庭で木を切っていたら、半分もきらないうちに筋肉が披露して、腕が動かなくなりました。エネルギーの枯渇です。若いときに比べたら、持続的に運動できる時間が本当に短くなったと思います。バス停まで走るときも、数十メートルも走ると足が止まってしまいます。20代後半にサッカーをやっていたときは、ボールを追いかけて長い距離を走ったら突然体が動かなくなってバランスを失い倒れたことがあります。小学校の頃にサッカーをやっていたときは、こういうエネルギーの枯渇を経験したことはありませんでした。サケが産卵で河を上ってきて、産卵を終えた後、ばたばたと死んでいくのも不思議でした。さっきまで普通に生きていたサケがなぜ、数分後にはエネルギーが枯渇して死んでしまっているのでしょうか。若いときは、エネルギーが枯渇するということがあまり実感できませんでしたが、歳をとると、突然エネルギーが切れて、体や頭が全く動かなくなるということを普通に経験するようになります。そうなって初めて、エネルギー代謝のありがたみを感じるのです。

エネルギー代謝って大事だなと思う今日この頃です。運動時のエネルギー代謝の変化としては、ATPの枯渇が数秒でおき、ATPを再生させる系であるクレアチンリン酸の枯渇が数十秒で起き（激しい運動時の話）、グリコーゲンが分解されてグルコースを供給しますが、それも数十分で枯渇し、酸化的リン酸化、脂肪酸などが大事になります。また、血中のBCAA(分岐鎖アミノ酸）を筋肉の細胞が取り込んでエネルギー源として使うほか、それも枯渇すると筋肉は筋肉のタンパク質を分解してBCAAを産生しそれをエネルギー源にします。

1. BCAAとは？大塚製薬
2. 必須アミノ酸BCAAとは　BCAAとは、筋肉のエネルギー源　大塚製薬　BCAAとは、運動時の筋肉でエネルギー源となる必須アミノ酸である、バリン、ロイシン、イソロイシンの総称です。
3. 筋肉量を増やすために必要な栄養素「分岐鎖アミノ酸（BCAA）」　環境再生保全機構　すこやかライフ　　BCAAは、筋肉の分解を抑制して、筋肉のエネルギー源となります。COPDの人は、呼吸筋を激しく使うため呼吸筋の損傷を防ぐ、という意味でもBCAAは欠かせない栄養素です。

実際どういうふうに使い分けされているのか、ChatGPTに聞いてみました。以下はその答え。

During exercise, the body utilizes a variety of energy sources to meet the increased energy demands. The specific utilization of energy sources can vary depending on factors such as exercise intensity, duration, and individual fitness level. Here’s a breakdown of how different energy sources are differentially used during exercise:

1. ATP (Adenosine Triphosphate): ATP is the primary source of energy for muscle contractions. It is stored in limited quantities within the muscles and is rapidly used during short bursts of high-intensity exercise. The body can generate ATP through different metabolic pathways, including the phosphocreatine system, glycolysis, and oxidative phosphorylation.
2. Creatine: Creatine phosphate, stored in muscles, serves as a quick energy source for immediate ATP production. During short-duration, high-intensity exercises, such as weightlifting or sprinting, creatine phosphate rapidly donates its phosphate group to ADP (adenosine diphosphate), forming ATP.
3. Glycogen: Glycogen is the storage form of glucose in muscle and liver tissues. It serves as an essential energy source during moderate to high-intensity exercise. Glycogen stored in the muscles can be broken down through glycolysis to produce ATP. The liver can also release glucose from glycogen into the bloodstream to maintain blood glucose levels during prolonged exercise.
4. Glucose: Glucose is a simple sugar derived from dietary carbohydrates or glycogen breakdown. It circulates in the bloodstream and is taken up by working muscles to be used as an energy source. Glucose can be metabolized through glycolysis to produce ATP, providing energy during moderate to high-intensity exercise.
5. Ketone Bodies: Ketone bodies, such as beta-hydroxybutyrate and acetoacetate, are produced by the liver during periods of prolonged fasting, low carbohydrate intake, or ketogenic diets. During exercise, the utilization of ketone bodies as an energy source becomes more prominent, especially during longer-duration, low-to-moderate intensity activities. However, the reliance on ketone bodies for energy may vary depending on an individual’s metabolic state and adaptation to utilizing fats for fuel.
6. Branched-Chain Amino Acids (BCAAs): BCAAs, including leucine, isoleucine, and valine, are essential amino acids that can be metabolized by muscles during exercise. They can serve as a minor source of energy during endurance exercise, particularly when glycogen stores become depleted. BCAAs can be oxidized within the muscles to produce ATP, helping to spare muscle glycogen.
7. Fatty Acids: Fatty acids are the main fuel source during low-to-moderate intensity and longer-duration exercises. Adipose tissue releases stored triglycerides, which are broken down into fatty acids and transported to the muscles for oxidation. Fatty acids undergo beta-oxidation in the mitochondria, generating ATP. However, high-intensity exercise relies more on carbohydrates and glycogen as a primary energy source, and fatty acid utilization decreases accordingly.

It’s important to note that the body dynamically shifts its reliance on different energy sources based on exercise duration, intensity, and individual factors. The interplay between these energy sources allows the body to meet the energy demands of various types of exercise.