Consumption of these drinks has more than doubled since the 1970s, and the rate of obesity among Americans during the same period reached 30% of the adult population, said the authors of a study published online by the New England Journal of Medicine.

The first study, which involved more than 33,000 American men and women, showed that drinking sugary drinks was affecting genes that regulate weight and increased the genetic predisposition of a person to gain weight.

The researchers used 32 variations of genes known to affect the weight to establish a genetic profile of the participants. They also determined the participants' eating habits, their consumption of sweetened beverages and exercise practices.

The other two studies showed that giving to children and adolescents calorie-free drinks like mineral water or soft drinks sweetened with artificial sweeteners resulted in weight loss.

The first was conducted at Children's Hospital Boston, which examined 224 overweight adolescents who were encouraged to consume water or light sodas for a year.

These teens gained only 0.68 kilograms of weight during this period compared to 1.5 kilograms in another group that consumed sugary drinks.

Yet another study was conducted by researchers at the VU University Amsterdam (the Netherlands) and involved 641 children aged 4 to 11.

Half of the group drank sweet and fruity drinks while the other half the same drinks with sugarless sweeteners.

After 18 months, children who consumed the low-calorie drinks gained 6.39 kilograms on average compared to 7.36 kilograms in the group that drank sugary fruit drinks.

"Taken together, these three studies suggest that calories from sugar-sweetened beverages do matter," said Doctor Sonia Caprio of Yale University writing in the New England Journal of Medicine.

"These randomized, controlled studies ... provide a strong impetus to develop recommendations and policy decisions to limit consumption of sugar-sweetened beverages, especially those served at low cost and in excessive portions, to attempt to reverse the increase in childhood obesity," she added.