(CNN) -- If you're searching for a villain in America's obesity epidemic, most nutritionists tell you to put one picture on the wanted poster: a cold, bubbly glass of soda pop.

Full of sugar, soda adds calories without making a person feel full, nutritionists say.

"Liquid candy" to detractors, sweetened soft drinks are so ubiquitous that they contribute about 10 percent of the calories in the American diet, according to government data.

In fact, said Dr. David Ludwig, a Harvard endocrinologist whose 2001 paper in the Lancet is widely cited by obesity researchers, sweetened drinks are the only specific food that clinical research has directly linked to weight gain.

"Highly concentrated starches and sugars promote overeating, and the granddaddy of them all is sugar-sweetened beverages," said Ludwig, who runs the Optimal Weight for Life Program at Children's Hospital in Boston.

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The rise in soft drink consumption mirrors the national march toward obesity. At the midpoint of the 20th century, Americans drank four times as much milk as soda pop. Today, the ratio is almost completely reversed, according to the U.S. Department of Agriculture. Meanwhile, in the past 30 years the national obesity rate has more than doubled, and among teenagers, more than tripled, reports the Centers for Disease Control and Prevention.

"Soda pop is a quintessential junk food," said Michael Jacobson, who heads the Center for Science in the Public Interest, which lobbies for government restrictions on foods it considers unhealthy. "It's just pure calories, and no nutrients. It's like a bomb in our diet."

Jacobson said the CSPI is pushing to require obesity warning labels on the sides of soda cans, like the surgeon general's warning on cigarettes.

While nutritionists are united in their dislike for nondiet soda, the "why" is controversial.

Some point a finger at high-fructose corn syrup, or HFCS, the sweetener used in most nondiet drinks. Last month, researchers at Rutgers University said they'd identified compounds in HFCS which may start a chemical chain reaction, leading to diabetes.

Most scientists, though, say there's little difference between HFCS and simple sugar, either in chemistry or the way they're handled by the body.

A bigger problem, doctors say, is simply the sheer number of calories. You'll find about 400 calories in a 32-ounce "extra-large" Coke, a fast-food staple. That's nearly a quarter of what the average adult woman needs in a whole day.

Scientists also say the body doesn't respond to liquid calories in the same way it would if those calories came in the form of French fries or chocolate cake. Appetite is controlled by a complex mix of hormones. Some signal the brain that your stomach is getting full. Others, including a hormone known as ghrelin, signal it's time to eat again.

If you eat a big burger, the level of ghrelin drops for a few hours. That drop doesn't happen if you drink a Big Gulp soda, even if it has more calories than the burger, according to Wayne Campbell, a professor in the Department of Foods and Nutrition at Purdue University.

"We're finding your hunger does not go down as far when you consume a beverage, as when you consume a solid," Campbell said. The result: Even with 400 liquid calories in your stomach, you polish off the burger too.

Campbell cautioned that the ghrelin theory isn't proven and that other factors -- such as a food's smell, or the sensation of chewing -- may affect appetite just as much, or even more. Our expectations also play a role.

"Soup is the anomaly to the liquid calorie research," he said. "People perceive soup as a meal, unlike drinking a Coke. So when we've done these types of studies, but used soup as the liquid, we don't see the same differences in [appetite] response."

The sugar in soda pop not only provides a massive dose of calories, but triggers a vicious appetite cycle, said Ludwig, who wrote "Ending the Food Fight," about healthy eating for children.

"It's rapidly absorbed, which raises blood sugar and in effect causes the body to panic." The body releases insulin to break down the sugar, "but the body overcompensates, and blood sugar drops below the fasting level," lower than it was in the first place.

Recognizing low blood sugar, the body releases ghrelin and other hormones, inducing hunger, inducing us to eat even more, Ludwig said.

The public is catching on, he said.

"In our obesity clinic, we used to routinely see patients coming in who were drinking four or five soft drinks a day. Now it's rare. That seems to be the first factor that comes to mind, when people are trying to lose weight."

Soft drink companies, under fire, are taking steps including a pledge last year to phase out nondiet soft drinks from America's schools.

A progress report issued Monday by the American Beverage Association said that shipments to schools of sweetened soda are down 45 percent since 2004, while shipments of bottled water are up 23 percent.

"There's no question the changes that are happening in schools are a mirror of what's happening in the larger marketplace," said Susan Neely, the ABA's president & CEO. "Adults, like kids, are reaching for lower-calorie beverages. ... As a consumer product company, we want to give consumers what they want."

While fighting obesity is complicated, Ludwig said, the first step is clear. "Giving up sugary soda for diet drinks, or water, will cause you to lose weight." E-mail to a friend

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