Sydnie Liev admits she's an addict.

A hit in the morning and another in the evening help Liev, a 19-year-old Washington University student and cross country runner, get through the day.

Even if she could — and Liev says she can't — she's not looking to give up this dependency, which began when she started college a year ago.

"It's a very controlled addiction," she says, a cup of her vice in hand.

Like many of us, Liev has fallen under the often sweet and always powerful spell of caffeine.

Health agencies estimate that 80 to 90 percent of Americans get a daily fix, usually from coffee, tea or soda. Without it, we'd be a nation of irritable zombies reaching for the aspirin bottle — at least for a couple of days, until we shook the stuff out of our systems.

But why would we do that, especially with some recent studies elevating caffeine to the status of a near miracle drug? Just last month Japanese scientists reported that drinking three or more cups of coffee a day may cut the risk of colon cancer in women by half.

Researchers in France chimed in with the finding that caffeine seemed to help preserve the cognitive skills of older women. That study came out just months after another on men showed similar results.

Earlier studies had already given caffeine lovers a rallying cry by showing that the stimulant — found in the beans and leaves of many plants, such as coffee shrubs and cacao trees — can keep them alert, help them concentrate and even improve athletic endurance.

THE DOWNSIDE

Sounds as if we might be doing our bodies a favor by downing Big Gulps of Pepsi with its 3 milligrams of caffeine per ounce, doesn't it?

Not so, say dietitians and other health experts. Don't even get them started on the obesity risk that comes from all the sugar, whipped cream and other extras that often spice up our caffeinated treats. Too much caffeine itself can cause heartburn, nausea, anxiety, muscle tremors and, in extreme cases, seizures, they point out.

In fact, in what appears to be a trend, caffeine overdoses are sending more and more adolescents and young adults to hospitals.

At the Illinois Poison Center, Dr. Michael Wahl has seen caffeine overdose reports for ages 13-30 rise steadily from 131 in 2002 to 186 last year.

Those numbers are going up just as the amount of caffeine in many consumer products popular among young Americans — especially diet supplements and so-called energy drinks — is climbing.

That parallel isn't lost on Wahl. The jump in caffeine overdose reports "has to do with availability," he said.

Wahl and others are quick to note that most caffeine consumers never get close to dangerous doses. The red flags go up when about a gram of caffeine gets into the bloodstream. Downing eight cups of brewed coffee could put about that much into your system.

But someone looking for a quick pick-me-up, say a college student preparing for an all-nighter, could get into the red zone with just a few 200 milligram tablets of NoDoz chased down by an energy drink such as Monster, with its 160 milligrams of caffeine per 16-ounce serving.

Even devotees of caffeine in its other forms say the wallops in energy drinks scare them off.

Washington University student Emily Guhl, 19, has a two-cup-a-day coffee habit that goes back to her high school years. She's sampled high-caffeine energy drinks but was put off by the weird feeling they left her with.

"I think those are a little too much," she said.

Calum Angus, a soccer player on the St. Louis University team, said he gave the energy drink Red Bull a whirl to see whether it improved his game. The energy drink packs 80 milligrams of caffeine in each 8.3-ounce can. That's less than the amount in a cup of coffee but more than double the 35 milligrams found in a 12-ounce can of Coke Classic.

The boost didn't amount to much for Angus.

"I never saw signs of enough change that I wanted to carry on drinking it," he said.

Jaime Rothermich, director of nutrition at the custom fitness center HammerBodies in Creve Coeur, Mo., would probably applaud Angus' decision. Rothermich says he is well aware of studies that show caffeine can give athletes a performance boost. Yet he never advises any of his clients to add caffeine to their diets.

He says caffeine comes with a downside — including diuretic effects, interference with sleep patterns and the potential to prompt an irregular heart rate — that makes strong doses of it unreliable and potentially dangerous for athletes.

Jonathan Burch, assistant athletics director and director of sports medicine at SLU, worries about the stress that the stimulant puts on the heart and lungs.

That stress can potentially overload a body already straining for maximum athletic output and even more so if the body is already being taxed by the heat and humidity of a Midwestern summer.

In their advice to athletes about how to fuel their bodies, both Rothermich and Burch say to stick to the basics.

"We kind of give old advice — a multivitamin, plenty of water and maybe a sports drink of some kind, like Gatorade," Burch said.

And, maybe, the sports trainers say, a cup or two of coffee and an occasional soda if it's already part of your routine. But don't look to those supercaffeinated sports drinks with their mysterious herbal supplements to send your game or your fitness regimen over the top.

"Making your body work on too many things will actually just slow it down," Rothermich said.

MODERATION IS URGED

In general, if there are any benefits for the body from caffeine, the consensus among health professionals is that they come from using it in moderation.

Dr. Alan Leviton, a professor of neurology at Harvard, has seen the studies that seem to extol the wonders of caffeine, such as links to reduced rates of diseases such as Parkinson's and diabetes. Nevertheless, he said, "I wouldn't encourage people to drink caffeine or coffee for that."

Your caffeinated beverage of choice is less a remedy and more a comfort drink, said Leviton, who is not only a coffee and tea consumer but a consultant to the scientific advisory group of the National Coffee Association.

And despite warnings in some circles about the proliferation of Starbucks turning us into overstimulated addicts or raising our risks for heart disease or other maladies, the doctor isn't worried.

He said he has seen "no convincing evidence that any disorder can be attributed to moderate use of coffee."

He points out that while we might crave our caffeine, our bodies also seem to have an amazing natural ability to know their limit and to stop sipping or slurping when the blood content reaches a familiar and comforting level.

That might explain David Stewart's occasional coffee habit. Stewart, a student at the St. Louis University School of Medicine, said he only tips a cup when he can't quite wake up.

"If I don't need the pickup for work," he says, "I don't find myself craving it."

UPSIDES OF CAFFEINE

• Improved alertness

• Caffeine crash or fatigue once the effects wear off