What is the relationship between amino acids and muscles that make up most of the body? Muscles are consumed during exercise!

When you play sports, you wants to perform well, don't you? Exercise cause to increase the muscle protein breakdown, which can lead to muscle loss. When you lose muscle, your condition gets worsen and you cannot achieve best performance.
Muscle is composed of proteins and these muscle proteins are made of amino acids. Hence, it is important to know about the relationship between muscles (that can be reduced during exercise) and amino acids, so that performance can be maintained.

1. Muscles are composed of amino acids! Amino acids are also related to the outcome of resistance training

What dose our body have to do with amino acids? Do you know what a human body is composed of?

As shown in Figure 1, 60% of the human body is made of water, 20% consists of protein, and the remaining 20% is composed of lipids, carbohydrates, minerals, etc. Excluding the water content, which accounts for 60% of the total content, half of the remaining amount is protein. Proteins are made up of just 20 kinds of amino acids that are attached to each other in large numbers. In other words, "half of the human body is composed of amino acids."

Muscle, which is important for exercise, is also primarily composed of protein. The 20 kinds of amino acids are divided into two groups:essential amino acids, which cannot be made in the body i.e, which must be taken from the diet, and non-essential amino acids that can be made in the body. Among essential amino acids, in particular, branched-chain amino acids(BCAA), which would be well-known to sports persons, are rich in the muscle. BCAA account for about 40% of the essential amino acids in the muscles.

"Resistance training" is essential to improve athletic performance. Resistance training can make muscle thicker and stronger. If you look at these muscles in detail under a microscope, you can see these muscles as a bundle of elongated cells called muscle fibers. During resistance training, the muscles increase in size, primarily because single muscle fibers become thicker.

Figure 1

2. Muscles are replaced every day and leave the body. It is important for Sportspersons to eat more protein.

We don’t sense, but in fact, the proteins in the body are constantly being replaced with new proteins while the old proteins are degraded simultaneously. As shown in Figure 2, in a normal adult weighing 60 kg, about 180 gram of protein is broken down everyday, and the same amount of new protein is being made1). In addition, the protein and amino acid is metabolized into urea and is excreted in urine. This is balanced out by consuming the protein that is needed everyday. Hence, it is important to eat properly the necessary protein from the diet.

Figure 2

When protein is consumed with the diet, it is broken down into amino acids, which are absorbed into the body, and spread across the body via the blood. These amino acids are used to make protein in various organs and tissues of the body every day. You may not realize it, but a certain amount of protein (amino acids) disappears from the body every day. In other words, to maintain the balance of proteins in the body, it is necessary to consume the same amount of protein daily as the amount that is excreted from the body. Similarly, in muscles, the protein is replaced by a little every day, and it takes about six months to replace half of the protein in the muscle. In other words, the muscles are completely replaced in about one year.

Recent publication by the American College of Sports Medicine (ACSM) 2) and the International Olympic Committee 3) recommend that athletes consume 1.2 to 2.0 g of protein per 1 kg of body weight per day (72 to 120 g per day for a person with 60 kg body weight). On the other hands, the protein intake for normal adults ranges from 0.95 to 1.09 g/day per kg of the bodyweight 4). It is, therefore, recommended that athletes consume 1.3 to 1.9-fold more protein than the general population.

In other words, it is very important for people who play sports to obtain a large amount of protein from their meals every day to maintain their muscles sufficiently.

1) Terada S, Sports Nutrition. University of Tokyo Press, pp 120 -121, 2017
2) Thomas DT, Med Sci Sports Exerc 2016;48(3):543-68.
3) International Olympic Committee. Nutrition for Athletes.2012.
4) Dietary Reference Intakes for Japanese (2015 Edition)

3. Are muscles reduced with high intensity and longer duration of exercise?

To exercise, you need to use energy stored in your muscles. This energy is made from carbohydrates and lipids as main energy sources. However, amino acids (proteins) are also used too.

The amount of energy used in the body composed of amino acids is 4-10% of the total amount, depending on the condition of the person exercising 5). However, if you continue to exercise with low levels of carbohydrates (glycogen) stored in your muscles, more proteins and amino acids are used to produce energy (Figure 3). As a result, there is an increase in the byproduct of protein (called urea nitrogen). In other words, any protein in your body that you want to maintain will be degraded and used as an energy source. Thus, you need to be careful.

Data 6)7) demonstrate that high-intensity exercise or a longer duration of exercise leads to a higher use of amino acids in the body (Fig. 4). It is clear that the harder the exercise, or the longer it lasts, the more amino acids are used.

Figure 3

Figure 4

In addition, eseveral types of amino acids are more likely to be depleted in the muscles when exercising. Among these amino acids , BCAA are well-known among sportspersons 8)9)10)11). During exercise, amino acids, and particularly BCAA, are used as an energy source and are degraded. (Fig. 5)

Figure 5

As we have discussed, protein is always being made in the muscles, and at the same time, old protein is broken down. During exercise, however, the production of new protein reduces, whereas the breakdown of old protein is increased 12). Exercise, therefore, results in higher breakdown of muscle protein. In other words, exercise leads to a gradual reduction in muscles which leads to a decline in your condition.

5) Lemon PW, J Appl Physiol Respir Environ Exerc Physiol 1980;48(4):624-9.
6) Haralambie G, Berg A, Eur J Appl Physiol Occup Physiol 1976; 36: 39-48
7) D J Millward, Proc Nutr Soc.1994;53(1):223-40
8) Gibala MJ, Sports Med 2007; 37: 337-340
9) Van Hall G, The Journal of physiology 1996;494 ( Pt 3):899-905.
10) Rush JW, J Appl Physiol 1995;78(6):2193-200.
11) Shimomura Y, J Nutr 2006;136(1 Suppl):250S-3S.
12) AJ Rose, J Appl Physiol 2009

4. Summary

In summary, (Figure 6)

  • ・Muscles are composed of amino acids (= proteins)..
  • ・Muscle protein is constantly being replaced…It is very important to eat sufficient amount of protein (amino acids) every day.
  • ・If exercise is longer or harder, amino acids in the muscles can also be used as an energy source.
  • ・The harder you exercise, the less new muscle protein is made and the more old muscle protein is broken down.
  • ・As a result, muscles gradually reduce during exercise, resulting in a decline in muscle condition.

Figure 6

What happens to your muscles when you exercise? Try to be conscious about improving the condition of your muscles in order to achieve satisfactory performance.

▼Here are the amino acids that solve this problem.
A "Leucine-enriched essential amino acid mixture" maintains the condition of muscles during exercise! It also supports recovery from fatigue after exercise!


Supervising Editor:Masaaki Sugita


Masaaki Sugita
Current position: Professor at the Faculty of Sport Science, Nippon Sport Science University. PhD (Academic).
As a member of the Japan Olympic Committee and scientific staff of the Japan Association of Athletics Federations, he has performed medical and scientific support activities for 30 years, contributing to the improvement of the international competitiveness of Japanese athletes. In 2010 FIFA World Cup and London and Rio de Janeiro Olympics, he provided scientific support for numerous athletes, including for race walking and marathon athletes. Currently, he is busy implementing initiatives aimed at Tokyo 2020.

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