Causes of fatigue and tiredness after exercise

[Digest video] Why does exercise cause damage to the stomach? / Learn with illustrations

Why exercise makes you tired – muscle inflammation and abdominal damage

As shown in Figure 1, training for a long period of time or strenuous exercise can damage the body. Many people are aware that when muscles are damaged, inflammation occurs, causing muscle stiffness and muscle pain. Inflammation is a normal reaction that protects the body due to stress, such as strenuous exercise. However, if the inflammation is too strong, it can cause pain and swelling. In fact, this inflammation does not only occur in the muscles, but throughout the entire body. When inflammation occurs throughout the body, you become more easily fatigued and it becomes difficult to perform at the level you had hoped.¹⁾.

Damage to the abdomen caused by exercise

It's hard to notice, but if you exercise for a long time or too hard, it can also damage your stomach.³⁾.

Damage to the stomach alone is a big problem for athletes, but when such problems occur, inflammation throughout the body can become even worse, which can lead to problems with not being able to perform as well as expected.

Figure 1

1) Cerqueira E, et al. Front Physiol. 2020;10:1550
2) Robson-Ansley P, et al.Int J Sports Physiol Perform. 2009;4(2):186-94
3) de Oliveira EP, Burini RC. Curr Opin Clin Nutr Metab Care. 2009;12(5):533-8

Causes of abdominal damage – Increased body temperature and decreased blood flow

We have seen that longer exercise times and increased intensity can cause damage to the stomach. So why does this damage occur? We will explain the possible causes: "internal body temperature rising too high" and "reduced blood flow to the stomach."

Effects of rising body temperature on the stomach

When you exercise, your body's internal temperature rises. In that case, you sweat, and the sweat evaporates from the surface of your body in an attempt to lower your body temperature. However, if you exercise for a long time or get too hard, sweating alone is not enough to lower your body temperature, and as a result, your body temperature rises.

There is actually a relationship between changes in body temperature during exercise and the damage done to the stomach (Figure 2). When body temperature rises to around 38.6-39.0℃, approximately 60% of people suffer damage to their stomachs, and when it rises above 39.0℃, data shows that this happens to 100% of people.⁴⁾.

Figure 2

Abdominal oxygen deficiency due to decreased blood flow to the abdomen

Let's take a look at why exercise reduces blood flow to the stomach. When you exercise, your heart rate increases, and the amount of blood sent from your heart to your entire body increases. So why does blood flow to the stomach decrease? In fact, during exercise, blood is sent preferentially to the muscles, so the amount of blood sent to the stomach decreases.

Figure 3

Figure 3 shows data that shows that the amount of blood flowing to the stomach (internal organs) decreases as you exercise. When you continue to cycle for about 60 minutes at a strength that a person feels is "a little hard," the blood flow to the stomach decreases by a whopping 80% compared to when you are not exercising.⁵⁾This reduction in blood flow to the stomach means that the stomach (internal organs) do not receive the oxygen they need. This causes the stomach (especially the small intestine and large intestine) to become oxygen-deprived, which can have adverse effects.

In other words, depending on the duration and intensity of the exercise, your body temperature will rise and blood volume will decrease, resulting in damage to your stomach.

4) Pires W, et al. Sports Med.2017;47(7):1389–403
5) ter Steege RW, Kolkman JJ. Aliment Pharmacol Ther.2012;35(5):516–28

Damage to the abdomen affects the whole body

We have explained how damage can occur to the stomach (particularly the small intestine and large intestine) depending on the duration and intensity of exercise. Next, let's look at how this damage affects the body.

Toxin production by intestinal bacteria

Our intestines are home to approximately 1,000 trillion intestinal bacteria, with approximately 100 different types. In fact, it has been found that half of these intestinal bacteria produce toxins.

Figure 4

As shown in Figure 4, if we look inside a healthy intestine, we can see that many cells are neatly arranged on the surface with no gaps. In this state, the intestine is fully protected, and the neatly arranged cells block the passage of intestinal bacteria and the toxins they produce.⁶⁾.

Decreased intestinal barrier function and worsening inflammation

When exercise damages the stomach, it also damages intestinal cells, creating gaps between them. This weakens the body's ability to protect itself, allowing intestinal bacteria and toxins to slip through these gaps and enter the bloodstream, worsening inflammation throughout the body. When inflammation worsens, you become more easily fatigued, which makes it harder to maintain good physical condition and perform at your best.

