An additional soft drink a day gives a child a 60 percent greater chance of becoming obese, new research suggests. The U.S. study, published this week in The Lancet medical journal, says the soft drink-obesity link is independent of the food children eat, how much they watch television or videos, and the amount they exercise.Experts, who called the findings "enormously important," have long thought that sweetened drinks were contributing to the obesity epidemic among children, but there had been no reliable evidence of a link.
"These are estimates, and the study doesn't tell us the importance of soft drinks relative to the other factors that contribute to obesity, but these data suggest that people aren't compensating" for the extra calories by cutting back on eating, said the study's lead investigator, Dr. David Ludwig, director of the obesity program at Boston Children's Hospital.
France Bellisle from France's Institute of Health and Medical Research said the study provided "convincing" evidence about the relationship between sugar and weight gain in children.
The prevalence of obesity among U.S. children doubled between 1980 and 1994.
A common measurement of obesity is the body mass index, or BMI, which takes into account weight and height. A BMI of 25 means a person is overweight. The threshold for obesity is a BMI of 30.
For children, experts disagree on what constitutes obesity. Some think that, in general, any child with a BMI above the 85th percentile for age and sex is obese, and others use the 95th percentile.
The study used the 85th percentile as the threshold. By that measure, scientists estimate that 24 percent of U.S. children are obese.
The soft drink study tracked 548 children ages 11 and 12 in public schools across Massachusetts for two school years until May 1997.
It found that each sugared soft drink the children were consuming each day at the beginning of the study contributed 0.18 points to their BMI.
If they increased their daily soft drink intake, each additional soda made them 60 percent more likely to become obese, regardless of how many sodas they were drinking before. All the children were already drinking some soft drinks at the beginning of the study, but the researchers extrapolated that the effect would remain consistent even if a child went from drinking none to one a day.
Only 7 percent of the children did not change their soft drink intake over the two years. Fifty-seven percent increased it, with a quarter of them drinking two or more additional cans a day, the study said.
Soft drinks tracked in the study included regular sodas, Hawaiian Punch, lemonade, Kool-Aid, sweetened iced tea and other sugared fruit drinks. Pure fruit juice intake also was tracked, but that did not account for the effect, the study said.
"The odds of becoming obese increased significantly for each additional daily serving of sugar-sweetened drink," the study said.
An increase in diet soda consumption made the children less likely to become obese.
Dr. Philip James, chairman of the International Obesity Task Force, an independent worldwide scientific organization not connected with the study, said the evidence so far indicates that sugar is slightly less fattening than fat, but that sugar in drinks can be deceptive because the beverages are less filling than food.
He said one explanation might be that while people tend to eat less at a meal if they have overeaten at a previous sitting, they don't tend to do that if the additional calories came from drinks. They tend to eat a normal-size meal despite having loaded up on sugar from soft drinks.
In the past 10 years, soft drink consumption has almost doubled among children in the United States, Ludwig said, adding that the average U.S. teenager consumes 15 to 20 additional teaspoons of sugar a day just from soda and other sugared drinks.
Childhood obesity has been linked to later development of diabetes, heart disease, cancer and arthritis.
