還能愉快地吃肉嗎？

By+Ian+Marber+++Fiona+Cai

Weve become rather used to categorising foods as good or bad. Its simpler that way. Some foods are easy—sugar is the most recent pantomime villain1） （boo?。?while kale2） and coconut water are the latest heroes （hurrah?。猙ut what happened to red meat， which used to be a baddy. Is it still worthy of a big boooo？

The vilification3） of red meat began in earnest in the 1960s following landmark postwar research entitled the Seven Countries Study4）， which established a link between meat consumption and cardiovascular5） disease. It was believed that saturated6） fat raised cholesterol7） levels， in turn increasing the incidence of coronary heart disease， and as red meat （along with butter and eggs） is a common source of saturated fat， it was directly implicated. More recently， we have understood that saturated fat intake alone is unlikely to raise cholesterol compared to that of sugars and carbohydrates—but does that mean meat is off the hook8）？

Historically， many of the dietary studies have researched the outcome of eating “meat，” a term that encompasses red and processed meat. The resulting reports included increased incidence of colorectal9） cancer as well as heart disease—and so challenging this strong belief has been a task. But processed meats are quite different from the fresh variety as they contain sodium nitrate10）， a preservative that is converted into a nitrite11） on contact with saliva12） or enzymes13） in the digestive system. Nitrites can then be further converted into nitric oxide14） or nitrosamine15）， the former being beneficial， the latter less so. Nitrosamines are created when nitrites are combined with amino acids16） （present in protein） and heat. In other words， fresh meat when cooked and preserved increases numbers of nitrosamines， which have been linked to colorectal cancer.

Concentrating on fresh unprocessed red meat （beef， lamb etc.）， there are distinct nutritional benefits on offer in the form of iron， together with zinc and the B group of vitamins including B12. But the concentration of nutrients is somewhat dependent on the type of feed used in rearing the animal. Some herds are fed with corn to bulk them up17） while others graze18） naturally on grass. Meat derived from grass-fed animals is generally nutritionally superior in terms of essential fats as it offers over four times the amount of omega 319） than grain-fed. Omega 3 is noted for its anti-inflammatory20） potential while omega 6， the prevailing fat in grain-fed meat， can encourage inflammation. Grass-fed also tends to be higher in iron， zinc， vitamin E and beta-carotene21） and is a worthy source of protein.

How the meat is cooked is a factor， too. Exposing fresh red meat to direct flames or allowing it to burn produces heterocyclic aromatic amines22） and polycyclic aromatic hydrocarbons23）. These increase the incidence of some forms of cancer and so over-barbecuing even the finest organic grass-fed red meat from a herd raised by nuns， massaged by professionals and certified kosher24） by the Chief Rabbi will turn it into the nutritional equivalent of a slice of iffy25） ham from some discount value range.

Although eating processed meat carries more of a risk than red meat， does that make the latter “safe”？ A meta-analysis of studies published in the British Journal of Nutrition last year considered data from 13 studies involving over 1.6 million individuals. This concluded that those eating the most processed meat had a 22 per cent increase in mortality from any cause and 18 per cent increase from cardiovascular disease. Even small amounts of processed meat might carry risk， as eating an extra 50g a day showed a positive association with mortality rates， while eating an extra 100g of red meat showed only a weak positive， but nonetheless a positive. Bear in mind that eating x means we dont eat y， and so eating large amounts of processed or red meat means that we are likely to eat fewer vegetables and so less fibre， fewer antioxidants26） etc. and to some extent this will affect the outcome.

In short， processed meat intake carries increased risk of mortality， cancer and cardiovascular disease while red meat intake showed a slight increase in risk. Where does this leave us？ All things considered， I think that processed meats should be a rare occurrence. A slice of ham or bacon here and there when added to a decent diet shouldnt result in ending up on an NHS27） waiting list. Nor should 200/250g of red meat twice a week as long as it is cooked carefully and not burnt. Ideally， the red meat would be grass-fed but that can cost anything from 25 per cent upwards more. （Grain-fed is still a decent enough choice.）

To my mind， the joy of eating a rib-eye steak， medium rare， with some fiery28） horseradish sauce once a week is a pleasure， which means that red meat， enjoyed sparingly， is a nutritional hero. Hurrah.

我們?cè)缫蚜?xí)慣將食物按好壞分類。這樣做更簡(jiǎn)單。有些食物很容易歸類：糖類是最近的“大反派”（噓?。?，羽衣甘藍(lán)和椰汁則是最新的“英雄”（吼吼！）。然而，紅肉又如何呢？曾經(jīng)是反面角色的它是否仍應(yīng)招來(lái)噓聲一片呢？

對(duì)紅肉的詆毀真正始于20世紀(jì)60年代一項(xiàng)名為“七國(guó)研究”的戰(zhàn)后研究。這項(xiàng)研究具有里程碑式的意義，它確立了肉的食用與心血管疾病間的關(guān)聯(lián)。該研究認(rèn)為飽和脂肪會(huì)使膽固醇水平升高，從而增加冠心病的發(fā)病率。由于紅肉（還有黃油和雞蛋）是飽和脂肪的常見(jiàn)來(lái)源，所以直接受到了牽連。最近我們了解到，相比糖和碳水化合物，單是攝取飽和脂肪未必會(huì)使膽固醇水平升高。然而，這是否意味著肉類可以擺脫其惡名了呢？

