Having just one fizzy drink a day could equate to putting on almost a stone in a year, a study of teenagers suggests.
The number of overweight children is set to increase, experts predict
US researchers were assessing if home deliveries of 'healthy' drinks such as bottled water helped.
The study, published in Pediatrics, showed those who did not receive such drinks put on weight.
A second study, by the International Obesity Task Force, warns childhood obesity will almost double by the end of the decade.
The IOTF, which studied data on childhood obesity from around the world, warns that the number of children who are overweight in the EU is set to rise by 1.3m a year, with more than 300,000 of those likely to be obese.
By the end of the decade, it is estimated that 26m children in the EU will be overweight, including 6.4m who will be obese - double the current number.
In the Middle East, the proportion of obese children is set to rise to 11.5%, and in both North and South America, it will be up to 15.2%.
In Asia, the proportion of obese children is set to treble to 5.3% - up from 1.5% now.
Dr Tim Lobstein, co-ordinator of the IOTF's childhood and adolescent obesity research programme, said: "The obesity estimates are very cautious, but extremely worrying."
'No nutritional value'
In the US study, researchers from the Children's Hospital Boston studied 103 children aged 13 to 18.
Half received weekly deliveries of healthy drinks.
They were instructed to avoid drinks containing sugar and received reminders via monthly phone calls and refrigerator magnets to "think before you drink".
The rest of the teenagers were asked to continue their usual eating and drinking patterns.
At the end of six months, those receiving deliveries had cut their consumption of sugary drinks by 82%, while that of the other group remained unchanged.
Researchers found that the heavier the teenager had been initially, the stronger the effect on body weight.
Among the heaviest third, those who had drinks delivered saw a decrease in their body mass index (BMI), while the control group had a slight increase - the difference equated to almost one pound per month.
Other factors affecting obesity, such as the amount of physical activity levels and time spent watching television, did not change in either group.
Dr Cara Ebbeling, who led the study, said: "Sugary beverages have no nutritional value and seem to make a huge contribution to weight gain."
She said the study showed it was relatively easy to have a significant impact on teenagers' habits.
"People often get overwhelmed by nutrition advice and give up.
"We opted to study one simple, potentially high-impact behaviour, and made it easy for adolescents to replace sugary drinks with noncaloric [no or low calorie] beverages."
She added: "It should be relatively simple to translate this intervention into a pragmatic public health approach.
"Schools could make noncaloric beverages available to students by purchasing large quantities at low costs."
Last week, the School Food Trust set out proposals for the removal of unhealthy drinks and snacks from UK schools.