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It's MORNING EDITION from NPR News. I'm Steve Inskeep.

RENE MONTAGNE, host:

And I'm Rene Montagne.

In your health today, how to be a savvy caffeine consumer, and also a look at why, despite lattes and espressos, adults don't have as much energy as kids.

First, caffeine. Experts say a small amount of the stimulant can give many people the lift they want without producing the anxious jitters. NPR's Allison Aubrey looks at how experts calculate how much is too much.

ALLISON AUBREY: People who love coffee say it's more than just a drink. The morning cup is part ritual, part pick-me-up. At Washington coffee shop, Rich Warwinsky enjoys his.

Mr. RICH WARWINSKY (Coffee Drinker): It's a habit. It's a little bit of a jones. It sort of gives me a lift. And if I'm manage it well, I drink two or three times a day, half and half, and I'm not too crazed.

AUBREY: Not too crazed when he manages it well. If that description sounds like he could be talking about a drug - but caffeine, a drug? Absolutely, says researcher Laura Juliano, who's taking a break at a coffee shop near her lab.

Ms. LAURA JULIANO (Researcher, American University): Caffeine is a drug that can affect our central nervous system and have effects on some pretty important areas of our lives.

AUBREY: Juliano runs a caffeine research lab at American University. She says after years of an on-again/off-again relationship with caffeine, she's still trying to figure out exactly how much she can personally tolerate.

Ms. JULIANO: Can I order a decaf?

Unidentified Woman: Sure.

Ms. JULIANO: I'll have a small decaf, please.

AUBREY: Juliano says she's already had a small amount of regular coffee at home, so she's laying off.

Ms. JULIANO: Even having it now may have an effect on me being able to fall asleep tonight.

AUBREY: Even though it's only 10 o'clock in the morning, though?

Ms. JULIANO: Yeah, sure. There's actually research showing that people who have caffeine in the morning can have a longer latency to fall asleep that evening.

AUBREY: Sensitivity to caffeine varies widely from person to person. Part of the equation is genetic makeup, part is body weight, and a meal can blunt or slow down the stimulant's effect. Smokers can often tolerate more caffeine because nicotine is thought to stimulate the enzymes that break caffeine down. Juliano says another factor is age.

Ms. JULIANO: We require liver enzymes to break down caffeine. And as one ages, there's just changes in our metabolism.

AUBREY: All these factors help explain individual differences, and how they can change over time. But Juliano says as a general rule of thumb, most caffeine users who enjoy the pleasant effects of the stimulant need surprisingly little. Studies show that 100 milligrams, which is just six ounces of a typical automatic drip coffee, produces a lift.

Ms. JULIANO: People report increased well-being, better mood. They become more sociable and talkative.

AUBREY: But it's not all good.

Ms. JULIANO: As that dose increases - let's say above 200 milligrams, certainly by about 400 milligrams - many people report anxious, jittery feelings of uneasiness.

AUBREY: Staying under 400 milligrams can be tricky, mainly because it's hard to know how much caffeine is in any given cup of coffee or tea. One published study of Starbucks coffee found the caffeine content in one of its drinks can vary from 250 milligrams to as high as 560 milligrams. Teas generally have about one third as much caffeine as coffees, but they range too, depending on how long the bag is steeped. A Diet Pepsi has just 35 milligrams.

When people ask Juliano about choosing a coffee, she sometimes turns them on to a espresso drinks.

Ms. JULIANO: Let's say you order a cappuccino and it's made with one shot of espresso. You're actually getting far less caffeine in that drink than you would be if you just ordered, let's say, a drip coffee.

AUBREY: Juliano says this always seems to surprise caffeine drinkers.

Ms. JULIANO: People often think, oh, espresso - you know, I had some espresso and that really gave me jolt. But in actuality that could be a better amount of caffeine, actually, to consume, because you're keeping the level under 100 milligrams a day. And that's what we recommended in terms of trying to use caffeine to get a little boost but not using so much that you are dependent on it and you need it every day.

AUBREY: The challenge of our caffeinated society, where eight in ten adults report using caffeine, is that many people fall into dependency without realizing it. What starts as one cappuccino a day escalates. When this happens, Juliano says, it's not terribly alarming. Caffeine is considered safe relative to classic drugs of dependence. But the downside becomes apparent when you skip a day and go into withdraw.

Ms. JULIANO: The number one symptom you hear people say is, oh, I get this pounding headache.

AUBREY: So for the caffeine dependent it's an every day habit. On weekend days, when people don't want the morning pick-me-up, Juliano says cutting back to just a half a cup may work.

Ms. JULIANO: You don't have to have the same amount of caffeine every day. But if you had at least a little bit, it may prevent withdrawal.

AUBREY: If you go cold turkey, research shows the headaches will last anywhere from a few days to several weeks. Juliano says the other option is to just break the caffeine habit. This is what she plans to do, one day soon.

As for longtime coffee drinker Rich Warwinsky, who's finishing his morning joe...

Mr. WARWINSKY: It's probably better not to have it.

AUBREY: But for now you're sticking with it?

Mr. WARWINSKY: Absolutely.

AUBREY: Allison Aubrey, NPR News, Washington.

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