Diet in a can? 2 new "negative-calorie" drinks claim to do the hard work for you

The shoppers looked skeptical.

"This is the first drink that can actually help you lose weight," sales representative Anthony Monforte said confidently, handing out tiny samples of a new soft drink, Celsius, at a Vitamin Shoppe in Aliso Viejo, Calif.

Leslie Bedford and Marsha McDonogh took cautious sips. "Hmmm. It does taste like RC Cola," McDonogh said, agreeing with Monforte's description. Sold on the taste — and the promise — she plunked down $6.99 for a four-pack.

"If it really works, that's great," Bedford said. "Everyone in our office wants to lose weight one way or another."

Beverage makers are counting on it. Stung by falling sales and criticism that sugar-sweetened soft drinks raise the risk of obesity, they're reaching into scientists' laboratories to come up with healthier products — vitamin waters, sports drinks, fortified juices and now so-called negative-calorie drinks.

The drinks, most notably Celsius and Coca-Cola's and Nestlé's Enviga, promise to boost metabolism and burn calories.

The key ingredients are green tea and caffeine. Celsius' manufacturer says its particular combination will increase metabolism enough to burn up to 77 calories per 12-ounce bottle; Coke states that three 12-ounce cans of Enviga will burn 60 to 100 calories. Snapple has also introduced green-tea beverages, with labels that claim they boost metabolism.

Intriguing, but ...

The effects of the green-tea drinks go beyond those of caffeine-laden zero-calorie sodas, the manufacturers of Celsius and Enviga say. An antioxidant found in green tea — epigallocatechin gallate, or EGCG — significantly increases metabolism, they say, which boosts the body's ability to burn fat.

Although scientists still aren't sure just how EGCG works, some suggest it triggers greater production of a neurotransmitter called norepinephrine, elevating metabolism. Caffeine also raises the metabolic rate, and early research suggests combining EGCG with caffeine is the key to a measurable increase.

The concept is intriguing — but far from proven, pharmacology experts point out.

Jeffrey Blumberg, a senior scientist in pharmacology at Tufts University, is skeptical. "In really carefully controlled studies, you can actually find an increase in metabolic rate," he says. "But if the effects are modest, it might be hard to see them in the real world."

Other studies have shown that the antioxidant does have potential to help prevent some types of cancer, they say, but the effects on metabolism shouldn't be counted on at this point.

Feel a burn?

The makers of Enviga and Celsius say they have research to support their weight-loss claims.

In a study of Celsius, which contains five to 10 calories a bottle depending on the flavor, 20 people were divided into two groups, with one group consuming 12 ounces of Celsius and the other group consuming 12 ounces of Diet Coke.

The volunteers' metabolic rates were measured before and after consumption. The study showed an average increase of 12 percent in metabolic rate over a three-hour period among those drinking Celsius compared with a 4 to 6 percent rise with Diet Coke.

Depending on the person's own metabolism (which varies by fitness level, weight, gender and age, among other factors), a 12 percent increase could result in burning up to 77 calories a bottle, says Elite FX, the manufacturer of Celsius, which funded the study.

The research was conducted at Ohio Research Group of Exercise Science and Sports Nutrition and was presented last year at a meeting of the International Society of Sports Nutrition.

A gentle boost

A study of Enviga, which contains five calories per can, showed that drinking three 12-ounce cans a day increased calories burned by 60 to 100 per day. The study, performed at the University of Lausanne in Switzerland, isn't published.

"The data show the green-tea extract appears to enable this gentle boost in the metabolic rate," says Rhona Applebaum, chief scientist of Coca-Cola. "The second mechanism is caffeine. The two together ... produce this synergy that allows for this gentle boost in rate."

Enviga was launched in October in the northeast U.S. and will become available nationwide this month. Sweetened with aspartame, it comes in three flavors — green tea, berry and peach — and sells for about $1.29 to $1.49 a can.

Celsius, which sells for roughly $1.99 a bottle, was launched in June 2005. Sweetened with sucralose, or Splenda, it, too, is available in a variety of flavors.

Celsius contains 200 milligrams of caffeine and Enviga 100. A Coke or Pepsi contains about 35 milligrams of caffeine; a 5-ounce cup of coffee has about 100 milligrams. Celsius also contains seeds of the guarana, an Amazonian berry, which contains caffeine.

But EGCG's effect on metabolism is stronger than that of caffeine, the companies say.

Of course, simply exercising and replacing regular sodas with low- or no-calorie drinks could help Americans shed excess pounds.