A much debated issue here(taiwan), yet the term 瘦肉精 is very unintuitive. After searching for a while, i finally found that it is actually called Ractopamine. Here's some explanation of this chemistry term, taken from wikipedia:Ractopamine is a drug that is used as a feed additive to promote leanness in pigs raised for their meat. The feed additive Paylean®, produced by the U.S. company Elanco Animal Health, contains ractopamine hydrochloride. Paylean was approved by the United States Food and Drug Administration on December 22, 1999, and has also been approved in more than 20 countries, including Australia, Brazil, Canada, Mexico, and Thailand.[1]
Ractopamine in feed for animals is responsible for dramatic muscle growth, yet it is not a steroid or hormone, but rather a compound known as a beta agonist. Only a trace amount of ractopamine need be added for a marked increase in protein and decrease in fat accretion in animals, in particular swine. For the last 90 pounds of live weight gain, a mere 18.5 grams of ractopamine added to a ton of feed (20 ppm) will increase protein by 24% and decrease fat by 34%.[2]
In July 2007 Chinese officials seized U.S.-produced pork for containing ractopamine residues.or in chinese:
俗語說的好，豬養肥了就要殺來吃，但是怎麼讓豬能夠產出最多的肉，賣更多的錢，我們必須讓豬吃的飼料裡面，含有讓它可以更紥實的物質。目前主要用於改進動物體組成的方法，有以下幾類：一、生長激素；二、腎上腺素受體促進劑﹔三、營養調節；四、合成代謝類的類固醇。
最近鬧的沸沸洋洋的瘦肉精就是屬於第二類，事實上瘦肉精是一個統稱，有很多種化學物質都可以有這個效果﹔目前衛生署公布可以核准使用的就是 Ractopamine(Paylean培林)，Paylean是在在一九九九年經美國ＦＤＡ核准上市，在肉豬上市前二十八天加二十ppm的 Paylean於飼料中，可使每頭豬隻增加瘦肉五公斤，降低脂肪三公斤，節省飼料十八點五公斤，並且可提早四天達到上市。體重另外一個毒性較高的是 Clenbuterol，衛生署正打算核准中，天呀!!為何要核准毒性較高的瘦肉精，台灣人真是眾人騎，沒人疼嗎??(參考電影悲情城市)。無能的政府你 們在幹麻??
Ractopamine本來是禮來藥廠預計研發在治療氣喘的用藥，但實驗中發現Ractopamine要達到能治療氣喘的藥量會產生極大的副作用，因此不適用人類使用，但在動物試驗中偶然發現，少量用於豬隻時卻有提高豬隻精質瘦肉的效果，就如GSK公司開發憂鬱症藥物Zyban，而意外發現可以戒煙一樣。
在台灣較常使用的則是Salbutamol最為普遍，因價格便宜，它的毒性比Ractopamine高，比Clenbuterol低。
腎上腺素受體促進劑(agonists)主要有類似腎上腺素和正腎上腺素的結構，能夠大量提高瘦肉率，在近二十年來深受重視。但是世界各國幾乎都已經禁止使用，剩下美國還採用大量的高標準，只是不知道是不是都用來輸出給落後的第三世界國家，例如台灣。
