The limit of human physical endurance is set

Recent scientific studies have shown that the level of human endurance during physical activity does not depend at all on the strength and pumping up of the muscles, but on … the intestines. But the revolutionary discoveries do not end there. Scientists were also able to establish where the very edge of human physical endurance is.

What is the gut

Probably, many are familiar with the feeling when you watch some athletes (for example, during the same marathon race) and it seems that these are superhumans with an inexhaustible supply of strength and energy. In fact, as recent research published in the journal Science Advances has shown, even the best athletes are not all-powerful. Everyone has an endurance limit. In addition, it is not determined by muscles or willpower, as it may seem, but by the digestive system.

A group of American and Scottish researchers studied what happens to the human body in different states: from running several marathons in a row to pregnancy. Studies have shown that the limit of calories that the body can spend in a day, and therefore the threshold of a person’s physical capabilities, is very limited. This is due to the fact that, as it turned out, the ability of the intestine to absorb calories from food (and therefore form energy reserves that can then be spent) is also not unlimited. That is, the threshold of physical endurance of a person directly depends on the functioning of the intestines.

Scientists went further in their research and determined the very maximum calories that the human body can spend per day. They turned out to be a very real number. The researchers found that the metabolic rate at the limit of physical capacity is about 2,5 times higher than that at rest (also known as the basal metabolic rate or basal metabolic rate).

To make it easier to perceive this information, you will have to deviate a little from the topic and remember what basal metabolism is. In simple terms, the basic metabolism is the number of calories that the body will spend per day in any case, even if a person sleeps all day. That is, this is the energy that our bodies will use to maintain their functionality (breathing, thermoregulation, blood circulation, etc.). For each person, this figure is unique and is calculated taking into account many individual characteristics.

So, if we take an average of 1600 kcal as the norm of the metabolic rate at rest, then the limit of a person’s capabilities will be approximately 4000 kcal. Simply put, a person with a basal metabolism of 1600 kcal will not be able to spend more than 4000 kcal of energy in a day without harming his body. Beyond this threshold, the body will begin to “eat” itself: break down fat, muscle and connective tissue.

How was the study

Scientists calculated the limit of physical endurance using the example of people engaged in the most energy-intensive activities. To test the theory, a team of researchers followed up with athletes participating in a 5-month marathon, during which they had to run 3000 miles.

Scientists examined urine samples from runners in the initial and subsequent stages of the marathon and found a surprising pattern. It turned out that at the initial stage, the energy expenditure of athletes reached a fairly high level (runners burned calories about 15,6 times faster than at rest), but then the figures dropped sharply to a 2,5-fold threshold. At the final stage of the race, scientists once again examined the athletes’ urine samples and found that marathon runners burned about 600 fewer calories per day than was predicted based on the initial indicators. But at the same time, the researchers agreed: if athletes continued to burn the same number of calories during the marathon as they did at the beginning of the run, they would not be able to complete even half the distance.

Does air temperature affect endurance?

The scientists compared the data obtained with the results of analyzes of people who took part in other marathons, swimming, Arctic hikes, the Tour de France cycling race, etc. In all cases, the researchers got identical results: the longer the grueling ordeal lasts, the slower the body burns calories.

For example, the metabolism of Tour de France cyclists on the 23rd day of riding dropped to 4,9 times their resting metabolic rate. Even more scientists were surprised by the results obtained after a survey of tourists who took part in a 93-day trip to Antarctica. They burned calories about 3,5 times faster than when they were relaxed. That is, in each case, scientists got a similar result: at first, calories were consumed in very high quantities, but then the metabolism of people gradually decreased to the already familiar 2,5-fold figure.

The researchers admitted that the results for them came as a surprise. The fact is that it was previously assumed that the energy consumption depends on the temperature factor. That is, it was believed that the maximum energy consumption of the body is determined by its ability to dissipate heat. But the results of the study confirmed that regardless of the conditions under which an athlete competes – in the heat or the arctic cold, the maximum possible energy loss is the same for everyone.

Endurance limit and pregnancy

This study allowed scientists to discover another pattern. It turned out that the number of calories burned by the body of a pregnant woman is also approaching an indicator that is 2,5 times higher than the basic metabolism. To be completely accurate, this figure is 2,2.

The foregoing allowed scientists to conclude that the endurance threshold remains the same regardless of which muscles or organs are involved in energy expenditure.

Thanks to the study, it became clear that regardless of the activity (be it running a marathon, cycling or even bearing a baby), the human body has a very limited amount of energy that can be used up in the long term. Simply put, our bodies cannot burn calories at the same rate indefinitely. The body, even at the highest physical exertion, consumes energy rather quickly at first, but then it seems to turn on the “Stop” command and begins to slow down the metabolism.

According to scientists, the reason for this severe restriction is the digestive system and the number of calories that the intestines can absorb in a day. And this figure also has its limits. At least one of the leaders of the study, Herman Pontzer, an assistant professor of evolutionary anthropology at Duke University, is sure of this.

Spend now, recover later

Professor Brent Ruby and his colleagues from the Center for Occupational Physiology and Metabolism at the University of Montana analyzed what level of physical activity a person can withstand for a long time without losing body weight.

Prior to the study, it was known that short-term endurance athletes typically lose more calories than they consume, thus creating a deficit. Now experts are interested in how weight loss occurs against the background of long-term physical exertion. As it turned out, participants in the Ironman triathlon and the 100-mile ultramarathon lost an average of 2,5 kg and 1,5 kg, respectively, during the competition. And according to scientists, this is a pretty good result, considering that during the race they burned about 9000 and 16000 calories, which, of course, cannot be fully restored during the competition. But it turned out that the human body quickly restores the accumulated energy deficit immediately after the completion of such a competition. Within a few days after the marathon, the body requires to remain calm and eat a lot of food, and then, having restored strength, it returns to its usual mode of work.

In addition, scientists realized that a person will not die if he goes beyond the 2,5-fold threshold for energy expenditure. He will continue to move on, but as a result of this, the balance between consumed and expended calories will be upset. As a result, the body begins to “eat” itself, to maintain its functionality at the expense of its own resources. But of course, this can’t go on forever. Moreover, so far, scientists have not been able to record cases when a person could burn calories above the 2,5-fold threshold for a long time.

Facts supporting the discovery

It is easy to verify that the limit of a person’s physical capabilities actually exists by analyzing the records that athletes have set in different years. In fact, when someone breaks a previously set record, it is usually a difference from a few seconds to a few tenths or even hundredths of a second. That is, the maximum level of physical capabilities (subject to certain training) for athletes is approximately the same.

Take at least the world records set in the short sprint. To date, the best world record among men is 9,58 seconds. And it has been holding for 10 years. If we analyze how this record has changed, then over the past 30 years it has grown by only 3,5 hundredths of a second. The first record in the short sprint among men was set in 1896, and then it was 12 seconds. That is, in more than a hundred years, the record has increased by only 2,5 seconds.

Approximately the same picture is observed in long-distance running – marathon. If you look into history, you can find information that in 1896 at the Olympic Games in Athens, a record was set in the marathon run. The athlete managed to overcome approximately 42 km in 2 hours 58 minutes 50 seconds. The modern marathon world record is 2 hours 1 minute 39 seconds. That is, over the century, this indicator, as in the case of a short sprint, has not increased significantly.

The conclusion from this suggests itself: no matter how much a person trains, he will not be able to do more than his physical endurance allows. And now we know at what level the very limit of endurance is located.

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