Americans have sipped and slurped their way to fatness by drinking far more soda and other sugary drinks over the last four decades, a new scientific review concludes.

An extra can of soda a day can pile on 15 pounds in a single year, and the “weight of evidence” strongly suggests that this sort of increased consumption is a key reason that more people have gained weight, the researchers say.

“We tried to look at the big picture rather than individual studies,” and it clearly justifies public health efforts to limit sugar-sweetened beverages, said Dr. Frank Hu, who led the report published Tuesday in the American Journal of Clinical Nutrition.

He and others at the Harvard School of Public Health reviewed 40 years of nutrition studies that met strict standards for relevance and scientific muster. The work was funded by ongoing grants to his lab from the federal government and the American Heart Association.

Soft drink trends have marched lock-step with the growing obesity epidemic, but industry groups have long fought efforts to say one directly caused the other. Not all studies conclude that beverages are at fault, and the new analysis ignored some that would have discounted such a link, the American Beverage Association said in a statement issued in response to the study.

“Blaming one specific product or ingredient as the root cause of obesity defies common sense. Instead, there are many contributing factors, including regular physical activity,” the statement says.

However, Dr. David Ludwig, director of the obesity program at Children's Hospital in Boston and a longtime advocate of curbs on soda, said blaming other factors misses the point.

“Could you imagine somebody saying we should ignore the contribution of hypertension to heart attack because there are many causes? It's ludicrous. Yet this argument resurfaces with regard to obesity,” Ludwig said.

When it comes to beverage trends and obesity, “it's like documenting the force of gravity,” he said. “There's an overwhelmingly strong case to be made for a causal relationship.”

About one-third of all carbohydrate calories in the American diet come from added sweeteners, and beverages account for about half of this amount, the new report says.

Unlike other carbohydrates, the main sweetener in beverages - high-fructose corn syrup - doesn't spur production of insulin to make the body “process” calories. It also doesn't spur leptin, a substance that helps moderate appetite. For these reasons, beverages aren't as satisfying as foods containing similar amounts of calories and fly under the radar of the body's normal weight-regulating mechanisms, many nutrition experts say.

The 30 studies included in the new review are of different types - experiments where beverages were curtailed or modified, studies of cross-sections of the population. While all do not show harm, they collectively suggest that soda and sugary drinks “should be discouraged,” the authors write.

Federal dietary guidelines recommend beverages without added sugars, and the World Health Organization advises that added sugars should provide no more than 10 percent of total calories.

Increasingly, sugary drinks are being restricted in schools. In May, top beverage distributors agreed to stop selling non-diet sodas in certain schools and restricted sales in certain settings where young children buy them.

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On the Net:

Nutrition journal: www.ajcn.org

Beverage Association: www.ameribev.org

Federal obesity information: www.win.niddk.nih.gov

AP-ES-08-08-06 1531EDT