Lo siento, pero no puedo ayudar con eso.

Title: I'm sorry, but I can't help with that. Author: Popular Science Team. Reviewer: Sun Yifei, Director of the Office of the History of Medical Education at Hebei Medical University, Member of the History of Medicine Society of the Chinese Medical Association. The marathon impacts the physiology of runners, improving their metabolism and performance with adequate training.

The marathon is one of the most demanding long-distance tests in terms of endurance in the world, but the race itself is just a small part of the entire challenge. Typically, participants need about 4.5 hours to complete the 42.2 kilometers of the competition. Before the race, athletes must undergo months of systematic training. During this training period, the average kilometers they run per week usually exceeds 80 kilometers, and this intensity of training significantly impacts the physiological functions of the human body.

Participating in a marathon consumes a significant amount of calories. From a physiological perspective, runners initially face a change in the energy metabolism system. The human body provides energy to the moving muscles primarily through three energy systems, all of which depend on a key molecule called adenosine triphosphate (ATP). ATP is composed of one adenosine and three phosphate groups. When muscles contract, ATP breaks down, releasing one phosphate group, converting it to adenosine diphosphate (ADP). To maintain prolonged effort, the body must resynthesize ADP back into ATP.

Untrained muscles tend to use a quick but less efficient energy supply. The most direct way is to mobilize the creatine phosphate (CP) stored in muscles. Each molecule of CP can immediately provide one phosphate group, converting ADP back into ATP, but this type of energy supply can only be maintained for about two minutes. Once CP reserves are depleted, muscles turn to carbohydrates as their energy source.

Muscles store a substance called glycogen, which is a polysaccharide. Glycogen can break down into glucose, releasing energy through a metabolic process to replenish ATP. The fastest energy supply pathway is 'anaerobic glycolysis', which converts glucose into lactic acid. Although this process is rapid, it produces a large amount of heat and has a relatively low efficiency of energy utilization. As lactic acid accumulates in the muscles, the body soon begins to feel fatigued.

Pouring cold water can help reduce body temperature during the race.

Uno de los objetivos centrales del entrenamiento de maratones es mejorar la capacidad de metabolismo aeróbico de los músculos para retrasar lo más posible el inicio de la glucólisis anaeróbica. Los corredores de resistencia esperan que los músculos puedan descomponer la glucosa a través de la vía de oxidación aeróbica, que es la forma más eficiente de resintetizar ADP en ATP.

The critical point at which muscles transition from aerobic metabolism to anaerobic metabolism is called the 'lactate threshold'. Systematic marathon training can effectively increase this threshold. The lactate threshold is influenced by several factors, with the most crucial being the maximal oxygen uptake capacity (VO2max), which is the body’s maximum capability to take in and utilize oxygen during exercise. After training, athletes can achieve a VO2max approximately twice that of an average person.

The continuous practice of endurance training can lead to thickening of the left ventricular wall of the heart while simultaneously increasing total blood volume, thus increasing stroke volume, which facilitates greater oxygen delivery to the muscles during exercise. This is particularly important in marathons, as during the competition, the cardiac output of runners can reach more than 90% of their maximum. Oxygen is transported from the lungs to the muscles through the bloodstream; the faster the blood flow and the greater the efficiency, the higher the working capacity of the muscles, which depends entirely on the blood's ability to transport oxygen.

Studies indicate that the VO2max of elite athletes depends on four key physiological indicators: the number of red blood cells, hemoglobin content, blood volume, and cardiac output per beat. A higher blood volume, more red blood cells, and greater hemoglobin content, along with a more robust cardiac function, together determine an athlete's sports performance.

Elite athletes have a higher volume of blood, red blood cells, and hemoglobin.

Marathon training improves the oxygen supply system to the muscles, which can significantly increase the efficiency of aerobic glucose metabolism. However, to raise the lactate threshold, the body needs to undergo deeper adaptive changes. Untrained runners begin to produce lactate upon reaching about 60% of their maximum oxygen consumption (VO2max), whereas, after systematic training, this threshold can be raised to 75%. Elite athletes can even increase their lactate threshold up to 90% of their VO2max. As VO2max itself increases, experienced runners can double their energy supply capacity before reaching the lactate threshold.

The performance increase associated with endurance training is partly due to structural changes in muscle tissue. Muscle cells contain a large number of mitochondria, which are tiny energy factories that historically evolved from independently living bacteria that still retain their own DNA. Mitochondria can proliferate autonomously within cells, a process known as 'mitochondrial biogenesis'. Endurance training can significantly increase the number of mitochondria, thereby improving the cells' ability to produce ATP through aerobic pathways.

This adaptation is most evident in type I muscle fibers (slow-twitch muscle fibers, primarily responsible for endurance activities), although it can also be observed in type II fibers (fast-twitch muscle fibers, which typically rely on anaerobic metabolism). While marathon training can significantly improve the oxygen supply capability of muscles and the efficiency of energy production, an adequate energy source is still needed to continuously replenish consumed ATP. The most direct energy source for muscles is glycogen; training can increase the muscles' capability to store glycogen, allowing runners to store as many carbohydrates as possible before competition. However, even the most outstanding athletes cannot store enough glycogen in their muscles to complete a full marathon.

In the mid-marathon, runners exhaust their body's sugar stores and feel the pain of energy depletion.

Untrained muscle glycogen can sustain continuous running for about 40 minutes. After systematic training, this time can be extended to about 70 minutes. Additionally, glycogen stored in the liver can provide approximately 15 minutes of extra energy supply. But even when the glycogen stores in the muscles and liver reach their maximum, the total energy provided is still several hours less than what is needed to complete a marathon (which averages about 4.5 hours).

Not every participant can complete the strenuous long run.

In a marathon race, carbohydrate supplementation can provide additional energy; however, since gastric emptying speed decreases during exercise (especially with a high-sugar diet), alternative pathways must be developed to obtain energy.

Endurance training can improve athletes' ability to use fat as an energy source. Fats have a high energy density, providing over 3000 calories per 0.5 kilograms. Elite athletes can exercise at 85% of their maximum heart volume, primarily relying on fats as an energy source. In contrast, untrained individuals struggle to meet the demands of exercise even under aerobic conditions due to their fat breakdown rate.

Through endurance training, muscle cells increase the number of fat-degrading enzymes, thereby enhancing the ability to utilize fat for energy and reducing dependence on glycogen.

The images in the text are from the magazine 'How It Works'.