精读文本：为什么咖啡因对人的效果差异巨大
“I’ll normally have two cups a day and then maybe an energy drink too. And then I can drink coke all night and feel completely fine.”
“我通常一天会喝两杯咖啡，偶尔还会喝点能量饮料。至于可乐，我可以整晚都喝，而不会觉得有什么不适。”
We’ve all got a friend like Emily. The one who can knock back a double espresso or even an energy drink in the evening and still drift off to sleep an hour or so later.
我们每个人都有像艾米丽这样的朋友。那种人就算在晚上喝下双份浓缩咖啡或能量饮料，也能在一小时后安然入睡。
For many people, though, the effects of caffeine can linger for far longer.
不过，对许多人来说，咖啡因的影响会持续更长时间。
Why then can some people drink coffee before bed and sleep like a baby, while others lie awake staring at the ceiling?
那么，为什么有些人可以在睡前喝咖啡，然后睡得香甜，而另一些人却无法入睡，只能躺在床上盯着天花板发呆呢？
What exactly is caffeine?
咖啡因到底是什么呢？
Caffeine is by far the most used psychoactive substance in the world.
咖啡因无疑是世界上使用最广泛的精神活性物质。
> 注释：原文译文误写为“致幻物质”，psychoactive substance标准译法为精神活性物质。
More than two billion cups of coffee are drunk each day and that’s before we start talking about cola, energy drinks, plus some chocolate, chewing gum and sports supplements, which also contain the chemical.
每天有超过 20 亿杯咖啡被饮用。而这还不包括可乐、能量饮料，以及那些含有这种化学物质的巧克力、口香糖和运动补充剂。
It can make us feel more alert, improve concentration and stave off tiredness. Too much can cause headaches, jitters, a racing heart and restless nights.
它能让我们更加清醒、提高注意力，同时还能缓解疲劳。不过，摄入过量则可能导致头痛、紧张手抖、心跳加速以及失眠等问题。
Caffeine doesn’t give your body extra energy, it helps mask tiredness, experts say.
专家表示，咖啡因并不能为身体提供额外的能量，它只是能暂时掩盖疲劳感而已。
It does this by blocking the action of adenosine, a chemical that builds up in the brain throughout the day and makes us feel sleepy.
它是通过阻断腺苷（adenosine）的作用来达到这一效果的。腺苷是一种在一天中不断在大脑中积累的化学物质，它会让我们感到困倦。
Why does it affect people differently?
为什么它对不同人的影响各不相同呢？
One key factor is how long caffeine stays in the body.
一个关键因素是咖啡因在体内停留的时间长短。
After drinking a cup of coffee, it is absorbed into the bloodstream, where it quickly reaches the brain.
喝下一杯咖啡后，咖啡因会被吸收到血液中，然后迅速到达大脑。
All this time it’s also being broken down – or metabolised – by an enzyme in the liver and then flushed out in our urine.
在整个过程中，这种物质还会被肝脏中的某种酶分解或代谢掉，之后通过尿液排出体外。
That clean‑up process can take between six and 20 hours depending on the individual, says Dr Marilyn Cornelis, an associate professor of nutrition at Northwestern University in Chicago.
芝加哥西北大学营养学副教授玛丽莲·科尼利斯博士表示，根据个人情况的不同，这个代谢清除过程可能需要 6‑20 小时不等。
“It’s possible to have a first cup of coffee and maybe not feel the effects,” she says.
“喝第一杯咖啡时，可能感觉不到任何效果，”她说。
“But then two or three hours later, the caffeine is still lingering in the system, and, if you add a second cup, that might be enough to tip the scales and you start feeling a little anxious.”
“但两三个小时后，咖啡因仍然残留在体内。如果你再喝一杯咖啡，体内咖啡因累积就可能足以让人感到焦虑。”
How quickly the liver can processes the caffeine depends on a single gene – known as CYP1A2.
肝脏处理咖啡因的速度主要取决于一个基因——CYP1A2（细胞色素P450酶基因）。
Intriguingly it might be influenced, at least to some extent, by your ancestry.
有趣的是，这一因素至少在某种程度上可能受到祖先遗传背景的影响。
A study (PMC12954933) found that Europeans and South Americans tend to break down caffeine more quickly on average.
一篇研究（PMCID:PMC12954933，复旦大学2026综述）发现：平均来看，欧洲、南美人群分解咖啡因的速度普遍更快。
> 背景补充：该综述指出东亚人群中，咖啡因快速代谢等位基因频率显著低于欧洲人群，人群间CYP1A2基因多态性差异是核心原因。
What other factors are at play?
还有哪些因素在起作用呢？
Your DNA is far from the whole story. Smoking, but probably not vaping, appears to stimulate the liver to break down caffeine much more quickly, speeding up the process by as much as 65%.