6) Clerk A, Mach N. J Int Soc Sports Nutr.2016;13:43.

Increased fatigue and lethargy caused by exercising in hot and humid environments

We have explained that long periods of exercise or strenuous exercise can raise body temperature and reduce blood flow in the stomach, causing stomach damage. It is said that such problems can be exacerbated in hot and humid conditions, such as those during the Japanese summer.⁷⁾.

Figure 5 shows data showing how body temperature changes after 22 minutes of running at a level that feels "a little strenuous" in normal temperature (room temperature 33°C) and high temperature (room temperature 60°C) conditions.

Figure 5

Changes in body temperature during exercise in high temperatures

When you start exercising, your body temperature rises gradually in both normal and high temperature conditions. However, when exercising in a high temperature condition, after about 30 minutes your body temperature becomes increasingly higher than in normal temperature conditions, and the more you continue exercising, the higher it rises. When you finish exercising, your body temperature is about 38°C in normal temperature conditions, but in high temperature conditions it has risen to over 39°C.

In other words, if you exercise in these conditions, your body temperature may rise quickly, exacerbating the damage to your stomach. When exercising in hot and humid conditions, be sure to stay hydrated and take frequent breaks, and always be careful to protect your body.

7) Yeh YJ, et al. Eur J Appl Physiol. 2013;113(6):1575–83

Causes of fatigue and lethargy after exercise and key points to prevent it

Here are some important points to know about the damage that can occur to your body during exercise:

• ・Long-term or strenuous exercise can cause inflammation throughout the body and damage the stomach.
• When body temperature rises or blood flow to the stomach decreases, the stomach's ability to protect itself decreases.
• ・If the body's ability to protect the stomach weakens, there is a risk that inflammation throughout the body will worsen.
• - Exercising in hot and humid conditions makes your stomach more susceptible to damage

Not many people are aware of the damage that can be done to the body through long periods of exercise or strenuous exercise. Exercising in high temperature and humidity, like the Japanese summer, makes you more susceptible to this damage.

Make sure you have all the knowledge that athletes should have, including what has been explained here, do what you think is appropriate training even in tough situations, always pay attention to your physical condition, and work hard to perform at the level you are aiming for.

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<Supervisor>Masashi Miyashita

In 2006, he completed his doctoral studies at the School of Sport and Exercise Sciences, Loughborough University, UK. In 2006, he became a researcher at the Graduate School of Comprehensive Human Sciences, University of Tsukuba, an assistant professor at the Graduate School of Sport Sciences, Waseda University in 2009, an associate professor at the Department of Arts and Sport Sciences, Faculty of Education, Tokyo Gakugei University in 2012, and an associate professor at the Faculty of Sport Sciences, Waseda University in 2016. He has been in his current position since 2021. His specialties are exercise metabolism, exercise physiology, exercise nutrition, and applied health science. He is conducting international collaborative research with the main themes of "exercise and postprandial metabolism" and "exercise and appetite regulation," aiming to form an international research hub.

About Professor Masashi Miyashita Laboratory (Exercise Metabolism Laboratory)
The main research theme of the Exercise Metabolism Laboratory is the prevention of arteriosclerosis through physical activity (exercise and daily activities) and the elucidation of appetite regulation mechanisms. Through basic and applied research using physiological, nutritional, and public health methods, we are conducting research with the goal of "translating impactful research into better health" in order to disseminate useful information to society. In addition, our laboratory is also committed to international academic exchange, aiming to form an international education and research hub in the academic field of "Physical Activity and Public Health", deepening collaboration with domestic and international collaborators and promoting academic exchange for education and research. We also accept many international students who wish to study in English degree programs (master's and doctoral programs) and are conducting research activities. Specific research content includes:
1) Research into the prevention of arteriosclerosis through physical activity and diet
2) Basic research (clarification of regulatory mechanisms) and applied research (eating behavior) on physical activity and appetite
3) Research on functional foods and their effects on exercise performance, fatigue, and prevention of metabolic diseases

Exercise Metabolism Laboratory website:https://miyashita.w.waseda.jp/