- It helps us stay focused and alert, happy, and energetic.

And it's the world's most widely consumed, psychoactive substance.

I'm Sheril Kirshenbaum.

And on this episode of Serving Up Science, we're going to explore the chemistry of caffeine.

85% of Americans consume at least one caffeinated beverage daily.

And worldwide, an estimated 2.25 billion cups of coffee are enjoyed per day.

That's more than 26,000 cups every second.

Clearly caffeine has global appeal, but why?

It all starts with evolution, specifically evolution in plants.

Joining me in the kitchen today is my favorite evolutionary plant biologist, self-identified coffee addict, and husband, Dr. David Lowry.

(relaxing music) (camera shutter) Welcome back to Serving Up Science.

- Thanks for having me back to my kitchen.

- You like coffee, right?

- I love coffee.

- [Producer] Could we do that one more time?

But maybe say, you know that he loves coffee because you're married to him.

- But what?

- Keeping a secret from you.

- [Crew] You know my secret!

- You like coffee, right?

- I do like coffee, but you should know that, you live with me.

- It's true, but it works for the episode.

So, why don't you brew us a cup and we'll talk a little bit about the evolution behind caffeine.

All right, so why did plants evolve to have caffeine?

- Plants evolved to have caffeine in order to defend themselves from voracious insects.

That's probably the primary reason.

(evil laughing) - So you're saying herbivory?

- Herbivory, yes.

Let's get technical.

- Is that it or anything else?

- Well, we actually think that some insects like caffeine and especially pollinators.

- [Producer] Could you maybe ask him about that and say like, you know, "so what's the buzz" or like, you know, - Oh gosh.

"Caffeine buzz."

- My producer wants me to ask you what the buzz is around pollinators.

- We actually think now they produce a little bit of caffeine in their flowers, so the pollinators will come by repeatedly to get a little bit of a dose of caffeine.

- Oh, the pollinators themselves feel that buzz and they want more.

- Exactly.

All right.

Now we're doing a full pour over here.

There's a third reason that plants might have evolved caffeine.

We also think that plants use it in order to defend themselves from other plants, by producing caffeine in their leaves that drop to the ground and then poison other plants.

- So caffeine on the soil might be keeping all of those other plants out of the vicinity.

- That's right, through convergent evolution.

- And what exactly is convergent evolution?

- So convergent evolution is when a trait evolves multiple times across different organisms at different parts of the evolutionary tree.

- Like wings in birds and bats?

- Exactly.

- Coffee likes growing near the equator, but the equator is an area of the world that's changing really quickly.

Do coffee lovers like you, or tea lovers like me, have anything to fear with climate change?

- Unfortunately, yes.

Arabica coffee, which makes up about 60% of the world's coffee production, is becoming harder to grow as temperatures rise and droughts become more frequent.

- Hmm.

Is there a reason for hope?

Are we looking at other kinds of coffee?

- Sorry, I don't know the answer to this question.

- Just be like, "scientists are looking at other species" - This is like totally - written by you.

Like you're like, "oh, just go to tea."

Like tea is okay.

Tea is no substitute for coffee.

- Is there any hope?

- Well, some scientists are trying to find alternative sources to produce coffee, like related species.

And of course you could always drink tea.

- Certain kinds of tea, less susceptible to climate change.

- Well, I think we're done.

- All right.

Well, thanks so much for joining us.

And we're done with you.

- [Producer] How about we do something a little bit nicer than "we're done with you."

- Thanks so much for joining us today.

- Okay.

See you at dinner.

(slurping coffee) - Now let's turn to caffeine and our bodies.

What exactly happens when we ingest it that makes us feel good?

Caffeine has a very similar molecular structure to another molecule called adenosine.

You may remember adenosine from high school chemistry as the "A" in ATP, or adenosine triphosphate.

Because caffeine has such a similar structure to adenosine, it gets in the way of adenosine receptors which would otherwise be involved in slowing down nerve activity in our brains and making us feel sleepy.

When caffeine docks into an adenosine receptor, it also leaves room for dopamine to lock in and take effect nearby, which is a neurotransmitter that makes us feel happy.

And that's why coffee can be a real science-based mood booster.

Still, over time we need more caffeine to have the same effect because our brains adapt to how much we consume.

If our adenosine receptors are always clogged with caffeine, our bodies create more, maintaining our ability to calm down.

These additional adenosine receptors aren't necessarily permanent, though.

If we stop ingesting caffeine, we leave lots of receptors available with little or no caffeine competition.

That's when we might experience what some people popularly refer to as caffeine withdrawal.

Thankfully, within a few days the extra receptors disappear.

After that shaky readjustment, we can feel as alert as ever, no caffeine required.

But what does caffeine actually do for us, beyond the rush of alertness and mood boost?

More than you might realize.

Once in the body, it breaks into three different molecules: theobromine, which increases oxygen and nutrient flow to the brain, Theophylline, which boosts heart rate and our ability to concentrate, and Paraxanthine, which can enhance athletic performance, increasing the rate of fat breakdown to fuel muscle activity.

There's also evidence that caffeine's effects on adenosine and dopamine receptors may have long term benefits like reducing the risk of Parkinson's, Alzheimer's, and some types of cancer.

With perks like these, maybe we should be caffeine loading.

Not so fast.

Caffeine only benefits us to a point, which scientists say is about 400 milligrams for adults.

But of course, that varies by individual based on weight, height, and speed at which we metabolize the molecule.

For reference, 400 milligrams is about three 8 ounce cups of coffee or eight cups of black tea.

But high doses definitely don't give our health a boost.

Lots of caffeine can raise heart rate and blood pressure, and cause us to feel jittery and anxious.

Too much also leads to increased urination, diarrhea, and insomnia.

And eventually caffeine becomes toxic at around 10 grams.

Still, to get to that level, you'd need to drink 75 cups of coffee or approximately 200 cups of black tea.

And once again, any so-called "lethal limit" depends on each person's unique body chemistry and metabolism.

So don't worry too much about overdosing on caffeine, but maybe just avoid that cup right before bedtime.

- [Crew] Can you sip louder?

(slurping coffee) - Rob!

No one's gonna watch Serving Up Science after that.

They're gonna be like "this woman is just slurping her coffee."

(kettle screaming) (relaxing music)