在過去很長時間裡，支鏈氨基酸(BCAA)常被視為抵抗肌肉分解的神奇補劑，但隨著近年來相關研究的增多，其神話地位已經越來越受質疑。
發表在2021年2月4日《營養學雜誌》(Journal of Nutrition)上的一項新研究報告稱，補充支鏈氨基酸(BCAA)並不能使肥胖成年人在減肥過程中保持瘦體重。高蛋白飲食可能更有利於保持瘦體重。
這項新的隨機對照減肥試驗包括132名成年人(63名男性和69名女性)，他們被隨機分配到三種低熱量飲食中。受試者經歷了16周被規定的飲食和補劑干預，在隨後的8周體重維持期僅規定了補劑。
在經歷16周的能量不足後，三組受試者在體重(7.97%)、脂肪(13.8%)和腰圍(7.27%)上出現了相似的顯著下降。研究發現，在肥胖成年人減肥期間，補充BCAA並不能保持瘦體重。
支鏈氨基酸(亮氨酸、異亮氨酸和纈氨酸)是肌肉蛋白質合成的必需氨基酸。亮氨酸是一種強大的合成代謝觸發器——它是最有效的支鏈氨基酸(BCAA)和mTOR通路的關鍵啟用劑，對促進肌肉生長的肌肉蛋白質合成至關重要。
亮氨酸有許多生長益處：防止肌肉流失，增加胰島素敏感性，促進脂肪代謝和恢復。亮氨酸是開啟mTOR通路的最重要化學物質，所以在運動後攝取亮氨酸可能比攝取BCAA更有效(也更便宜)。
由於在轉運到肌肉細胞方面存在競爭，異亮氨酸和纈氨酸的加入可能會阻礙亮氨酸的作用——在進入細胞和肌肉細胞方面，BCAA共享相同的主動轉運系統。事實上，異亮氨酸和纈氨酸已被證明能抑制亮氨酸的吸收。
氨基酸研究人員羅伯特·R·沃爾夫在《國際運動營養學會雜誌》(2017年)中說：BCAA還與其他氨基酸競爭運輸，包括苯丙氨酸，這種競爭可能影響肌肉內其他EAA的可用性。作為轉運蛋白競爭的結果，單獨使用亮氨酸可能對肌肉蛋白質合成有短暫的刺激作用，BCAA無法引起這種反應。
研究表明，肌肉蛋白質合成的增加依賴於亮氨酸的濃度。亮氨酸刺激肌肉蛋白質自身的合成代謝作用。
2018年10月18日發表在《營養學雜誌》上的一項日本研究發現，與透過一餐補充相同數量的亮氨酸相比，單獨服用亮氨酸補劑可能更有利於肌肉蛋白質的合成代謝。
營養研究表明：基於這些發現，可以推測，與單獨攝取蛋白質或遊離氨基酸相比，從混合飲食中攝取蛋白質可能導致血漿中最大亮氨酸濃度較低。然而，迄今為止，還沒有研究調查攝取混合飲食與攝取相似量遊離氨基酸後氨基酸濃度的變化。
在一項隨機交叉研究中，10名健康的日本年輕男性在不同條件下進行了試驗：單獨攝取2克亮氨酸；含2.15克亮氨酸的混合飲食(不含任何亮氨酸補劑)；飯後立即攝取2克亮氨酸；飯後180分鐘攝取2克亮氨酸。
該研究得出結論：基於上述討論，單獨攝取遊離亮氨酸顯著增加血漿亮氨酸濃度。然而，食用含有相同亮氨酸量的混合飲食後，亮氨酸濃度的增加顯著低於單獨攝取遊離亮氨酸。
此外，當在混合飲食後攝取遊離亮氨酸以增加血漿亮氨酸濃度時，儘管總亮氨酸含量加倍，但最大血漿濃度降低。這些結果表明，當以提高血漿氨基酸濃度為目的攝取遊離氨基酸時，需要考慮混合飲食攝取的時間。
為了達到最佳的合成代謝觸發效果，應該在練後餐的前30分鐘服用5克亮氨酸(空腹)或蛋白質奶昔。一項綜合了7項研究結果的薈萃分析(Nutrition, 2017)顯示，BCAA補劑最好在運動後服用，而不是運動前或運動期間。
與隨食物一起服用相比，空腹服用純亮氨酸會使血液水平更好，因為食物可以減緩亮氨酸的吸收。由於轉運肌肉細胞方面的競爭，異亮氨酸和纈氨酸的加入可能會阻礙亮氨酸的作用。
當空腹攝取亮氨酸時，它是開啟蛋白質合成的一個強大代謝開關。亮氨酸可以在訓練後的幾個小時內增加mTOR活性。當在抗阻運動後，和富含必需氨基酸的含蛋白質練後餐前服用亮氨酸，它會促進蛋白質合成，改善恢復，獲得更大的收益。
總之，最近發表在《營養學雜誌》(Journal of Nutrition)上的這項新的隨機對照減肥干預研究證實，BCAA補劑在超重和肥胖成年人減肥過程中並不能保持瘦體重。
高蛋白飲食和氨基酸亮氨酸結合有氧運動和抗阻訓練，可能更有利於在節食期間保持瘦體重。記住，最重要的是，健康的減肥是在保持肌肉質量的情況下逐漸減少脂肪。
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