Lisa Shuster is craving her first Diet Coke of the day.

She is slender, 5-foot-6 with blond good looks, and the only hint that something is wrong is the way she gnaws at her thumbnail, fighting off that jittery feeling.

It's 9:30 in the morning. She's been up for three hours and is drinking her third cup of coffee. But she hasn't had a Diet Coke. Three hours and no Diet Coke. Most days, she has already driven to McDonald's by now and indulged. Super Size. 42 ounces. A cup the size of a child's bucket.

This morning, her 16-year-old daughter slept in late with a headache and Lisa has been stuck at home.

Finally, at 10:30, Kathleen is ready for school.

Finally, Lisa can get her fix.

As she cruises toward Myers Park High School in her blue Lincoln Navigator, she's thinking about how good it will taste. Cool. Fizzy. With that sweet tang of aspartame mixed with saccharin found only in a fountain drink.

Without intending to, she turns left off Colony Road onto Fairview Road, away from her destination.

"I'm headed to McDonald's!" Lisa blurts out, laughing at her mistake.

"Tough luck," she calls over her shoulder to Kathleen. "You're second. My car is trained to go to McDonald's."

For Lisa, who's 42, Diet Coke is more than a drink. It's a way of life.

"I don't drink alcohol," she says. "I don't smoke. I don't cuss. This is my addiction."

In the United States last year, we drank nearly 3 billion gallons of Coke. "It's the real thing." Two billion gallons of Pepsi. "You got the right one baby." One billion gallons of Diet Coke. "Do what feels good." Dr. Pepper, Mountain Dew and Sprite, 7 UP, Mello Yello and Sun-drop, Pepsi Vanilla, Pepsi Twist and Pepsi Blue.

We drink more soda than water.

It costs us $63 billion a year and, some researchers say, our health.

Super Size it!

Lisa pulls into the drive-through lane at the McDonald's on the corner of Sharon and Fairview roads."Oh my God. I hope I have money." She fumbles through her purse and fishes out two dollar bills.

Relief.

"Super Size Diet Coke," she calls into the intercom.

From the back seat, Kathleen interrupts: "You never get Super Size. You get large."

"I've moved on. Large doesn't last long enough."

Kathleen, who prefers milk and juice, stares at her mother with an I-don't-believe-you're-saying-this look. Lisa tells her, "I just never realized they have a bigger one."

She hands over the $2 through the window.

"With the money she spends on Diet Coke," Kathleen says, "she could buy a car."

Lisa reaches for her drink and 21 cents in change.

She sips.

Zing.

She sighs.

"Ah. That's so good."

She drives around the building to the stop sign and sips again before pulling out into traffic.

"You have no idea."

She grins.

"My day can begin."

Down, down, down

You take that first sip, and your tongue tingles. The carbon dioxide added to sodas to make them fizzy is reacting with acid in your mouth. A ripple of bubbles tickles your gums, the hollows of your cheeks, the back of your throat.

You swallow. The soda streams into your esophagus, a hollow tube an inch wide and 12 inches long. Down, down, down it flows, tickling your muscles along the way, until it reaches your stomach. There, the CO2 mixes with more acid. More bubbles. When you burp after drinking a carbonated drink, that's why.

Most of the water in a soda goes into the small intestine, where it is absorbed into your blood. The caffeine and phosphorus also enter the bloodstream from the small intestine. They go into your liver and from there into tissues, including your brain and your heart.

If you haven't eaten, you'll feel a sensation from the caffeine within about five minutes: Your heart pumps faster. Your brain activity speeds up. You feel peppier. That's what caffeine does. On a full stomach, it will hit you later.

The feeling lasts about an hour, the time it takes Lisa to finish one Super Size Diet Coke and reach for another.

`You don't want to get fat'

Lisa drank her first diet soda in high school.She grew up in Greenville, S.C., and says she had buck teeth, cat-eye glasses and stringy hair. A dork. She went to private school, Christ Episcopal, with many of the same kids from kindergarten through 10th grade. Even when she transformed her looks with braces and contact lenses, their image of her stuck. Still a dork.

