BCAAs （Branched chain amino acids） 支鏈氨基酸

支鏈胺基酸是運動營養產業的新寵－而且大多有充分的理由。 眾所周知，支鏈胺基酸可直接被肌肉細胞和免疫系統細胞吸收－它們主要用作能量來源（而不是像某些其他胺基酸那樣作為蛋白質合成的主要驅動力）。

基於 BCCA 的產品的典型聲明包括：

· 增加耐力

· 預防疲勞 預防性疲勞

· 改善心理表現促進身體健康、增強意志力

· 增加能量水平 增強能量

· 改善免疫​​系統功能 加強免疫系統

· 改善運動後恢復（減少肌肉酸痛及減少上呼吸道感染） 促進運動後恢復

儘管中樞性疲勞和補充 BCAA 的一般理論是合理的，但研究結果並不全是正面的。 然而，一般而言，急性補充 BCAA（運動前或運動期間攝取高劑量 10-20 克）已被證明可以提高心理表現、提高自行車耐力並減少完成馬拉鬆的時間。 長期補充支鏈胺基酸（10-20克/天，持續兩週）也被證明可以有效提高訓練有素的自行車運動員的計時賽表現。

然而，對受過訓練和未經訓練的受試者進行的許多研究表明，支鏈氨基酸補充劑對運動表現或心理表現沒有影響——並且在許多這些研究中，支鏈氨基酸的苦味和大劑量導致了多種不良反應。胃腸不適。 在某些情況下，運動前和運動期間補充支鏈胺基酸的價值與運動期間補充碳水化合物的價值大致相當（結果相似，因為兩者都以相同程度延緩疲勞）。

當談到運動後使用支鏈胺基酸來增強免疫系統對肌肉和肺部損傷的修復時，科學證據更加明確。 許多研究表明，在劇烈或力竭的耐力運動後攝取支鏈氨基酸可以減輕肌肉酸痛，減少上呼吸道感染，更好地維持激素水平（主要是減少皮質醇，一種壓力激素），並改善整體情緒狀態。與更高的能量水平和更低的疲勞水平相關，這表明​​恢復更充分）。

與耐力增強研究中使用的大劑量（10-20g）和耐受性差的BCAA 相比，低劑量（750-1500mg）的BCAA 已被證明可有效增強運動後恢復（亮氨酸、纈氨酸的比例） ，異亮胺酸似乎對恢復效果很重要，在耐力運動員的研究中，3:1:1 的效果最好）。

研究表明，片劑形式的支鏈胺基酸每日補充攝取量高達40 克，液體形式的支鏈胺基酸補充攝取量為每公升1 至7 克，沒有明顯的不良副作用（除了高於幾克/劑量的胃痛之外）。 由於可能競爭性抑制飲食中其他胺基酸的吸收以及胃腸道不適的風險，因此應避免每天攝取較高的攝取量（5-10克/天）。

對於運動前或運動期間使用，延遲中樞疲勞的支鏈氨基酸有效劑量為每天10-20 克（片劑/膠囊），分次服用或在運動期間添加到含碳水化合物的飲料中（1 至7 克）。每公升液體中支鏈胺基酸的克數）。 請注意，這些水平可能會引起胃腸道不適，包括痙攣和腹瀉。

為了在運動後使用以促進恢復，750-1500 毫克支鏈氨基酸（亮氨酸：纈氨酸：異亮氨酸比例為3:1:1）（如Recover-Ease from Wicked Fast Sports Nutrition 中所述）已被證明可以減輕肌肉酸痛和上呼吸道症狀道感染，同時改善能量/疲勞水平、皮質醇水平和整體情緒。

支鏈胺基酸（BCAA）是三種必需胺基酸

氨基酸； 亮氨酸、纈氨酸和異亮氨酸。 「支鏈」名稱指的是它們獨特的化學結構 - 而「必需」術語表明您的身體無法製造它們，因此需要從飲食中獲取它們。

支鏈氨基酸的建議膳食攝取量約為每天 3 克——這個量應該很容易從蛋白質食物中獲取。 補充水準的使用量從低至 750 毫克/劑（以特定比例）到每天 20-40 克（通常透過某些運動飲料中的乳清蛋白來源進行補充）。

