经济学人下载:心脏疾病与微生物

Science and technology
科学技术

Heart disease and the microbiome
心脏疾病与微生物

High steaks
牛排吃越多，心脏病风险越高

Hardening of the arteries may be caused by a malign interaction of meat-eating and intestinal bacteria
肉食与肠道细菌的恶性作用可能会造成动脉硬化

THOSE who take part in clinical trials often have to do nasty things, from taking new drugs to forgoing sleep.
参加临床试验的受试者常常得要做一些令人难受的事儿，从试用新药到放弃睡眠。

Participants in a trial organised by Stanley Hazen of the Cleveland Clinic, in Ohio, had a decidedly easier task: eating steak.
克里夫兰医学中心的斯坦利?哈森在俄亥俄州组织的临床试验中的受试者所要做的，相对来说要简单的多：吃牛排。

After reading Dr Hazen's conclusions, though, they may be inclined to eat rather less of it.
但要是读过哈森博士的实验报告后，他们或许宁愿自己少吃一些。

A link between red-meat consumption and heart disease was perceived by epidemiologists several decades ago, but the nature of this link has never been properly explained.
几十年前就有流行病学家发现心脏疾病与食用红肉有关，但这种关联的原理一直没有得到合理的解释。

Blame's finger usually points at saturated fats and cholesterol.
饱和脂肪酸和胆固醇经常被指责为引起心脏疾病的罪魁祸首。

Red meat contains both.
红肉中这两种物质都有。

But a big recent study showed no connection between saturated fat and heart disease, so something else is probably involved.
最近一项大型研究发现饱和脂肪酸并不会引起心脏疾病，很可能是一些其他物质引起的心脏疾病。

Dr Hazen thinks he knows what.
哈森博士认为他发现了这些物质是什么。

As he outlines in a paper just published in Nature Medicine, he believes the blame actually lies with the microbiome—the collection of 100 trillion or so bacteria that live in the human gut.
正如他在《自然医学》上刚发表的文章中所说的那样，他认为罪魁祸首实际上是微生物菌群-人体肠道中生存着大约100万亿细菌。

Generally, members of the microbiome get on well with their host.
一般来说，肠道菌群可与宿主和谐共处。

They digest complex carbohydrates that human enzymes cannot handle, thus increasing the nutritional value of food.
它们可分解人体中的酶所不能分解的复杂碳水化合物，因而可进一步提高食物的营养价值。

They also fend off infections by hostile bugs.
它们还可阻挡病原菌对人体的侵害。

But, as with any partnership, the interests of the partners are not always in perfect alignment, and Dr Hazen thinks the processing of red meat is an example of such misalignment.
不过，像任何合作关系一样，双方的利益并不总是完美一致的，哈森博士认为红肉的消化吸收过程正是这种不一致的实例。

The mismatch in question revolves around a molecule called carnitine.
这种失衡主要是由一种叫做肉毒碱的化学物质引起的。

This chemical helps transport fatty acids inside cells, and red meat is rich in it.
肉毒碱可参与运输细胞内的脂肪酸，而红肉中富含肉毒碱。

Dr Hazen thinks that when it is metabolised by gut bacteria, a chain of events begins that results in atherosclerosis.
哈森博士认为肠道细菌代谢红肉时，由此引发人体内一连串的生物化学反应，从而导致动脉粥样硬化。

A gut feeling
肠道感觉

Dr Hazen has already demonstrated, in a paper published in 2011, that parts of the microbiome can promote atherosclerosis.
哈森博士在他2011年发表的文章中验证，部分肠道菌群可促进动脉硬化。

In that paper he showed that choline, a molecule found in eggs and meat, is digested by some gut bacteria to produce trimethylamine, which is then processed in the liver to create trimethylamine N-oxide, or TMAO—a substance that encourages atherosclerosis.
他在文中指出胆碱—在蛋类和肉类中发现的一种分子，被肠道细菌消化吸收后生成三甲胺，三甲胺在肝脏中转化为能够加速动脉硬化的三甲胺N-氧化物或氧化三甲胺。

That is bad in general.
这些化学物质通常对人体是有害的。

And if the arteries in question are the coronary arteries, which supply heart muscle with blood, it is bad in particular because it can lead to a heart attack.
并且如果有损伤的动脉是为心肌供血的冠状动脉，情况尤为糟糕，因为这将引起心脏病发作。

Carnitine is chemically similar to choline so, given its abundance in red meat, Dr Hazen wanted to know if it, too, might be linked with heart disease.
肉毒碱的化学作用与胆碱相似，由于肉毒碱在红肉中含量丰富，哈森博士试图通过实验探究肉毒碱是否也与心脏疾病有关联。

He also wanted to know how TMAO wreaks its havoc on arteries.
同时他也想搞清楚氧化三甲胺是如何损伤动脉血管的。

Attempting to answer these questions required several studies, on both mice and men.
为了得到答案，哈森博士对人和老鼠进行了多项研究。

The first involved five human volunteers eating 8oz sirloin steaks, plus a carnitine supplement.
第一组实验的五个志愿者食用了8盎司沙朗牛排以及一片肉毒碱补充剂。