She pleaded with her parents to let her transfer to public school. They refused.

Then one day her mother threatened Lisa and her sister and two brothers: If you make any Cs, you're going to public school. On her next report card, Lisa says, she brought home all Cs -- "I think I threw a `D' in there, too."

She transferred in 11th grade to J.L. Mann High School and surrounded herself with new friends.

"My life began."

She was named class flirt in 12th grade.

At parties, she was the skinny blond with the contagious laugh, sipping from a hot pink can of soda, the new diet drink: TaB. Two calories in every can. Sweetened with saccharin. Advertised in a shapely glass resembling a woman with a tiny waist. It was Coke's answer to women who didn't want the calories of Coca-Cola. Now they could keep tabs on their weight.

Lisa's best friends, Juli and Terri, drank TaB, too.

All the cool kids did.

"You're a teenager, and you're skinny, but you always think you're fat," Lisa says. "But you don't want to get fat so you always diet, even though your diet consists of a TaB, four Snickers bars and some hamburgers."

She resisted Diet Coke when it was introduced in 1982. She was a TaB girl. Actress Jayne Kennedy drank TaB. So did Elle McPherson, the model. And figure skater Dorothy Hamill.

But when scientists said saccharin, the sweetener in TaB, caused cancer in laboratory rats, Lisa looked for another diet drink. At Wendy's, where she worked the pick-up window after school and on weekends, it was Diet Coke or nothing. Nothing wasn't an option.

Soon, she craved the new taste.

Enhancing the flavor

It's no wonder, says a Johns Hopkins University study.

Caffeine is added to soft drinks, the study says, to addict consumers the way nicotine is added to cigarettes. Adults and children become physiologically and psychologically dependent on the caffeine. If they quit drinking, they get headaches, become lethargic or suffer other withdrawal symptoms.

The soft-drink industry says it adds caffeine to enhance flavor -- not to addict consumers. But the study, funded by the National Institute on Drug Abuse, found that only two of 25 adults tasted the difference between colas with caffeine and those without.

"If somebody doesn't want caffeine, they don't have to drink it," says Lauren Steele of Coca-Cola Bottling Company Consolidated of Charlotte. "We sell what anybody wants."

Coca-Cola. Caffeine-free Coca-Cola Classic. Diet Coke. Caffeine-free Diet Coke. Cherry Coke. Diet Cherry Coke. Diet Coke with Lemon. Vanilla Coke. Diet Vanilla Coke ...

The first Coca-Cola was concocted in 1886 by a pharmacist in Atlanta looking for a cure for headaches. He sold a glass for a nickel. Those first Coca-Colas contained a chemical component of cocaine, Steele says, but not at narcotic levels. Extracts from the coca leaf are still used, but without the cocaine alkaloids.

The recipe is secret, protected by the U.S. Supreme Court.

Where to find the best Coke

When Lisa and her first husband, Robert Rockholt, were saving for a Jeep Cherokee in the late 1980s, they factored Diet Coke into the budget. She allowed herself one fountain drink a day. (They're more expensive than cans or bottles.)"I felt so guilty drinking that one fountain drink ... . I'd tell myself, `You really don't have to have this.' And then I'd think, `Yes, I do!' "

In Lisa's world, there's a hierarchy to Diet Coke.

Fountain drinks over ice taste best.

Then cans. At home at night, she drinks several.

As a last resort, or when they're on sale, plastic bottles.

There's a hierarchy, too, of places to find the perfect Diet Coke.

McDonald's, Lisa says, serves the best.

If she can't find a McDonald's and is desperate, she'll settle for Chick-fil-A or Jack in The Box.

Wendy's? "Disgusting."

He knew her so well

Lisa worked 15 years as a nurse in Augusta, Ga., and always carried a stethoscope, bandages, hemostats -- and a Diet Coke. After a change in policy banned carrying food or drinks into patients' rooms, she would leave her Diet Coke at the nurses' station, and steal sips on her way in and out.

