Finally, after two decades of trying and failing to lose weight with (you name it) Weight Watchers, NutriSystem, a nutritionist, a personal trainer—not to mention the therapist who derided her for being fat—it has come down to this: Linda Hay is sitting in an examining room at the Virginia Commonwealth University Medical Center in Richmond with Harvey Sugerman, the surgeon who will perform a gastric bypass operation on her in two weeks.

Gastric bypass is major surgery that shrinks the stomach's capacity from wine bottle to shotglass size and reconfigures the small intestine. Most patients lose about two-thirds of their excess weight within a year of surgery. "Gastric bypass surgery is a tool," Sugerman says. "It reduces the stomach. The patient can't eat as much. In most instances, if a bypass patient eats sugar or fatty foods, it provokes a dumping syndrome that causes flushing, nausea, sweating." You could say it's almost like Antabuse for the obese. Even so, the operation fails in 15 percent of cases. Some patients can subvert the surgery. They overeat by snacking continuously.

And the surgery is risky. The list of possible complications includes blood clots in the lung, pneumonia, infection, leakage from the reshaped intestinal tract, and—in one out of a hundred cases—death.

Hay, 39, is five feet five (1.7 meters) and weighs 314 pounds (142 kilograms); she is morbidly obese, which makes her a candidate for the surgery. Her managerial level job in the human resources department of a financial company demands tact, efficiency, and organization—qualities she exudes. She has a close circle of friends who would do anything for her, a clear sense of who she is, and few illusions of who she is not. She dresses stylishly, has long blond hair swept back by a headband, a classic oval face, and fair complexion. But she is—let's face it—huge.

When I ask about her decision to have surgery, she describes the humiliation of asking for a seat-belt extender on a plane; her reluctance to go to movies because the seats are too narrow; the time she signed up for a dating service, put down as body type "a few extra pounds," got a few responses, and then, opting for honesty, changed it to "large." This time she got none. She lists health problems associated with her weight: high blood pressure, varicose veins, pain and swelling in her feet and ankles, depression. "You take control for a while," she says, "then you fail yet again, and you're more depressed than ever."

Linda Hay has considered the risks and decided to have surgery. Nonetheless she is anxious. "No one at the office knows I'm going to do this," she confesses. "Someone said, 'Have a good week,' and my mind kept racing to the worst-case scenario…What if?"

It seems, I say, turning to Sugerman, that this is surgery for the desperate.

He nods. "Surgery is a drastic solution," he says, "but then obesity is a drastic problem."

It's become a far too familiar headline: Today one out of three Americans is obese, twice as many as three decades ago, and enough for the Centers for Disease Control and Prevention to declare obesity an epidemic. More disturbing are statistics relating to children: 15 percent of children and teens are overweight, a nearly three-fold jump from 1980. Obesity is defined by your body mass index, or BMI, a fancy calculation in which your weight is divided by your height. If it's 25, you're overweight. If it's 30, you're obese. Over 40, you're morbidly obese.

The broadening of America is everywhere you look, or sit. The Puget Sound ferries in Washington have increased the width of their seats from 18 to 20 inches (20 to 51 centimeters) to allow squeeze-in room for people with bigger bottoms. In Colorado an ambulance company has retrofitted its vehicles with a winch and a plus-size compartment to handle patients weighing up to half a ton (0.45 metric tons). Even the Final Resting Place has had to accommodate our growing girth. An Indiana manufacturer of caskets now offers a double-oversize model—38 inches (97 centimeters) wide, compared with a standard 24 inches (61 centimeters).

Being overweight is associated with 400,000 deaths a year and an increased risk of heart disease, type 2 diabetes, and colon, breast, and endometrial cancers. Most poignant is the psychological pain of those stigmatized by obesity. In one study at Michigan State University, undergraduates said they would be more inclined to marry an embezzler or cocaine user than an obese person.

