NEW YORK (Reuters) - As Americans debate what is most to blame for the nation's obesity epidemic, researchers say they have the strongest evidence yet that sugary drinks play a leading role and that eliminating them would, more than any other single step, make a huge difference.

Three studies published Friday in the New England Journal of Medicine represent the most rigorous effort yet to see if there is a link between sugar-sweetened beverages and expanding U.S. waistlines.

"I know of no other category of food whose elimination can produce weight loss in such a short period of time," said Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children's Hospital, who led one of the studies. "The most effective single target for an intervention aimed at reducing obesity is sugary beverages."

Previous research on the subject has been mixed, and beverage makers fiercely contest the idea that a single source of daily calories can bear so much responsibility.

"We know, and science supports, that obesity is not uniquely caused by any single food or beverage," said the American Beverage Association (ABA) in a statement. "Studies and opinion pieces that focus solely on sugar-sweetened beverages, or any other single source of calories, do nothing meaningful to help address this serious issue."

The NEJM studies, as well as an editorial and opinion pieces on the topic of sugary drinks and obesity, land as concern about obesity and its impact on public health is rising.

A report released this week projected that at least 44 percent of U.S. adults could be obese by 2030, compared to 35.7 percent today, bringing an extra $66 billion a year in obesity-related medical costs.

Last week, New York City adopted a regulation banning the sale of sugary drinks in containers larger than 16 ounces at restaurants and other outlets regulated by the city health department.

Sugary drinks are in the crosshairs because from 1977 to 2002 the number of calories Americans consumed from them doubled, government data show, making them the largest single source of calories in the diet. Adult obesity rates, 15 percent in the late 1970s, more than doubled in that period. The ABA points out, however, that consumption has since fallen, yet obesity rates keep rising.

Although most observational studies find that people who drink sugary beverages are more likely to be obese than people who do not, no cause-and-effect has been proved. People who drink sugary beverages, especially children, also watch more TV and eat more calorie-dense fast food, raising the possibility that liquid sugar is not the main culprit.

A 2008 analysis of 12 studies, led by a scientist who went on to work for the ABA, concluded that the association between sugary drinks and body-mass index (BMI) "was near zero."

Studies in which children cut their intake of sugary drinks found modest benefits, but "they were considered unconvincing," said Martijn Katan of VU University in Amsterdam: "Most had a small number of subjects and followed them for only a short time." He and his colleagues aimed to do better.

BUILDING A BETTER STUDY

For DRINK (Double-Blind Randomized Intervention in Kids), they gave 641 children aged about 5 to 12 and with a healthy BMI of just under 17 one 8-ounce (250 milliliter) noncarbonated drink per day, sweetened artificially or with sugar. The sugar-free drinks were specially formulated to look and taste like sugary ones so the kids would not know which they had.

About a quarter of the kids stopped drinking the beverages. Among those who stuck it out for 18 months, the sugar-free kids gained less body fat, 2.2 pounds (1 kilogram) less weight, and 0.36 units less BMI than the sugary-drink kids, the researchers report in the NEJM.

Why? There is good evidence that liquid sugar does not produce a feeling of fullness that other calories do. "When children substituted a sugar-free drink, their bodies did not sense the absence of calories, and they did not replace them with other food or drinks," said Katan.

DRINK doesn't answer whether switching to zero-calorie drinks would help obese kids. But another study in the same issue of NEJM suggests it might.

Researchers at Boston Children's had zero-calorie drinks delivered to 110 obese 15-year-olds who had BMIs of about 30 (where obesity starts), counseled them not to drink sugary beverages and offered other support.

After a year the teens had cut their intake of sugary drinks from almost two a day to zero and their daily calorie intake by 454. They had gained an average of 3.5 pounds (1.6 kilograms). By comparison, 114 teens who continued to consume sugar-sweetened beverages gained 7.7 pounds (3.5 kg) on average and ten times the BMI units: 0.63 compared to 0.06.

Once the deliveries stopped the two groups diverged less. After two years, teens who had received the no-cal drink deliveries had gained 9.5 pounds (4.3 kg) and 0.71 unit of BMI, compared to the control group's 11.2 pounds (5.5 kg) and 1.0 unit of BMI.

"It isn't surprising that after the intervention stopped, old behaviors crept back," said Ludwig of the New Balance Center. An "obesogenic" environment that promotes calorie-laden foods "overwhelms individuals' ability to maintain behavioral change" such as avoiding sugary drinks.

Hispanic teens benefited the most: Those receiving no-cal deliveries gained 14 fewer pounds after one year and almost 20 fewer pounds after two. That raised the possibility that genetic factors influence the effect of sugary drinks.

To investigate gene-environment-obesity links, scientists at Harvard School of Public Health looked at 33,097 people from long-term ongoing health studies, such as the Nurses' Health Study, identifying how many sugary drinks they consume and whether they have any of 32 genes linked to obesity.

The effect of genes on the likelihood of becoming obese was twice as large among people who drank one or more sugary drinks per day as among those who had less than one a month, the scientists report in the NEJM. In other words, belting back soda and sugary tea may turbocharge the genetic risk of obesity.

Conversely, eating a healthy diet devoid of sugary drinks keeps fat genes inactive. People with "fat genes" can be thinner if they avoid sugary drinks and other high-calorie foods.

(Editing by Michele Gershberg and Prudence Crowther)