This diet resulted in high levels of both carnitine and TMAO in the volunteers' blood.
实验结果是，志愿者血液中都出现了高含量的肉毒碱和氧化三甲胺。

But when the same people were given antibiotics to kill their gut microbes, a subsequent steak meal produced little TMAO, even though their carnitine levels went even higher than before.
但当这些志愿者食用可杀死肠道细菌的抗生素后，食用同样的牛排却只产生了少量的氧化三甲胺，尽管之前他们血液中肉毒碱含量升高过。

Production of TMAO, then, seems to need bacteria.
这表明氧化三甲胺的产生似乎确实需要肠道菌群参与。

Those bacteria are not, however, always present.
然而，这样的肠道细菌并不总是存在的。

In the interests of science, Dr Hazen persuaded a vegan volunteer to eat a steak.
出于对科学真理的探索，哈森博士说服一名严格素食主义志愿者食用一块牛排。

He compared this man's subsequent TMAO levels with those of a meat eater after a steak meal.
他将这位志愿者与食肉志愿者吃牛排后的血液中氧化三甲胺含量相比较，

The carnivore's TMAO levels jumped.
肉食志愿者的氧化三甲胺含量飙升，

The vegan's did not.
而素食志愿者的却没有。

That suggests the vegan's particular gut flora—adapted to his usual meat-free diet—did not contain species that can digest carnitine.
这表明素食者特有的肠道菌群—已经适应了他日常的素食饮食—不含可分解肉毒碱的细菌。

Some vegetarian volunteers, given carnitine pills but spared the steak, showed similar results.
一些素食志愿者没吃牛排，但服用肉毒碱制剂，实验结果与吃牛排相似。

To find out whether all this matters, Dr Hazen looked at the association between heart disease and levels of carnitine and TMAO in more than 2,500 people.
为证实所有这些因素是否相关，哈森博士调查了2500多人，试图发现心脏疾病与肉毒碱和氧化三甲胺之间的关系。

He found there was indeed an association between heart disease and carnitine, but only when TMAO levels were also high.
他发现心脏疾病确实与肉毒碱的含量有关，但只有当氧化三甲苯含量也同样很高时才可以。

Experiments in mice confirmed the broader hypothesis: TMAO production relies on microbes that digest carnitine, and prolonged consumption of that chemical by susceptible mice leads to atherosclerosis.
老鼠实验证实了进一步的猜想：氧化三甲胺的产生需要分解肉毒碱的肠道细菌参与，易感小鼠如果长期食用肉毒碱会导致动脉硬化。

Exactly how this happens is still hazy.
确切的作用机理目前尚不清楚。

Dr Hazen has shown, in yet further experiments on mice,that microbe-generated TMAO interferes with liver enzymes that make bile acids—substances that help remove excess cholesterol.
在接下来的老鼠实验中，哈森博士指出，细菌分解产生的氧化三甲胺可干扰产生胆汁酸的肝酶的活性—胆汁酸可帮助转移多余的胆固醇。

TMAO also influences cholesterol metabolism in other parts of the body, including the artery wall.
氧化三甲胺还影响包括动脉壁在内的身体其他部位中的胆固醇代谢。

But the precise chain of events linking microbes to heart disease is still unclear.
但使得肠道细菌与心脏疾病有关的一系列相关的精确反应至今还不清楚。

Another unknown is which of the microbiome's many species are responsible for promoting TMAO production.
肠道菌群中是否有很多种细菌参与氧化三甲胺的生成目前也并不明确。

Based on an analysis of the faeces of both vegetarians and meat eaters, Dr Hazen suspects bacteria of several genera, including Clostridium and Fusibacterium.
基于对素食与肉食志愿者的粪便研究，哈森博士猜想有几种不同属的细菌参与反应，其中包括梭状芽胞杆菌属和梭菌属。

Regardless of the details, though, this study suggests that people looking for the link between heart disease and the eating of meat have been ignoring two culprits, carnitine and bacteria.
但不论生化反应细节如何，这项研究表明人们长期以来寻找心脏病与食肉之间的关联，却忽略了肉毒碱和肠道细菌这两大元凶。

That does not absolve cholesterol of blame—on the contrary, carnitine boosts cholesterol's pernicious effect.
不过这并不意味着胆固醇是无害的—相反，肉毒碱的存在大大增长了胆固醇的害处。

But the study is yet another reminder that medical science's past failure to take proper account of humanity's bacterial guests has stopped researchers understanding how bodies actually work.
这项研究同时也提醒人们，之前医学研究未能把人体肠道内的细菌考虑在内，使得研究者们没能真正搞明白人体是如何运转的。

Heart disease is the biggest killer around.
现在心脏疾病是威胁人类健康的头号杀手。

Though Dr Hazen's analysis does not change advice about how to prevent atherosclerosis, it may radically revise the treatment of those for whom prevention has failed.
虽然哈森博士的研究未能改变预防动脉硬化的医嘱，但却彻底改变了动脉硬化患者的治疗方案。