around the world every year.

超過十萬噸，

That's equivalent to the weight of 14 Eiffel Towers.

相當於 14 座艾菲爾鐵塔的重量。

Most of this caffeine is consumed in coffee and tea,

大多咖啡因都攝取自咖啡和茶，

but it's also ingested in some sodas,

但有些也來自汽水、

chocolate,

巧克力、

caffeine pills,

咖啡因錠，

and even beverages labeled decaf.

甚至是標示無咖啡因的飲料。

Caffeine helps us feel alert, focused, happy, and energetic,

就算昨晚沒睡飽，咖啡因仍使我們機警、

even if we haven't had enough sleep.

專注、愉悅且充滿活力；

But it can also raise our blood pressure, and make us feel anxious.

但它也會增加血壓，使我們感到焦慮。

It's the world most widely used drug.

它是全球使用最廣的藥。

So how does it keep us awake?

所以⋯⋯咖啡因到底如何提神？

Caffeine evolved in plants where it serves a few purposes.

在植物演化的進程中，咖啡因有幾種功能：

In high doses, as it's found in the leaves and seeds of certain species,

例如某些植物的葉子和種子中，咖啡因的劑量很高，

it's toxic to insects.

對昆蟲有毒；

But when they consume it in lower doses, as it's found in nectar,

但當昆蟲攝取花蜜中低劑量的咖啡因，

it can actually help them remember and revisit flowers.

卻能因此記得花叢的位置並再次找到它。

In the human body, caffeine acts as a stimulant for the central nervous system.

人體中，咖啡因是中樞神經的興奮劑。

It keeps us awake by blocking one of the body's key sleep-inducing molecules,

它藉由阻絕導致睡眠的分子之一

a substance called adenosine.

——腺苷酸——使我們清醒。

Your body needs a constant supply of energy,

人體所需的持續性能量

which it gets by breaking down a high-energy molecule called ATP.

是由分解叫做腺苷三磷酸的高能量分子所獲得。

In the process,

在分解過程中，

it liberates adenosine, ATP's chemical backbone.

腺苷三磷酸會釋放腺苷酸——腺苷三磷酸的化學鍵骨幹。

Neurons in your brain have receptors perfectly tailored to this molecule.

腦神經元有專門接收腺苷酸的受體。

When adenosine docks to these receptors,

當腺苷酸進入受體時，

it activates a cascade of biochemical reactions

便活化了一連串的生化反應，

that cause neurons to fire more sluggishly

使神經元的活動趨緩，

and slow the release of important brain-signaling molecules.

並漸少釋放重要的腦內訊號傳遞分子。

In other words, you get sleepy.

也就是說，它使你想睡。

Caffeine is what's called an adenosine receptor antagonist.

咖啡因可說是腺苷酸進入受體時的競爭對手：

That means it derails this process of slowing your neurons down

它藉由阻擋腺苷酸進入受體

by blocking adenosine receptors.

使放鬆神經的機制失效。

Caffeine and adenosine have a similar molecular structure,

咖啡因和腺苷酸有相似的分子結構，

close enough that caffeine can wedge into the adenosine receptors,

足以使它嵌入腺苷酸的受體，

but not close enough to activate them.

但因結構不完全相同，所以並不會活化受體。

To summarize, adenosine inhibits your neurons.

總之，腺苷酸抑制你的神經，

Caffeine inhibits the inhibitor, so it stimulates you.

而咖啡因則制約了抑制神經的腺苷酸，所以可以提神。

Caffeine can also boost positive feelings.

咖啡因也能提升正面感受。

In some neurons, the adenosine receptors

某些神經元中，腺苷酸受體

are linked to receptors for another molecule called dopamine.

和多巴胺受體是相連的。

One of dopamine's roles in the brain is to promote feelings of pleasure.

多巴胺的功能是增進愉悅感。

When adenosine docks in one of these paired receptors,

當腺苷酸進入這類雙重受體，

that can make it harder for dopamine to fit in its own spot,

多巴胺就比較難進入它的位置，

interrupting its mood-lifting work.

也就無法使心情變好。

But when caffeine takes adenosine's place, it doesn't have the same effect,

但咖啡因取代腺苷酸進入受體不會產生排擠效應，

and dopamine can slide in.

多巴胺可以順利進入受體。

There's evidence that caffeine's effects on adenosine and dopamine receptors

證據指出咖啡因對於腺苷酸和多巴胺受體的正面影響

can have long-term benefits, too, reducing the risk of diseases

是長期的，因此可以減少罹患

like Parkinson's, Alzheimer's, and some types of cancer.

帕金森氏症、阿茲海默症以及特定癌症的機率。

Caffeine can also ramp up the body's ability to burn fat.

咖啡因也能提升身體燃脂能力。

In fact, some sports organizations think

事實上，有些運動協會認為

that caffeine gives athletes an unfair advantage

運動員攝取咖啡因之後，將擁有對比賽不公平的身體優勢，

and have placed limits on its consumption.

因此將它列為管制藥物。

From 1972 until 2004,

從 1972 到 2004 年，

Olympic athletes had to stay below a certain blood-caffeine concentration

奧運選手血液中的咖啡因含量必須低於標準濃度

to compete.

才能比賽。

Of course, not all of caffeine's effects are so helpful.

當然咖啡因也不全然對身體有益。

It might make you feel better and more alert,

它使你感覺好、精神好，

but it can also raise your heart rate and blood pressure,

但也可能增高你的心率和血壓，

cause increased urination or diarrhea,

造成頻尿或腹瀉，

and contribute to insomnia and anxiety.

也和失眠和焦慮有關。

Plus, the foods and beverages caffeine is found in

此外，包含咖啡因的食物和飲料本身

have their own impacts on your body that have to be taken into account.

也會影響身體，不容忽視。

Your brain can adapt to regular consumption of caffeine.

人腦會適應經常攝取的咖啡因。

If your adenosine receptors are perpetually clogged,

當腺苷酸進入受體不斷被阻擋，

your body will manufacture extra ones.

人體會製造更多受體，

That way, even with caffeine around,

如此一來，即使受咖啡因阻擋

adenosine can still do its job of signaling the brain to power down.

腺苷酸也能提醒大腦關機。

That's why you may find you need to consume more and more caffeine

這就是為何你會需要越來越多咖啡因

to feel as alert.

來保持清醒。

There are more and more adenosine receptors to block.

因為需要阻擋的腺苷酸受體會越來越多。

It's also why if you suddenly quit caffeine,

這也是為什麼某天你突然戒掉咖啡因

you may experience an unpleasant withdrawal.

會感到不舒服、不愉快的原因。

With plenty of receptors and no competition,

當受體增加，卻沒有咖啡因來競爭受體的位置，

adenosine can work overtime,

腺苷酸會過度工作，

causing symptoms like headaches,

導致頭痛、

tiredness,

疲倦、

and depressed moods.

和憂鬱的心情。

But in a few days, the extra adenosine receptors will disappear,

但幾天之後，多餘的腺苷酸受體會消失。

your body will readjust,

身體重整之後，

and you'll feel just as alert as ever,

即便沒有咖啡因的幫助，

even without an infusion of the world's most popular stimulant.

你也能恢復精神。