How did Americans get so fat? Where did we go wrong? It depends on whom you ask. I asked Robert Atkins last year, a month before the purveyor of today's hottest diet died from a head injury suffered in a fall. He sat in guru-like serenity behind a black leather-top desk in his Manhattan office. His expression remained impassive, with an occasional lapse into wryness. He seemed to float, as if hovering above the storm of contention his diet provokes.

"We went wrong by allowing the American Medical Association and the United States Department of Agriculture to say: 'You've got to go on a low-fat diet.' They failed to take into account that when people do that, they increase their carbohydrates."

For breakfast that morning, Atkins (who said he was six feet [two meters] tall, 189 pounds [86 kilograms]) had eaten a sausage-and-cheese omelet, two ounces (0.06 liters) of tomato juice, and tea without sugar. His wonderland diet books say yes to bacon, eggs, and lobster dripping with butter and tell readers to lay off the bread and fruit. Slashing carbohydrates and sticking to protein and fat, Atkins claimed, prompts the body to burn fat through a metabolic process known as ketosis. Another purported advantage: Remaining in near ketosis makes it easier for people to control hunger.

In the post-Atkins era, pork rinds have become a snack sensation, egg consumption has risen, and "doing Atkins" is now synonymous with adhering to a high-protein, low-carb diet. Since 1972 his Diet Revolution and the updated version published in 1992 have sold 18 million copies. The latest edition has been translated into 25 languages. But not, as yet, in Italian. "They didn't want to give up their pasta," he said.

To be sure, Americans are filling up on carbohydrates like pasta, potatoes, and bread. In the early '70s we ate 136 pounds (62 kilograms) of flour and cereal products per capita, and now it's 200 pounds (91 kilograms). Most of those products are highly processed grains, like white bread, that are low in fiber and absorbed into the bloodstream more quickly than high-fiber whole grains. Such foods have a high glycemic index, which means they prompt a sharp spike in glucose and trigger a corresponding spike in insulin production from the pancreas. Atkins and other advocates of low-carbohydrate diets claim that surges in insulin cause blood sugar to plummet, which in turn creates cravings for more carbs—and on and on in a spiraling raise-you-one war between glucose and insulin. The trouble is, research doesn't back that up: Low blood sugar hasn't been directly linked to hunger. And unless you have diabetes, blood sugar remains generally stable anyway.

Not everyone has converted to the Atkins gospel. Dean Ornish, director of the Preventive Medicine Research Institute in Sausalito, California, is one of the original advocates of a low-fat diet as a way to lower heart disease risk. He contends that following Atkins's diet might help you lose weight in the short run, but at the cost of "mortgaging your health." He cites an increased risk of breast cancer, prostate cancer, and heart disease, not to mention headaches, constipation, and even bad breath as the price you pay for the Atkins diet.

"Atkins is right about us eating too many simple carbohydrates," he says. But Ornish argues the solution is to replace them with complex carbohydrates like whole grains and vegetables, not more fat. "Atkins gets into trouble when he says to eat bacon and go into ketosis. It's a toxic state. Look, I'd love to tell people it's OK to eat bacon and sausage, but it's not. You can lose weight in ways that aren't good for you. Smoking causes you to lose weight, as do amphetamines. But it's not just about losing weight, it's losing weight in a way that is helpful. There are no long-term studies to support this diet."

As a closing question for Atkins, I had asked him how he wanted to be remembered. "As a person who changed mainstream medicine's approach," he replied. "I hope I live long enough to see that." A month after his death, the New England Journal of Medicine reported that in the short run, people on the Atkins diet did lose more weight than those on a low-fat diet, and there was no real difference in cholesterol between the two groups. The catch: Those on Atkins started regaining weight after six months, and by year's end were on par with the comparison group. The jury is still out on the diet's long-term effects, but the National Institutes of Health is funding a five-year study that may render a verdict.

Even so, not even Atkins's death has silenced the critics. "I want to know why he didn't have himself autopsied, so we could see for ourselves what his coronary arteries looked like," one nutrition expert hissed when I broached the subject of Atkins's death. "That's what a real scientist would have done."

If even the experts can't agree which diet is best, who are we supposed to believe?

