还能愉快地吃肉吗?

We’ve become rather used to categorising foods as good or bad. It’s 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！）―but 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 don’t 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 shouldn’t 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.

我们早已习惯将食物按好坏分类。这样做更简单。有些食物很容易归类：糖类是最近的“大反派”（嘘！），羽衣甘蓝和椰汁则是最新的“英雄”（吼吼！）。然而，红肉又如何呢？曾经是反面角色的它是否仍应招来嘘声一片呢？

对红肉的诋毁真正始于20世纪60年代一项名为“七国研究”的战后研究。这项研究具有里程碑式的意义，它确立了肉的食用与心血管疾病间的关联。该研究认为饱和脂肪会使胆固醇水平升高，从而增加冠心病的发病率。由于红肉（还有黄油和鸡蛋）是饱和脂肪的常见来源，所以直接受到了牵连。最近我们了解到，相比糖和碳水化合物，单是摄取饱和脂肪未必会使胆固醇水平升高。然而，这是否意味着肉类可以摆脱其恶名了呢？

以往的许多饮食研究都调查过吃肉的后果，这里所说的“肉”包括红肉和加工肉制品。在研究得出的结论报告中，心脏病及结肠直肠癌的发病率均呈上升趋势。因此，要挑战如此坚定的观念已经成了一件困难的事。不过，加工肉制品与新鲜肉类大相径庭，因为加工肉制品中含有硝酸钠，这是一种防腐剂，在接触到消化系统中的唾液或酶之后，会转化成亚硝酸盐。而后，亚硝酸盐能进一步转化成一氧化氮或亚硝胺，前者有益于健康，后者则不然。亚硝胺是亚硝酸盐与（存在于蛋白质中的）氨基酸及热量结合时产生的。也就是说，新鲜肉类在烹饪和保存的过程中，其亚硝胺的含量会增加，而这与结肠直肠癌存在关联。

让我们将注意力集中到那些新鲜且未经加工的红肉（牛、羊肉等）上，它们可以提供独特的营养价值――铁、锌及维生素B族，包括维生素B12。但这些营养物质的浓度在某种程度上取决于牲畜的饲养方式。有些牲畜被喂以玉米来增重，有些牲畜则食用天然牧草。就必需脂肪而言，草饲动物的肉通常要比谷饲动物的肉营养价值高，因为前者所含的Ω3脂肪酸比后者所含的四倍还多。Ω3脂肪酸因其抗炎潜能而著称，而普遍存在于谷饲动物肉中的Ω6脂肪酸却会促使炎症发展。草饲动物还富含更多的铁、锌、维生素E和β-胡萝卜素，并且还是很好的蛋白质来源。

肉的烹饪方式也是一个影响因素。将新鲜的红肉暴露于明火之上或者把肉烧焦都会产生异环式芳香胺和多环芳香烃。这些物质会增加某些癌症的发病率。因此，即便是由修女饲养、专业人士按摩并经首席拉比认证过的最上等的有机草饲动物红肉，在过度烧烤之后，其营养价值也会变得和一片打折的问题火腿相差无几。

尽管食用加工肉制品的风险比食用红肉的风险更高，但这是否意味着后者是“安全的”呢？去年（编注：英文原文发表于2015年12月）发表在《英国营养学杂志》上的一篇对相关研究进行元分析的文章考虑了13项研究中的数据，其中涉及的研究个体超过160万。分析得出的结论是，那些食用加工肉制品最多的个体因各种原因致死的死亡率会增加22%，因心血管疾病致死的死亡率会增加18%。即便是少量的加工肉制品也可能存在风险，因为每天多食用50克加工肉制品的行为与死亡率呈现正相关关系，而每天多食用100克红肉的行为只与死亡率呈现出较弱的正相关性，但即便如此也是正相关。请记住，吃了X就意味着我们不会再吃Y，因此，食用大量加工肉制品或红肉就意味着我们很可能会少吃蔬菜，减少植物纤维、抗氧化剂等物质的摄入。这在一定程度上会对结果产生影响。

简而言之，加工肉制品的摄入会增加死亡和罹患癌症及心血管疾病的风险，而红肉的摄入只会使风险略有提高。这意味着我们该怎么办？考虑到所有因素，我认为应该少吃加工肉制品。在丰盛的日常饮食里偶尔加入一片火腿或培根是不会导致人们出现在英国国民健康保险制度的就诊等候名单上的。同样，每周食用两次200克或250克的红肉也不会让你上这个名单，只要这些肉小心烹制，不被烧焦。要是这些红肉来自草饲动物，那就完美了，但这会导致花销上涨25%，甚至更多。（所以，谷饲动物的肉仍是蛮不错的选择。）

在我看来，每周食用一次三分熟的肋眼牛排，佐以辛辣的辣根酱，这是一件乐事。这意味着只要有节制地享用，红肉也是提供营养的“英雄”。吼吼。