ALCHEMISTS SOUGHT to turn base metals into gold. They failed. But for body builders, Eugene Schiff, a pharmacist from Detroit, managed the next best thing(次优选择). In 1936 he turned whey(乳清), the liquid waste from cheesemaking, into a protein powder. Athletes and gym rats(健身狂) now pay top dollar(花大价钱) for the stuff. And whey has been joined by powders made from its fellow dairy product, casein(酪蛋白), and from leguminous(豆科的) plants, namely peas and soyabeans. Which is best?
炼金术士曾试图将贱金属变成黄金，但失败了。不过，对健美者而言，底特律药剂师尤金·希夫做到了次优之事。1936年，他将奶酪制作产生的液体废料——乳清，变成了蛋白粉。如今，运动员和健身狂们为此一掷千金。乳清蛋白粉也不再孤单，它的同类乳制品酪蛋白，以及豆科植物（即豌豆和大豆）制成的蛋白粉也加入了阵营。哪种最好？
A crucial ingredient to look for is leucine(亮氨酸). This is about 12% of whey powder’s mass and one of the 20 amino acids(氨基酸) from which proteins are composed. It is one of the nine dubbed “essential”, because humans cannot make them, so they must arrive in food.(它属于九种被称为“必需”的氨基酸之一，因为人体无法合成它们，所以必须从食物中获取。) But leucine has many talents. It also activates a biochemical pathway, called the mechanistic target of rapamycin (mTOR(雷帕霉素机制靶点，调控细胞生长的关键通路)), that encourages protein synthesis(合成).
关键要看亮氨酸。它约占乳清蛋白粉质量的12%，是构成蛋白质的20种氨基酸之一。它属于九种“必需”氨基酸，因为人体无法自行合成，必须从食物中获取。但亮氨酸本领多多。它还能激活一条名为“雷帕霉素机制靶点”（mTOR）的生化通路，从而促进蛋白质合成。
After exercise, mTOR gets to work(开始工作) replenishing(补充) muscle fibres. Leucine thus helps provide not only the signal, but also the raw materials. Put another way(换言之), says Stuart Phillips, a kinesiologist(运动机能学家) at McMaster University, leucine is both brick and bricklayer. In younger, active adults, leucine’s muscle-building might goes only so far, says Brad Schoenfeld of Lehman College in New York: there is no additional benefit to taking more than that found in a standard 25g scoop of whey powder.(纽约莱曼学院的布拉德·舍恩菲尔德表示，对年轻活跃的成年人来说，亮氨酸的增肌效力有限：摄入量超过标准25克一勺乳清蛋白粉所含的量，并无额外益处。) Older people’s muscles are tougher to stimulate. For them, whey with extra leucine can be beneficial for building brawn(肌肉；体力), according to a study of 20 people in Nutrition Journal in 2014.
运动后，mTOR开始补充肌肉纤维。因此，亮氨酸不仅提供信号，还提供原料。麦克马斯特大学运动机能学家斯图尔特·菲利普斯说，换言之，亮氨酸既是砖块，也是砌砖工。纽约莱曼学院的布拉德·舍恩菲尔德表示，对年轻活跃的成年人来说，亮氨酸的增肌作用有限：摄入超过标准25克一勺乳清蛋白粉所含的量，并无额外益处。老年人的肌肉更难被刺激。2014年《营养学杂志》一项针对20人的研究显示，对他们而言，额外添加亮氨酸的乳清蛋白粉有助于增肌。