On trips to see her parents in Greenville, she would get a Diet Coke for the road and her husband would pull over in Greenwood, halfway, so she could get another, then again in Greenville for a third.

Robert knew her so well. They had fallen in love when she was at Greenville Tech, studying to be a nurse, and he was at Clemson University. They married in 1984. After Robert took a job as a computer systems programmer with Delta Air Lines, she quit nursing and moved to Atlanta.

Life was good. She and Robert were happy. Their three children were getting bigger.

Late one evening in February 1999, Kathleen, Kelly and Preston were in bed and Lisa was reading on the sofa when Robert walked over to the CD player. He spun around and faced her as if he wanted to say something, his knees buckled and he fell down.

Lisa knew from the look on his face that he had suffered a heart attack or an aneurysm. As she dialed 911, she performed CPR. He was dead. A heart attack at 37.

One by one the next morning, the children woke up and came downstairs, first Kathleen, who was 11, then Kelly, 9, then Preston, 5. Lisa had to tell each one that Daddy was dead.

She barely remembers anything from the next few months, she was so numb with grief. The day she forced herself to drive to the supermarket a mile away, she got lost and went back home empty-handed.

A neighbor recommended Charlotte stockbroker Roger Shuster to help invest her husband's life insurance. She talked with him on the telephone, but doesn't remember a word they said. They e-mailed back and forth. After a few months, their e-mails turned flirtatious and eventually she drove to Charlotte to meet Roger.

They fell in love.

In August 2000, they flew to Disney World and married. With them were his two daughters and her three children.

And Diet Coke for the bride.

`She's a terrific mother'

Roger Shuster isn't picky about what he drinks, except for one thing.No diet soda.

He hates the taste. He drinks water, sodas and fruit juice -- in that order. He likes Coke and Sunkist Orange, but he doesn't drink as many soft drinks as Lisa. Nobody they know drinks as many soft drinks as Lisa.

In August, on their third wedding anniversary, Roger rented a room at The Park Hotel and greeted her with a 20-ounce bottle of Diet Coke.

"She's a very well-disciplined, extremely well-balanced human being except for that area," says Roger, 45. "She's a terrific mother. I've learned a lot from her."

It seems like Lisa is always working on something: re-tiling the tub with Roger, helping him build a stone fountain in the back yard, sewing curtains, smocking dresses, cross-stitching, studying Henry VIII online and forensic nursing at Queens University of Charlotte.

She's tried to make her children as self-sufficient as she is. Kathleen, Kelly and Emily run a baby-sitting service.

She doesn't care whether they make a lot of money in life. She cares whether they're happy. Because of her, they love to read. She read to them every night: "Old Hat New Hat," "Go, Dog. Go!" "The Cat in the Hat," often the same books again and again and if she tried to skip a page, they caught her.

She taught them to brush their teeth, take vitamins and limit the amount of fat they eat. She has even tried to limit soda, but it's hard to say "no" when she's sipping a Diet Coke.

"It's the sugar," she says. "I don't like them having all that sugar. Emily will drink diet. The others won't."

Liquid candy

The average American drinks 1 1/2 cans of carbonated soft drinks every day -- 18 ounces, more than 2 cups. Students at most middle and high schools can buy them at lunch. Some parents even put soft drinks in their babies' bottles.

The soft drink industry pours hundreds of millions of dollars into advertising, much of it targeted toward teenagers. They are at an age when they are asserting their independence, developing their own tastes. They often take those habits with them through life, the way Lisa did with Diet Coke.

Soft drinks are especially bad for children, nutritionists say, because they take the place of healthy food.

A 12-ounce glass of orange juice or milk contains vitamin A, vitamin C, folate, calcium, potassium and magnesium, which growing bodies need for strong bones, teeth and hearts and cells and eyes and skin.

A 12-ounce can of Coke contains no vitamins.

A 16-ounce bottle of Dr. Pepper contains no vitamins.

A 32-ounce fountain Sun-drop contains no vitamins.

"Parents and health officials need to recognize soft drinks for what they are -- liquid candy," says the Center for Science in the Public Interest, "and do everything possible to return those beverages to their former, reasonable role as an occasional treat."

