Low-fat, low-cal, low-carb. Atkins, South Beach, The Zone. Food fads may be distracting attention from something more insidiously piling on pounds: beverages.

One of every five calories in the American diet is liquid. The nation’s single biggest “food” is soda, and nutrition experts have long demonized it.

Now they are escalating the fight.

In reports to be published in science journals this week, two groups of researchers hope to add evidence to the theory that soda and other sugar-sweetened drinks don’t just go hand-in-hand with obesity, but actually cause it. Not that these drinks are the only cause – genetics, exercise and other factors are involved – but that they are one cause, perhaps the leading cause.

A small point? In reality, proving this would be a scientific leap that could help make the case for higher taxes on soda, restrictions on how and where it is sold – maybe even a surgeon general’s warning on labels.

“We’ve done it with cigarettes,” said one scientist advocating this, Barry Popkin at the University of North Carolina in Chapel Hill.

Barry Popkin, a scientist at the University of North Carolina, is one of several researchers who think consumption of sweetened sodas leads to obesity.

Comparing soda and obesity to tobacco and lung cancer is a baseless crusade, industry spokesmen say.

“I think that’s laughable,” said Richard Adamson, a senior science consultant to the American Beverage Assn. Lack of exercise and poor eating habits are far bigger contributors to America’s weight woes, he said.

“The science is being stretched,” said Adam Drewnowski, director of nutritional sciences at the University of Washington in Seattle. He owns stock in beverage companies and has done extensive research in the field, much of it financed by industry but also some by government.

However, those making the case against soda include some of the nation’s top obesity researchers at prestigious institutions like Harvard and Yale.

“There are many different lines of evidence, just like smoking,” said Dr. David Ludwig, a Harvard pediatrician who wants a “fat tax” on fast food and drinks.

Beverage companies seem worried. Some are making sodas “healthier” by adding calcium and vitamins, and pushing fortified but sugary sports drinks in schools that ban soda. This could help them duck any regulations aimed at “empty calorie” drinks, said Jennifer Follett, a USDA nutritionist at the University of California in Davis.

“Even defining ‘milk’ is getting tough these days,” with so many flavored varieties and sweetened liquid yogurts, she complained. “It tastes like you’re sucking on ice cream.”

Proving that something causes disease is not easy. It took decades with tobacco, asbestos and other substances now known to cause cancer, and met strong industry opposition. It would be especially difficult for a disease as complex as obesity.

Diet is difficult to study. Most people drink at least some sweetened beverages and get calories from other drinks like milk and orange juice, diluting the strength of any observations about excess weight from soda alone.

Children are growing and gaining weight naturally, “so we have this added complication” of trying to determine how much extra gain is from sweet-drink consumption, said Alison Field, a nutrition expert at Harvard-affiliated Children’s Hospital in Boston.

“Given these caveats, it’s amazing the association we do see,” she said.

Opinions divided on diet soda

Nutrition experts who are soured on soda often take a sweeter view of diet soda, which doesn’t contain the sugar calories that many of them blame for weight problems.

Barbara Rolls, an author and nutrition expert at Penn State, is among its fans.

“Diet soda gives you a tool if you use it right” to wean off sweets and reduce calorie intake, she said.

However, others think it may encourage people to feel they’ve “earned” more dessert because they’ve cut calories from beverages.

Although diet soda consumption is soaring, “we haven’t seen the benefit” in lower rates of obesity yet, noted Dr. Louis Aronne, a Weill-Cornell Medical School nutrition expert who is president of the Obesity Society.