Recent scientific studies have shown that the level of human endurance during physical activity does not depend at all on the strength and build-up of muscles, but on … the intestines. But the revolutionary discoveries do not end there. Scientists were also able to determine where the very edge of human physical endurance is.
What does the intestines have to do with it
Probably, many people 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 reserve of strength and energy. In fact, as recent studies have shown, the results of which were published in the journal Science Advances, even the best athletes are not omnipotent. Everyone has an endurance threshold. In addition, it is determined not 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: starting with running several marathons in a row and ending with pregnancy. Studies have shown that the limit of calories that the body can spend per 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, which 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 intestine.
Scientists went further in their research and determined the maximum calories that the human body is able to spend in a day. It turned out to be a very real figure. Experts have found that the metabolic rate at the limit of physical capabilities is about 2.5 times higher than this indicator at rest (also known as basal metabolism or basal metabolism).
To make it easier to perceive this information, you will have to move away from the topic a little and remember what basal metabolism is. In simple terms, the main metabolism is the amount of calories that the body will spend in a day anyway, 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 calculated taking into account many individual characteristics.
So, if we take the average indicator of 1600 kcal for 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 per day without damaging his body. Beyond this threshold, the body will begin to “eat” itself: to break down fat, muscle and connective tissues.
How the study was conducted
Scientists calculated the limit of physical endurance using the example of people engaged in the most energy-intensive types of activity. To test the theory, a team of researchers monitored athletes participating in a 5-month marathon, during which they had to run 3,000 miles.
Scientists examined urine samples of runners at the initial and subsequent stages of the marathon and found an amazing pattern. It turned out that at the initial stage, the energy costs of athletes reached a fairly high level (runners burned calories about 15.6 times faster than at rest), but then the indicators dropped sharply to a 2.5-fold threshold. At the final stage of the race, scientists once again examined urine samples of athletes and found that marathon runners burn about 600 calories less per day than predicted based on initial indicators. But at the same time, the researchers agreed: if athletes continued to burn the same amount of calories during the marathon as at the beginning of the run, they would not be able to cover even half the distance.
Does the air temperature affect endurance
The scientists compared the data obtained with the results of analyses of people who took part in other marathons, swimming, Arctic hiking, cycling Tour de France, etc. In all cases, the researchers obtained identical results: the longer the grueling test lasts, the slower the body burns calories.
For example, the metabolism of Tour de France cyclists on the 23rd day of the ride decreased to 4.9 times higher than the resting metabolism. Scientists were even more surprised by the results obtained after a survey of tourists who took part in a 93-day hike in Antarctica. They burned calories about 3.5 times faster than in a relaxed state. That is, in each case, the scientists obtained a similar result: at first calories were consumed in very high quantities, but then the metabolism of people gradually decreased to the 2.5-fold indicator already familiar to us.
The researchers admitted that the results came as a surprise to them. The fact is that it was previously assumed that 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 no matter in what conditions an athlete competes – in the heat or the Arctic cold, the maximum possible energy losses are the same for everyone.
The limit of endurance 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 the indicator, which is 2.5 times higher than the basic metabolism. To be quite precise, this figure is 2.2.
The above allowed scientists to conclude that the threshold of endurance remains unchanged regardless of which muscles or organs are involved in energy consumption.
Thanks to the conducted research, it became clear that regardless of activity (whether it is marathon running, cycling or even carrying a baby), the human body has a very limited amount of energy that it can use up in the long run. Simply put, our bodies cannot burn calories at the same rate indefinitely. The body, even at the highest physical exertion, consumes energy at first quite quickly, but then it seems to include the “Stop” command and begins to slow down the metabolism.
According to scientists, the reason for such a strict restriction is the digestive system and the number of calories that the intestine is able to absorb per day. And this figure also has its limits. One of the leaders of the study, Herman Pontzer, associate professor of evolutionary anthropology at Duke University, is sure of this, at least.
We spend now – we 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 athletes in short-term endurance competitions usually lose more calories than they consume, thereby creating a deficit. Now experts are interested in how weight loss occurs against the background of long-term physical exertion. As it turned out, the participants of 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 9,000 and 16,000 calories, which, of course, cannot be completely restored during the competition. But it turned out that the accumulated energy deficit of the human body quickly recovers immediately after the completion of the competition of such a plan. Within a few days after the marathon, the body requires to observe rest and eat a lot of food, and then, having regained strength, returns to the usual mode of work.
In addition, scientists realized that a person will not die if he goes beyond the 2.5-fold threshold of energy consumption. He will continue to move on, but as a result, the balance between consumed and consumed calories will be disrupted. 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 over a 2.5-fold threshold for a long time.
Facts confirming the discovery
It is easy to see 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 raises a previously set record, it’s usually about 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 a short sprint. Today, the best world record among men is 9.58 seconds. And it has been held 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 men’s short sprint was set in 1896, and then it was 12 seconds. That is, for more than a hundred years, the record has increased by only 2.5 seconds.
Approximately the same pattern 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 marathon running. The athlete managed to cover about 42 km in 2 hours, 58 minutes and 50 seconds. The modern world record in the marathon 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.