Four sodas, six sodas, 12 even

One evening in spring, Lisa sat on the sidelines at a Little League baseball game at Randolph Park. Preston, 9, played on the Green Berets team and Roger coached. Kathleen, Kelly, 14, and Emily, 13, helped.It was a warm, sticky night. Lisa sipped a can of Diet Coke. Finished, she sent Kelly to get another. Finished with that one, she sent Kelly off for a third.

If someone asked Kelly to draw her mom and include what's important to her, a Diet Coke would be in the picture.

"Diet Coke," Kelly says, "is pretty much part of her family."

Barbara Green, whose son played on the Green Berets, was sitting next to Lisa.

How many of those do you drink a day? Barbara asked. Lisa laughed and rolled her eyes. You have no idea, she said. I'm addicted.

Barbara Green is a physical therapist and asks patients the same question.

"I just see so many people who are consuming soft drinks who have physical issues, whether they be migraine headaches, osteoporosis, even muscular-skeletal problems," she says. "When I asked them what they drank every day, they'd tell me four sodas, six sodas, 10 sodas, 12 even."

She warned Lisa that she could be at risk for osteoporosis, a disease in which bones become fragile and more likely to break. Studies have linked drinking sodas with calcium loss, and you need calcium for strong bones. Other studies link sodas with obesity, cavities and caffeine dependence. Some even link diet drinks with weight gain.

Soft drinks are bad for you, Barbara told Lisa.

I know, I know, Lisa said. I should quit.

Dueling studies

Teenage girls who drank soda were three times more likely to suffer bone fractures than girls who didn't, says a study by the Harvard School of Public Health. The risk was higher for active girls who drank sodas.

Grace Wyshak, who wrote the study, believes soda weakens bones either indirectly because people substitute soda for calcium-rich milk or directly because phosphorus (an acid that gives soda its tartness) may leech calcium from bones.

It is cause, she says, for "national concern and alarm."

It is hogwash, says the soft-drink industry.

"For every study out there by some activist group that says soft drinks have health issues," said Steele, the Coca-Cola spokesman, "I can give you another study that says the exact opposite thing by credible scientific researchers. It depends on what you're looking for in a study."

Soft drinks, the industry points out, contribute 2 percent of the average phosphorus intake in the U.S. diet. Most phosphorus comes from meat and cheese.

The only concession the industry makes is that soda contributes to tooth decay -- but soft drink companies say sodas aren't the only cause.

Dieticians say we should drink water.

Water doesn't have vitamins. But it doesn't have calories or caffeine or additives either. And from the tap, it's inexpensive.

More than a gallon

Lisa drinks water when she works in the yard. Otherwise, it's coffee in the morning, then Diet Coke until bedtime.The day she turned toward McDonald's by mistake on her way to Myers Park High, it took her about an hour to finish the first Super Size Diet Coke. 42 ounces. Then she was on her way to SouthPark mall so she stopped by Chick-fil-A for a large Diet Coke. 21 ounces.

After the mall, she went to the library (no Diet Coke allowed), then rushed home to meet Preston's school bus. She didn't have a chance to get another drink until mid-afternoon on the way back from the car dealership. She bought a McDonald's Super Size and finished it on the drive home.

From 5 to 8 p.m., she drank 2 1/2 glasses of bottled Diet Coke over ice while she painted the deck. At 9 p.m., the family went for pizza at Mellow Mushroom and she drank two more.

By bedtime at 10:30, she had drunk 159 ounces of Diet Coke. That's about thirteen 12-ounce cans. More than a gallon of Diet Coke.

Giving up Diet Coke

"I can't give it up," Lisa says. "I know all the risk factors. I'm like the smoker who thinks he'll never get emphysema."

Like a smoker, she is thinking about the next one even while she finishes the first.

"I probably should get tested for osteoporosis. It's a family history for me."

She pauses and sips her Diet Coke.

"But if they find something, they're going to tell me to stop drinking."

She sips her Diet Coke.

"I don't want to stop drinking."