欢迎来到肠道超级城市

想象一下，在你的身体深处，有一座比任何城市都更加繁忙的超级都市。这座城市的常住人口超过100万亿——没错，是“万亿”这个单位，比地球上的人口还要多出数千倍。这座城市没有高楼大厦，没有车水马龙，但它有自己的居民、自己的街道、自己的社会秩序，甚至还有自己的“外交关系”。欢迎来到你身体里的肠道超级城市。

Every single day, without you noticing, an incredible metropolis operates silently within your body. This city houses more than 100 trillion inhabitants—that's 100,000,000,000,000, to be exact—dwarfing the entire human population of Earth by thousands of times. There are no skyscrapers or bustling traffic here, but there are residents, streets, social orders, and even "diplomatic relations." Welcome to the Gut Super City inside your body.

这座城市的地理位置非常独特。它蜿蜒盘踞在你的腹腔之中，从一端到另一端足有三层楼那么高（如果把它拉直的话）。想象一下，你的肠道展开后的总面积可以达到250到400平方米，相当于一套三居室公寓的居住空间，有的科学家甚至说它相当于一个小型网球场的大小。这么大面积的内表面，每天都在忙着吸收营养、排出废物，同时还要维持整个“城市”的秩序与安全。

The location of this city is quite unique. It winds through your abdominal cavity, stretching as tall as a three-story building if uncoiled. Imagine the surface area of your intestine, unfolded, measures about 250 to 400 square meters—equivalent to a three-bedroom apartment, though some scientists compare it to a small tennis court. This vast inner surface works daily, absorbing nutrients, eliminating waste, while maintaining order and security throughout the entire "city."

🏙️ 肠道城市的一天：从早餐到入睡

🏙️ A Day in Gut City: From Breakfast to Bedtime

让我们跟随一位名叫小明的中学生的早餐，来看看这座超级城市是如何运转的。

Let's follow the breakfast of a middle school student named Xiao Ming to see how this metropolis operates.

当小明咬下第一口全麦面包时，肠道城市立刻忙碌起来。面包的主要成分是碳水化合物，其中含有复杂的淀粉分子。这些淀粉分子顺着食道滑落，经过胃的初步处理，最终来到小肠——这座城市的商业中心。在这里，它们会遇到一群特殊的“工人”：消化酶。胰淀粉酶、麦芽糖酶、蔗糖酶……这些酶就像工厂里的流水线机器，把复杂的淀粉分子切割成小小的葡萄糖分子。葡萄糖是身体的能量货币，可以直接被小肠细胞吸收，进入血液，输送到全身各处。

When Xiao Ming takes his first bite of whole wheat bread, Gut City immediately springs into action. The bread contains carbohydrates, including complex starch molecules. These molecules slide down the esophagus, undergo preliminary processing in the stomach, and finally arrive at the small intestine—the commercial center of this city. Here, they encounter special "workers": digestive enzymes. Pancreatic amylase, maltase, sucrase... these enzymes work like assembly line machines in a factory, cutting complex starch molecules into tiny glucose molecules. Glucose is the body's energy currency, directly absorbed by small intestine cells and transported through the bloodstream to every corner of the body.

但是，这还不是故事的全部。

But this isn't the whole story.

在小明的早餐里，除了面包，还有牛奶或者酸奶。牛奶中含有一种特殊的糖，叫做乳糖。消化乳糖需要一种叫乳糖酶的酶。有趣的是，全球约有65%到70%的成年人体内缺乏足够的乳糖酶，这意味着他们很难消化牛奶中的乳糖。当未被消化的乳糖到达大肠——也就是肠道城市的居民区时，事情就变得有趣了。

However, Xiao Ming's breakfast also includes milk or yogurt. Milk contains a special sugar called lactose. Digesting lactose requires an enzyme called lactase. Interestingly, about 65% to 70% of adults worldwide lack sufficient lactase, meaning they struggle to digest the lactose in milk. When undigested lactose reaches the large intestine—the residential district of Gut City—things get interesting.

👥 100万亿居民：肠道城市的形形色色

👥 100 Trillion Residents: The Diverse Population of Gut City

大肠里住着肠道城市最庞大的居民群体：微生物。严格来说，这些微生物包括细菌、古细菌、真菌、病毒，甚至还有一些微小的寄生虫。它们共同构成了一个复杂的生态系统，科学家们给这个生态系统起了一个专有名词——肠道微生物组。

The large intestine houses the largest population of Gut City: microorganisms. Strictly speaking, these include bacteria, archaea, fungi, viruses, and even some microscopic parasites. Together they form a complex ecosystem, which scientists have given a specific name—the gut microbiome.

当你听到“微生物”这个词，脑海中可能会浮现出致病的细菌、引发感冒的病毒这些坏家伙。没错，有些微生物确实会让人生病——金黄色葡萄球菌可以引起皮肤感染，大肠杆菌的某些菌株会导致严重的腹泻。但请记住，微生物世界远比这复杂得多。就像人类社会中既有犯罪分子也有善良的公民，微生物世界里同样有好有坏，而且大多数微生物既不好也不坏，它们就是存在着，以自己的方式生存着。

When you hear the word "microorganism," you might immediately think of disease-causing bacteria or cold-inducing viruses—the bad guys. Yes, some microorganisms can make people sick—Staphylococcus aureus can cause skin infections, certain strains of E. coli can lead to severe diarrhea. But remember, the microbial world is far more complex than that. Just as human society contains both criminals and kind citizens, the microbial world has good and bad actors, and most microorganisms are neither—they simply exist, living life on their own terms.

