the athletes can expend an energy in multiples of the basal metabolic rate, once the duration of the event increases the energy expenditure starts falling rapidly levelling off for events of very long duration. The authors of the article infer an asymptotic value of 2.5 times the basal metabolic rate. The explanation of this limit is not quite clear. One possible source of this limitation is a long-term fatigue of the cardio-vascular system as well as of the organs involved in waste excretion. The authors of the article argue that the limitation is due to the digestive process. Energy intake implicates the alimentary system and the latter poses limits to the maximum energy uptake. Once the energy demand exceeds the maximum rate, the body starts drawing on its reserves leading to a non-sustainable situation.
One interesting finding of the study of Thurber et al. was that towards the end of the race the athletes were burning per day fewer calories than what could be expected based on the distance run. While a part of this reduction could be attributed to a reduction in non-exercise activity it appears that a reduction of non-muscular physiological activity was also in play. According to the authors this energy expenditure reduction was essential in allowing the athletes to complete the race.
This is a remarkable finding since it adds support to the constrained energy expenditure model. Put in a nutshell, the additive energy expenditure model posits a linear increase of the total energy expenditure in response to physical activity. The constrained model on the other hand suggest a subtler relation. The body adapts to an increase of physical activity by reducing the energy spent on other physiological tasks, leading to a total energy expenditure which levels off at high activity loads. Energy balance models focusing solely on the effect of physical activity on total energy expenditure may provide a biased measure of the latter. In particular additive total energy expenditure approaches will tend to overestimate the effect of physical activity at higher activity levels.
I find it really funny that a study on ultra-marathoners yields results which could provide insights on weight control strategies, but, after all, physical activity is a continuum ranging from that of the week-end jogger to the one of cross-continent runners.
The champions of prolonged effort are migrating birds. Several studies have been devoted to the measure of the energy expenditure during their long treks, the consensus being that migrating birds can sustain energy budgets of the order of 5 times their basal metabolic rate. So the natural question is where do humans stand in this.
Clearly humans can produce energy at rates much high than the basal metabolic one, provided the efforts are of short duration. There are however events which require weeks' or even months' long efforts. The article that inspired this post was based on a study of the energy expenditure of a group of athletes who ran six marathons a week for five months as part of the Race Across the USA, from west to east coast, a race of almost 5000 kms. The findings are summarised in the figure below.
While for short-duration events the athletes can expend an energy in multiples of the basal metabolic rate, once the duration of the event increases the energy expenditure starts falling rapidly levelling off for events of very long duration. The authors of the article infer an asymptotic value of 2.5 times the basal metabolic rate. The explanation of this limit is not quite clear. One possible source of this limitation is a long-term fatigue of the cardio-vascular system as well as of the organs involved in waste excretion. The authors of the article argue that the limitation is due to the digestive process. Energy intake implicates the alimentary system and the latter poses limits to the maximum energy uptake. Once the energy demand exceeds the maximum rate, the body starts drawing on its reserves leading to a non-sustainable situation.
One interesting finding of the study of Thurber et al. was that towards the end of the race the athletes were burning per day fewer calories than what could be expected based on the distance run. While a part of this reduction could be attributed to a reduction in non-exercise activity it appears that a reduction of non-muscular physiological activity was also in play. According to the authors this energy expenditure reduction was essential in allowing the athletes to complete the race.
This is a remarkable finding since it adds support to the constrained energy expenditure model. Put in a nutshell, the additive energy expenditure model posits a linear increase of the total energy expenditure in response to physical activity. The constrained model on the other hand suggest a subtler relation. The body adapts to an increase of physical activity by reducing the energy spent on other physiological tasks, leading to a total energy expenditure which levels off at high activity loads. Energy balance models focusing solely on the effect of physical activity on total energy expenditure may provide a biased measure of the latter. In particular additive total energy expenditure approaches will tend to overestimate the effect of physical activity at higher activity levels.
I find it really funny that a study on ultra-marathoners yields results which could provide insights on weight control strategies, but, after all, physical activity is a continuum ranging from that of the week-end jogger to the one of cross-continent runners.
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