支鏈胺基酸補充劑背後有兩個主要理論。 第一個與稱為「中樞疲勞」的現像有關，該現象認為大腦的精神疲勞會對耐力項目中的身體表現產生不利影響。 中樞疲勞理論表明，血液中支鏈胺基酸水平較低可能會加速血清素（大腦中的關鍵神經傳導物質）的產生，並過早導致疲勞。 大腦中血清素水平升高可能會抑制中樞神經系統功能，導致困倦和疲勞感。

色氨酸是一種在血液中循環的氨基酸，是血清素的前體，當血液中的支鏈氨基酸水平較低時，可以更容易地轉運到大腦中以增加血清素水平（因為血液中的支鏈胺基酸水平高可以阻止色氨酸轉運到腦部）大腦）。

在運動過程中，隨著肌肉和肝醣原的能量消耗，血液中的支鏈氨基酸水平也可能降低，脂肪酸水平增加，作為額外的能量來源。 血液中多餘脂肪酸的問題在於，它們需要附著在稱為白蛋白的載體蛋白上才能正確運輸。 在此過程中，脂肪酸取代了白蛋白上的色氨酸，從而促進色氨酸轉運到大腦中轉化為血清素。 因此，由於血液中支鏈氨基酸減少和脂肪酸升高，更多的色氨酸進入大腦並產生更多的血清素，導致疲勞。 在飲食中補充額外水平的支鏈氨基酸被認為可以阻止色氨酸運輸，從而延緩長時間運動期間的疲勞。

支持補充 BCAA 的另一個主要理論是支持免疫系統功能並加速運動後肌肉損傷和肺部損傷的修復。 在這種情況下，補充支鏈氨基酸被用作運動後恢復輔助劑，它們充當免疫系統細胞的燃料來源，然後免疫系統細胞能夠增加其活性並更徹底地修復肌肉/肺部損傷。

對於耐力運動員，尤其是參加馬拉松、鐵人三項、自行車、山地自行車和冒險賽車等較長比賽（2 小時以上）的運動員來說，BCAA 補充劑可能有助於延緩中樞疲勞，並且肯定有助於增強運動後的能力恢復。 參與持續時間較短的活動不太可能導致支鏈氨基酸、色氨酸或脂肪酸血液水平發生實質性變化，低強度運動也不會對肌肉或肺組織產生足夠的損傷，從而無法從支鏈氨基酸的補充來源中受益。

BCAAs are the new darling of the sports nutrition industry – and mostly with good reason. The BCAAs are known to be directly taken up by both muscle cells and by immune system cells – where they are used primarily as an energy source (rather than as a primary driver of protein synthesis as certain other amino acids might be).

Typical claims for BCCA-based products include:

·  Increased endurance 增加耐力 

·  Prevention of fatigue 預防疲勞 

·  Improved mental performance增進身心健康、增強意志力 

·  Increased energy levels 增強能量 

·  Improved immune system function 加強免疫系統 

·  Improved post-exercise recovery (less muscle soreness and fewer upper respiratory tract infections) 促使運動後恢復 

Although the general theory of central fatigue and BCAA supplementation is sound, the research findings have not all been positive. In general, however, acute BCAA supplementation (large doses of 10-20 grams consumed before or during exercise) has been shown to increase mental performance, improve cycling endurance, and reduce the time to complete a marathon. Chronic BCAA supplementation (10-20g/d for 2 weeks) has also been shown effective in improving time-trialing performance in trained cyclists.

There are, however, a number of studies in trained and untrained subjects that have shown no effect of BCAA supplements on exercise performance or mental performance – and in many of these studies, the bitter nature and large dose of BCAAs has led to a variety of gastrointestinal complaints. In some cases, the value of BCAA supplementation before and during exercise has been compared as roughly equivalent to carbohydrate supplementation during exercise (with similar magnitude of results because both delay fatigue to the same degree).

When it comes to the post-exercise use of BCAAs to enhance immune system repair of muscle and lung damage, the scientific evidence is a bit clearer. A number of studies have shown that BCAAs consumed following intense or exhaustive endurance exercise, result in reduced muscle soreness, fewer upper respiratory tract infections, better maintenance of hormone profile (primarily reduced cortisol, a stress hormone), and an overall elevated mood state (associated with higher energy levels and lower fatigue levels, which is indicative of fuller recovery).