DNA并不能完全解释个体差异。吸烟（电子烟大概率没有该效果）可以激活肝脏，大幅加快咖啡因分解，代谢效率最高可提升65%。
> 机制注释：烟草烟雾中的多环芳烃通过AHR通路上调CYP1A2酶活性；该效应是酶诱导效应，电子烟一般缺少这类诱导物。
That may help explain why smokers tend to drink more coffee and why people who quit can suddenly find their usual cup feels much stronger than before.
这或许可以解释为什么吸烟者往往喝更多的咖啡；同样，戒烟之后，人们会发现，往常的一杯咖啡，效果突然变得强烈很多。
Some antibiotics, contraceptive pills and other prescription medicines can have the opposite effect, allowing caffeine to stay trapped in the body for longer and prolonging the effects.
部分抗生素、口服避孕药以及其他处方药会起到相反作用：抑制CYP1A2酶活性，让咖啡因在体内停留更久，延长咖啡因的生理作用。
That’s also the case during pregnancy, when hormonal changes suppress the liver enzyme that breaks down caffeine.
怀孕期间同样如此：激素改变会抑制负责分解咖啡因的肝酶活性，咖啡因半衰期显著延长。
Then there is the question of tolerance.
除此之外还有咖啡因耐受度的问题。
Regular coffee drinkers become less sensitive to caffeine as the body adapts to the presence of the chemical.
长期喝咖啡的人，身体逐步适应咖啡因，对咖啡因的敏感度会下降。
If you then go without the drink for a few weeks, those adaptions in the brain begin to reverse, meaning the first coffee back might leave you feeling unusually wired or jittery.
如果连续几周停止摄入咖啡因，大脑已经形成的耐受适应性会发生逆转。重新喝咖啡的时候，第一杯就可能让你异常亢奋、手抖心慌。
What do experts advise?
专家们有什么建议呢？
Sleep researchers often recommend avoiding caffeine for at least six hours before bedtime.
睡眠研究人员通常建议：睡前至少6小时避免摄入咖啡因。
Dr Lindsay Browning, a psychologist and sleep expert, says the evidence suggests caffeine might not just stop you dozing off but can affect the quality of sleep as well.
心理学家、睡眠专家林赛·布朗宁博士指出：证据显示咖啡因不仅仅会让人难以入睡，还会破坏睡眠质量，即使能够睡着，深度睡眠也会变少。
“I’ll suggest to people that they might want to avoid caffeine after about 2pm if they are going to bed around 10pm,” she says.
“如果打算晚上10点睡觉，我会建议下午2点之后就不要再摄入咖啡因。”她说。
“But a blanket hard‑and‑fast rule isn’t helpful, because it doesn’t really allow for the intricacies and the individualisation of what’s really going on.”
“但一刀切的硬性规则并不合适，现实中个体差异错综复杂，不能一概而论。”
The NHS (https://www.nhs.uk/live‑well/eat‑well/food‑guidelines‑and‑food‑labels/water‑drinks‑nutrition/) advises to keep caffeine intake below 200 mg a day, about two mugs of filter coffee, as studies have linked it to low birth weight and other complications.
英国NHS官方指南建议孕妇每日咖啡因摄入不超过200毫克，约等于两杯过滤咖啡；过量咖啡因和新生儿低出生体重以及妊娠并发症存在关联。
For other adults the key message is moderation, as sensitivity to caffeine varies so much from person to person.
对于普通成年人，核心原则是适度饮用，每个人对咖啡因的敏感度差异极大。
“Caffeine is relatively safe,” says Peter Rogers, emeritus professor of nutrition at the University of Bristol.
布里斯托大学营养学荣誉教授彼得·罗杰斯说：“咖啡因整体安全性较高。”
“If you consume a great deal of it, you just start feeling jittery and unpleasant and you start realising, ‘I need to stop this, there’s no point taking any more’.”
“如果你摄入太多，身体会出现心慌、烦躁不适，你自己就会意识到‘不能再喝了’。”
Caffeine overdoses are very rare and severe poisonings almost always involve concentrated caffeine products, where several grams can be swallowed in a matter of minutes.
咖啡因普通饮品过量中毒非常罕见；严重中毒事件几乎全部来自高纯度浓缩咖啡因制品，数克咖啡因可在短时间内摄入，存在致命风险。
For coffee lovers the good news is that, if anything, that regular (if expensive) morning habit is linked to better health outcomes.
咖啡爱好者的好消息：每天早晨喝咖啡（虽然开销不小）和更好的健康指标存在相关性。
While researchers cannot prove that coffee is the only reason, decades of studies suggest that people who drink it in moderation tend to have lower risks of several major diseases and may even live slightly longer than non‑drinkers.
研究者尚不能证实咖啡就是健康获益的直接原因，但数十年流行病学研究提示：适量喝咖啡的人群，多种重大疾病风险更低，预期寿命相比不喝咖啡人群会略高。
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