The search for the culprit in rising obesity levels is increasingly narrowing on one source -- soft drinks. A new study amplifies those claims, finding that the increased consumption of sugary beverages corresponds with Americans' expanding waistline.

Researchers at the Harvard School of Public Health reviewed the most credible scientific nutrition studies conducted over the last 40 years. The work was conducted with support from the American Heart Association and various federal agencies.

The new study finds that one-third of all carbohydrate calories in the American diet come from added sweeteners. Of that total, the study claims, beverages account for about half those calories.

The study points the finger of blame at the main sweetener used in soft drinks, high fructose corn syrup. Not only does it contain more calories than regular refined sugar, but some studies suggest it reduces the body's ability to process calories. The study notes the increased availability of soft drinks has also been a contributing factor. It notes that consuming one extra soft drink each day would add 15 pounds in body weight to the normal person in a year.

Not surprisingly, the beverage industry isn't buying it.

"These authors conclude the evidence indicates greater consumption of sugar-sweetened beverages is associated with weight gain and obesity, but a recent review by others looking at the exact same studies concluded that there was a lack of evidence to support this idea," said Dr. Richard Adamson, senior scientific consultant for the American Beverage Association.

"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."

Adamson says obesity is a complex issue that has no single cause, but depends on genetics, lifestyle, eating habits and exercise.