In contrast to the large (10-20g) and poorly tolerated doses of BCAA used in studies of endurance enhancement, much lower doses (750-1500mg) of BCAA have been shown effective for enhancing post-exercise recovery (the ratio of leucine, valine, and isoleucine appears to be important to the recovery effects, with 3:1:1 demonstrating the greatest effects in studies of endurance athletes).

Supplemental intakes of the BCAAs have been studied up to 40 grams per day in tablet form and 1 to 7 grams per liter in liquid form with no significant adverse side effects (aside from stomachaches at levels above a few grams/dose). Higher intakes (5-10g/day) should be avoided on a daily basis due to the possibility of competitive inhibition of the absorption of other amino acids from the diet and the risk of gastrointestinal distress.

For use before or during exercise, effective doses for of BCAA for delaying central fatigue are in the range of 10-20 grams per day (tablets/capsules) in divided doses or during exercise when added to a carbohydrate-containing beverage (1 to 7 grams of BCAA per liter of fluid). Be aware that these levels can induce gastrointestinal complaints including cramps and diarrhea.

For use following exercise to enhance recovery, 750-1500mg of BCAAs in a 3:1:1 ratio of leucine:valine:isoleucine (as found in Recover-Ease from Wicked Fast Sports Nutrition) has been shown to reduce muscle soreness and upper respiratory tract infections, while improving energy/fatigue levels, cortisol levels, and overall mood.

Branched chain amino acids (BCAA) are three essen
tial amino acids; leucine, valine and isoleucine. The “branched chain” designation refers to their unique chemical structure – and the “essential” terminology indicates that your body cannot make them, so they need to be obtained in the diet.

The recommended dietary allowance for the BCAA is about 3 grams per day – an amount that should be easily obtained from protein foods. Supplemental levels have been used at levels from as low as 750mg/dose (in a specific ratio) to as much as 20-40 grams per day (typically supplemented from whey protein sources in some sports beverages.

There are two main theories behind BCAA supplements. The first relates to a phenomenon known as “central fatigue,” which holds that mental fatigue in the brain can adversely affect physical performance in endurance events. The central fatigue theory suggests that low blood levels of BCAA may accelerate the production of serotonin, a key neurotransmitter in the brain, and prematurely lead to fatigue. Increased serotonin levels in the brain may depress central nervous system function and lead to feelings of sleepiness and fatigue.

Tryptophan, an amino acid that circulates in the blood, is a precursor of serotonin, and can be more easily transported into the brain to increase serotonin levels when BCAA levels in the blood are low (because high blood levels of BCAA can block tryptophan transport into the brain).

During exercise, as muscle and liver glycogen are depleted for energy, blood levels of BCAA may also decrease and fatty acid levels increase to serve as an additional energy source. The problem with extra fatty acids in the blood is that they need to attach to a carrier protein called albumin for proper transport. In doing so, the fatty acids displace tryptophan from its place on albumin and thus facilitate the transport of tryptophan into the brain for conversion into serotonin. Therefore, because of the combination of reduced BCAA and elevated fatty acids in the blood, more tryptophan enters the brain and more serotonin is produced, leading to fatigue. Supplementing the diet with additional levels of BCAA is thought to block the tryptophan transport and, therefore, delay fatigue during long duration exercise.

The other main theory supporting BCAA supplementation is one of supporting immune system function and accelerating the repair of muscle damage and lung damage following exercise. In this scenario, supplemental BCAAs are used as a post-exercise recovery aid, where they serve as a fuel source for immune system cells, which are then able to increase their activity and more completely repair muscle/lung damage.

For endurance athletes, particularly those competing in longer races (2+ hours) such as marathons, triathlons, cycling, mountain biking, and adventure racing, BCAA supplements may be warranted to help delay central fatigue and are certainly warranted to help enhance post-exercise recovery. Participation in shorter duration events is unlikely to result in substantial changes in blood levels of BCAA, tryptophan, or fatty acids, nor would lower-intensity exercise generate enough damage to muscle or lung tissue to benefit from a supplemental source of BCAAs.