Harvard School of Public Health researchers looked at nutrition studies over the last 40 years and found that a person can gain as much as 15 pounds per year from drinking an extra can of soda a day -- the equivalent of 10 teaspoons of table sugar each day. The report also noted that one-third of America's carbohydrate intake comes from added sweeteners, and half of that from beverages.
"We tried to look at the big picture rather than individual studies," said lead researcher Dr. Frank Hu, who added that recent public health efforts to limit the availability of sugary beverages were justified by the results.
In the United States, most soft drinks and similar beverages are sweetened with high-fructose corn syrup, which contains slightly more fructose than table sugar. According to some studies, pure fructose may not trigger the production of either insulin -- necessary for calorie processing -- or leptin, which is an important appetite regulator.
Recently, recommendations from both federal and international health experts have inspired top beverage distributors to stop providing non-diet sodas in certain schools and to restrict sales in areas where young children tend to purchase the drinks. However some studies suggest diet sodas' primary sweetener, aspartame, can cause seizures, brain tumors and even other health problems.
Industry groups argue that not all of the studies have concluded that beverages and obesity are linked. The American Beverage Association released a statement saying the Harvard study had left out studies that would discount the link altogether.
"Blaming one specific product or ingredient as the root cause of obesity defies common sense," said ABA senior science consultant Richard Adamson in the statement. "Instead, there are many contributing factors, including regular physical activity."
The Grocery Manufacturers of America (GMA) has opposed the regulation of soft drink sales in states such as Texas, Oregon and California. The official GMA position is that a balance of physical exercise and nutrition is the only way to effectively combat obesity.
Director of the obesity program at Children's Hospital in Boston and soda restrictions supporter Dr. David Ludwig disagrees, saying that studying the link between beverage drinking and obesity is akin to "documenting the force of gravity."
"There's an overwhelmingly strong case to be made for a causal relationship," he said.
"The soft drink industry will always discount the link between their products and obesity," added Mike Adams, a consumer health advocate. "It's in their interests to make people second guess the enormous amount of scientific evidence demonstrating that sugary beverages do, indeed, contribute strongly to both obesity and diabetes."