"Me, of course," says Marion Nestle, professor of nutrition, food studies, and public health at New York University. "Beyond that, how about using some common sense? It's a simple matter of eating fewer calories. But nobody wants to talk about calories because doing so does not sell books." She's right. The government recommends 1,600 calories a day for the average sedentary woman and 2,200 for men. In 2000 our reported per capita daily calorie consumption was 1,877 for women and 2,618 for men—roughly 300 calories more than we need.

So in one sense, the obesity crisis is the result of simple math. It's a calories in, calories out calculation. The First Law of Fat says that anything you eat beyond your immediate need for energy, from avocados to ziti, converts to fat. "A calorie is a calorie is a calorie," says Lawrence Cheskin, director of the Johns Hopkins Weight Management Center, whether it comes from fat, protein, or carbohydrate. Cheskin, who is six foot one (1.85 meters) and weighs 160 pounds (72.8 kilograms), has never had a weight problem himself. "Who said life is fair?" he observes.

The Second Law of Fat: The line between being in and out of energy balance is slight. Suppose you consume a mere 5 percent over a 2,000-calorie-a-day average. "That's just one hundred calories; it's a glass of apple juice," says Rudolph Leibel, head of molecular genetics at Columbia University College of Physicians and Surgeons. "But those few extra calories can mean a huge weight gain." Since one pound (.45 kilograms) of body weight is roughly equivalent to 3,500 calories, that glass of juice adds up to an extra 10 pounds (4.5 kilograms) over a year. Alternatively, you'd gain 10 pounds (4.5 kilograms) if, due to a more sedentary lifestyle—driving instead of walking, taking the escalator instead of the stairs—you started burning 100 fewer calories a day.

"We know people get fat by overeating slightly more than they burn, but we don't know why they do it," Leibel says. "I'm convinced our overeating is not willful or the result of a deranged upbringing. It's the genes talking, but it's a very complicated language. Genetics are everything."

In the 1960s James V. Neel, a geneticist at the University of Michigan, listened in on one genetic conversation. In his "thrifty gene" hypothesis, Neel suggested that some of us inherited genes that make us exceptionally efficient in our intake and use of calories. Our bodies are good at converting food into fat and then hanging on to it. This trait may have helped our ancestors survive when calories were few and far between, Neel speculated.

But fast-forward to the 21st century, when calorie supply isn't a problem, and genes that favor gaining weight have outlived their usefulness. Evolution betrays us. We store fat for the famine that never comes. "If we understood the genetics well enough," says Anna Mae Diehl, a professor of medicine at Johns Hopkins School of Medicine, "we could fingerprint people when they are born and say: Ah, good genes. Lucky you. You can eat whatever you want. Or: Uh-oh. Poor kid. Better never have a doughnut."

A team led by Jeff Friedman at Rockefeller University discovered a piece of the genetic puzzle in 1994. In studies of obese mice, the scientists identified a gene that tells the body how to make leptin, a hormone that decreases appetite. Leptin, produced in the fat cell itself, turned out to be part of a thermostat-like system that maintains weight at a constant level. Think of it as a watchdog guarding against starvation by monitoring body fat. It doesn't wait for you to become skinny; it acts within a few days to correct any perceived imbalance. Lose weight and leptin levels fall, prompting you to eat more and gain back the weight. Put on some extra pounds and leptin goes up; you eat less. It's part of an intricate biochemical and neurological circuitry that flashes signals on and off like a sailor's semaphore: EAT! DON'T EAT! EAT!

So if we take enough leptin, we can all fit into our high school prom outfits? It didn't turn out that way. Injecting leptin into people with a rare congenital inability to produce it does cause them to lose weight, but it wouldn't do much for the rest of us. In clinical trials, what worked in mice didn't always translate to humans.

The discovery of leptin and a number of other promising hormones has not yet produced a miracle drug. But genetic research is providing clues about why some people are more likely to get fat than others. We tend to assume that people who overeat simply lack willpower. What seems increasingly clear, however, is that the drive to overeat has strong biological underpinnings. People who are genetically susceptible to obesity don't necessarily have slow metabolisms that help their bodies hang on to fat. Instead, they may have a stronger biological drive to eat, especially in an environment where food is tasty, cheap, and plentiful.