以往的許多飲食研究都調(diào)查過(guò)吃肉的后果，這里所說(shuō)的“肉”包括紅肉和加工肉制品。在研究得出的結(jié)論報(bào)告中，心臟病及結(jié)腸直腸癌的發(fā)病率均呈上升趨勢(shì)。因此，要挑戰(zhàn)如此堅(jiān)定的觀念已經(jīng)成了一件困難的事。不過(guò)，加工肉制品與新鮮肉類大相徑庭，因?yàn)榧庸と庵破分泻邢跛徕c，這是一種防腐劑，在接觸到消化系統(tǒng)中的唾液或酶之后，會(huì)轉(zhuǎn)化成亞硝酸鹽。而后，亞硝酸鹽能進(jìn)一步轉(zhuǎn)化成一氧化氮或亞硝胺，前者有益于健康，后者則不然。亞硝胺是亞硝酸鹽與（存在于蛋白質(zhì)中的）氨基酸及熱量結(jié)合時(shí)產(chǎn)生的。也就是說(shuō)，新鮮肉類在烹飪和保存的過(guò)程中，其亞硝胺的含量會(huì)增加，而這與結(jié)腸直腸癌存在關(guān)聯(lián)。

讓我們將注意力集中到那些新鮮且未經(jīng)加工的紅肉（牛、羊肉等）上，它們可以提供獨(dú)特的營(yíng)養(yǎng)價(jià)值——鐵、鋅及維生素B族，包括維生素B12。但這些營(yíng)養(yǎng)物質(zhì)的濃度在某種程度上取決于牲畜的飼養(yǎng)方式。有些牲畜被喂以玉米來(lái)增重，有些牲畜則食用天然牧草。就必需脂肪而言，草飼動(dòng)物的肉通常要比谷飼動(dòng)物的肉營(yíng)養(yǎng)價(jià)值高，因?yàn)榍罢咚摩?脂肪酸比后者所含的四倍還多。Ω3脂肪酸因其抗炎潛能而著稱，而普遍存在于谷飼動(dòng)物肉中的Ω6脂肪酸卻會(huì)促使炎癥發(fā)展。草飼動(dòng)物還富含更多的鐵、鋅、維生素E和β-胡蘿卜素，并且還是很好的蛋白質(zhì)來(lái)源。

肉的烹飪方式也是一個(gè)影響因素。將新鮮的紅肉暴露于明火之上或者把肉燒焦都會(huì)產(chǎn)生異環(huán)式芳香胺和多環(huán)芳香烴。這些物質(zhì)會(huì)增加某些癌癥的發(fā)病率。因此，即便是由修女飼養(yǎng)、專業(yè)人士按摩并經(jīng)首席拉比認(rèn)證過(guò)的最上等的有機(jī)草飼動(dòng)物紅肉，在過(guò)度燒烤之后，其營(yíng)養(yǎng)價(jià)值也會(huì)變得和一片打折的問(wèn)題火腿相差無(wú)幾。

盡管食用加工肉制品的風(fēng)險(xiǎn)比食用紅肉的風(fēng)險(xiǎn)更高，但這是否意味著后者是“安全的”呢？去年（編注：英文原文發(fā)表于2015年12月）發(fā)表在《英國(guó)營(yíng)養(yǎng)學(xué)雜志》上的一篇對(duì)相關(guān)研究進(jìn)行元分析的文章考慮了13項(xiàng)研究中的數(shù)據(jù)，其中涉及的研究個(gè)體超過(guò)160萬(wàn)。分析得出的結(jié)論是，那些食用加工肉制品最多的個(gè)體因各種原因致死的死亡率會(huì)增加22%，因心血管疾病致死的死亡率會(huì)增加18%。即便是少量的加工肉制品也可能存在風(fēng)險(xiǎn)，因?yàn)槊刻於嗍秤?0克加工肉制品的行為與死亡率呈現(xiàn)正相關(guān)關(guān)系，而每天多食用100克紅肉的行為只與死亡率呈現(xiàn)出較弱的正相關(guān)性，但即便如此也是正相關(guān)。請(qǐng)記住，吃了X就意味著我們不會(huì)再吃Y，因此，食用大量加工肉制品或紅肉就意味著我們很可能會(huì)少吃蔬菜，減少植物纖維、抗氧化劑等物質(zhì)的攝入。這在一定程度上會(huì)對(duì)結(jié)果產(chǎn)生影響。

簡(jiǎn)而言之，加工肉制品的攝入會(huì)增加死亡和罹患癌癥及心血管疾病的風(fēng)險(xiǎn)，而紅肉的攝入只會(huì)使風(fēng)險(xiǎn)略有提高。這意味著我們?cè)撛趺崔k？考慮到所有因素，我認(rèn)為應(yīng)該少吃加工肉制品。在豐盛的日常飲食里偶爾加入一片火腿或培根是不會(huì)導(dǎo)致人們出現(xiàn)在英國(guó)國(guó)民健康保險(xiǎn)制度的就診等候名單上的。同樣，每周食用兩次200克或250克的紅肉也不會(huì)讓你上這個(gè)名單，只要這些肉小心烹制，不被燒焦。要是這些紅肉來(lái)自草飼動(dòng)物，那就完美了，但這會(huì)導(dǎo)致花銷上漲25%，甚至更多。（所以，谷飼動(dòng)物的肉仍是蠻不錯(cuò)的選擇。）

在我看來(lái)，每周食用一次三分熟的肋眼牛排，佐以辛辣的辣根醬，這是一件樂(lè)事。這意味著只要有節(jié)制地享用，紅肉也是提供營(yíng)養(yǎng)的“英雄”。吼吼。