肠道中的绝大多数细菌都是无害的，甚至是有益的。它们帮助我们消化那些我们自己无法处理的食物成分，合成某些维生素，训练我们的免疫系统，甚至影响我们的情绪和思维。你没看错，肠道微生物确实在影响着你的大脑——这就是“肠脑超连线”这个书名的含义，我们会在后面的章节中详细探讨这个令人惊叹的发现。

The vast majority of bacteria in our gut are harmless, even beneficial. They help us digest food components we cannot process ourselves, synthesize certain vitamins, train our immune systems, and even influence our moods and thoughts. You read that correctly—gut microorganisms truly affect our brains, which is where the book title "The Gut-Brain Superhighway" comes from. We'll explore this amazing discovery in detail in later chapters.

📊 肠道居民的人口普查

📊 A Census of Gut Residents

科学家们花费了数年时间，试图清点肠道城市中到底有多少种居民。截至目前的研究表明，健康的成年人肠道中大约有1000到1500种不同的细菌物种。当然，每个人的肠道菌群组成都是独一无二的，就像世界上没有两片完全相同的树叶一样，也没有两个人的肠道微生物组完全相同。基因遗传、出生方式（顺产还是剖宫产）、婴儿期的喂养方式（母乳还是配方奶）、你生活的环境、你的饮食习惯，甚至你养的宠物，都会影响你的肠道菌群构成。

Scientists have spent years trying to count the residents of Gut City. Current research indicates that a healthy adult's gut contains approximately 1,000 to 1,500 different bacterial species. Of course, everyone's gut microbiota composition is unique—just as no two leaves are exactly alike, no two people have identical gut microbiomes. Genetic inheritance, birth method (vaginal or cesarean), infant feeding method (breast milk or formula), living environment, dietary habits, and even your pets all influence your gut microbiota composition.

在这1000多种细菌中，有几个“姓氏”特别常见，我们称之为“门”（Phylum，生物学分类中的一个级别）。最重要的几个门包括：

Among these 1,000+ bacterial species, several "surnames" are particularly common—we call these "phyla" (a level in biological classification). The most important phyla include:

厚壁菌门（Firmicutes）：这是肠道中数量最多的细菌群体，占据了大约60%到80%的江山。厚壁菌门的细菌种类繁多，形态各异，既有球形的，也有杆状的，还有些能形成坚韧的芽孢来抵御恶劣环境。这类细菌特别擅长分解复杂的碳水化合物，把它们变成身体可以利用的短链脂肪酸。有一种著名的厚壁菌叫*梭菌*（Clostridium），它们在维持肠道健康方面发挥着重要作用。

Firmicutes: This is the largest bacterial population in the gut, occupying approximately 60% to 80% of the territory. Firmicutes bacteria are diverse in species and morphology, ranging from spherical to rod-shaped, with some capable of forming resilient spores to withstand harsh environments. These bacteria excel at breaking down complex carbohydrates, converting them into short-chain fatty acids the body can utilize. A well-known Firmicutes is Clostridium, which plays an important role in maintaining gut health.

拟杆菌门（Bacteroidetes）：这是第二大细菌群体，约占肠道菌群的20%到30%。拟杆菌门的细菌同样是分解碳水化合物的能手，而且它们还擅长处理蛋白质和胆汁酸。有趣的是，拟杆菌门中有些成员能够合成一种叫维生素K的物质，这种维生素对血液凝固至关重要。如果你受伤流血，正是这些小小的肠道居民在帮助你止血。

Bacteroidetes: The second-largest bacterial group, comprising approximately 20% to 30% of the gut microbiota. Bacteroidetes bacteria are also experts at breaking down carbohydrates and excel at processing proteins and bile acids. Interestingly, some members of this phylum can synthesize vitamin K, a substance crucial for blood clotting. If you get injured and bleed, it's these tiny gut residents helping you stop the bleeding.

放线菌门（Actinobacteria）：虽然数量相对较少，但放线菌门中有一些非常出名的成员，比如双歧杆菌属（Bifidobacterium）。双歧杆菌是婴儿肠道中的优势菌种，它们能够帮助消化母乳中的特殊糖类，保护婴儿免受有害菌的侵害。许多益生菌补充剂中声称添加的所谓“有益菌”，往往就是双歧杆菌。

Actinobacteria: Although relatively small in number, this phylum includes some famous members, such as the genus Bifidobacterium. Bifidobacteria are the dominant species in infant guts, helping digest special sugars in breast milk and protecting babies from harmful bacteria. Many probiotic supplements claim to contain these "beneficial bacteria"—they're often Bifidobacteria.

变形菌门（Proteobacteria）：这个门包含了一些大家熟悉的细菌名字，比如大肠杆菌（Escherichia coli）、沙门氏菌（Salmonella）和幽门螺旋杆菌（Helicobacter pylori）。变形菌门在健康人体内的数量通常不多，但如果它开始大量繁殖，往往预示着肠道生态出了问题。许多研究已经注意到，在炎症性肠病、结肠癌等疾病状态下，变形菌门的丰度会显著增加。

Proteobacteria: This phylum contains familiar bacterial names like Escherichia coli, Salmonella, and Helicobacter pylori. Proteobacteria typically ex