The real question, says Friedman, isn't why so many of us are getting fat, but why, in our food-rich environment, is anyone thin at all?

One morning I click on the tube to see what food messages are reaching America's living rooms. According to a 2001 study published in the Journal of Nutrition Education, the average child in the U.S. will watch nearly 10,000 commercials touting food or beverages a year.

Click.

A huge bag of Double Delight Oreos swims into view…

Click.

Martha Stewart, looking as if she hadn't a care in the world, is making a chocolate ganache.

Click.

Finally, amid a sea of sugar, an image of healthy food flashes by:

It's Bugs Bunny, munching a carrot.

What's a broccoli-pushing parent to do?

"We live in a toxic environment," says Kelly Brownell, director of the Yale Center for Eating and Weight Disorders. "It's like trying to treat an alcoholic in a town where there's a bar every 10 feet (3 meters). Bad food is cheap, heavily promoted, and engineered to taste good. Healthy food is hard to get, not promoted, and expensive. If you came down from Mars and saw all this, what else would you predict except an obesity epidemic?"

Brownell favors the intervention of legislation that would, for example, suspend food advertising directed at children or remove soft drinks and snack foods from school vending machines. "The parallels with tobacco are interesting," he says. "We could search for a drug that would cancel out the effects of smoking, or we could go right to the cause and do everything possible to get rid of cigarettes."

Perhaps what this country really needs to fight fat is a mom. Make that a vintage mom, with a gingham apron tied around her waist as she places a bowl of vegetables on the table next to a skinless roast chicken. "Instead," says Harry Balzer, vice president of NPD, a marketing research firm that has been tracking what and where Americans eat for nearly a quarter of a century, "the restaurant has become the ultimate kitchen appliance."

Because I, too, deserve a break today, I am sitting inside a McDonald's in State College, Pennsylvania, with Barbara Rolls, professor of nutritional sciences at the Pennsylvania State University. For twelve dollars and change, Rolls and I have ordered a Happy Meal with cheeseburger, Coke, and fries; a Big Mac with medium fries and large Coke; and the Grilled Chicken California Cobb Salad with a packet of Caesar dressing. Inspection begins. Rolls peers at the Big Mac (600 calories) like an entomologist classifying a new species. The kid's Happy Meal cheeseburger turns out to be a regular size cheeseburger (330 calories). In total, 2,470 calories are sitting on the red plastic tray in front of us; if we clean our plates, we will each have consumed 77 percent of our daily caloric requirement in just one meal.

As we talk, I absently reach into a small bag of fries and scarf down the contents. Rolls calls this "mindless eating." "We pay little attention to the actual need for food," she says. In one experiment she and her students fed subjects baked pasta. "Some days we offered a normal portion. Some days we offered a portion 50 percent bigger for the same price. If we served them 50 percent more, they ate it. They just kept eating."

Betrayed by our genes, confused by the experts, we graze in endless pastures of food while the statistics grow more chilling. "Some of the earlier treatments like jaw wiring were extreme, but so is gastric bypass," Rolls says. "It's like the prefrontal lobotomy used to treat mental illness in the past."

Last year surgeons performed gastric bypass operations on 103,200 patients, with a complication rate of 7 percent. Linda Hay's four-hour operation was complicated by pneumonia. She stayed in the hospital five days longer than expected. A year and a half later, she has lost 162 pounds (73.5 kilograms). She feels full quickly and eats sparingly—a protein shake for breakfast; salad or sandwich for lunch; Lean Cuisine for dinner.

Hay has given away her size 4X clothes, buys size 10 pants, and can climb a flight of stairs without gasping for breath. "And I've caught the eye of men when I go out—especially in my new convertible."

Short of stapling our stomachs, will we ever solve the problem of fat? Meanwhile the struggle has turned global. For the first time, the Worldwatch Institute reports, there are as many overfed, overweight people in the world as those who are underfed and underweight.