Researchers provide the strongest evidence yet that soda and other sugary drinks contribute to the obesity epidemic in children.
The new findings, reported in a trio of studies published online in the New England Journal of Medicine, offer persuasive support for New York City’s first-in-the-nation ban on large-sized soft drinks at restaurants and sports arenas. Critics and the beverage industry immediately cried foul following the passage of the ban, arguing that there was little evidence that such drastic action would change people’s drinking habits — or their waistlines.
But the new research suggests that limiting children’s access to sugary beverages can indeed curb weight gain: one paper found that providing children with water or diet soda as an alternative to full-sugar soft drinks can lead to meaningful drops in children’s fat deposits and weight; another showed that drinking a single no-calorie drink a day, instead of a sugary one, slows weight gain, independent of other behaviors like overeating or failing to exercise. A third study finds that for people who are already genetically predisposed to obesity, drinking sugary sodas can make their weight problem worse.
Taken together, the papers provide the most robust evidence to date that sugary drinks are a significant driver of weight gain.
(MORE: BPA Linked with Obesity in Kids and Teens)
In the first study, led by Janne de Ruyter at the Vrije Universiteit Amsterdam, scientists followed 641 normal-weight schoolchildren ages 4 to 11 years old for 18 months. The students were randomly assigned to one of two groups: one group received an 8-oz. can of a sugar-sweetened fruit drink to consume every day during recess, while the other group got an identical can of an artificially sweetened, calorie-free drink. On weekends, the children were sent home with two cans to drink on Saturday and Sunday. The trial was double-blinded, meaning that neither the children nor the researchers knew who received which beverage; the scientists went to great lengths to work with a soda manufacturer to formulate beverages that tasted the same and were presented in cans that looked the same.
At the end of the 18-month trial, the children drinking the sugar-free beverage had gained less weight, about 14 lb. (6.35 kg) on average vs. 16.25 lb. (7.37 kg) for the sugar-sweetened drink group. What’s more, the sugar-free group also gained 35% less in body fat than the other kids, as measured by an electrical-impedance test that gauges fat accumulation.
The 2-lb. difference in weight gain between the groups may seem small, but it “is highly significant, especially since we are talking about an 8-oz. can,” says the study’s senior author Martijn Katan, emeritus professor of nutrition at the Vrije University Amsterdam. “A small amount of soft drink can shift the prevalence of obesity from something parents don’t need to worry about into an entirely different territory. It also means that if you take away sugar, you can shift the curve away from obesity, as we saw with the children in the sugar-free group.”
Because the study involved a large number of children who were followed over a relatively long period of time and because the it was blinded, Katan says, the results provide the clearest evidence of how drinking sugar-sweetened beverages can affect children’s weight gain. Moreover, the researchers purposefully refrained from counseling the children or their families about good nutrition or the risks of obesity, and did not ask them to change their eating or activity habits in any way. All they varied was one beverage a day. Katan acknowledges that some of the children, just by virtue of being in a study, may have altered their eating habits, but given the size of the study, he says that any amendments made by participants in one group were likely balanced out by parallel changes in the other group.
(MORE: Meet Big Soda — as Bad as Big Tobacco)
In the second study, scientists analyzed genetic data on 33,000 health professionals who answered questions about their diet, including their consumption of sugar-sweetened beverages, while participating in three large health surveys over many years. Lu Qi, assistant professor of nutrition at the Harvard School of Public Health and the senior author of the paper, and his colleagues calculated a score for each participant representing his or her genetic vulnerability to obesity, based on 32 genetic markers known to be involved in weight.
People who had higher genetic scores for obesity were more likely to be obese than those who had fewer such genes, and those who drank a lot of soda were also more likely to be obese than those who drank less — but people who had both a high genetic risk for obesity and drank a lot of soda were at much higher risk of being obese than they would have been given either factor alone. In fact, among people with a high number of obesity genes, the risk of obesity was more than twice as great in those consuming the most sugary drinks (at least one serving a day) as in those who consuming the least (less than one serving a month).
(MORE: Study: Obese Kids Have Less Sensitive Taste Buds)
Finally, in the third study, doctors revealed more encouraging evidence that relatively small interventions can alter children’s soda-drinking habits and reduce weight gain. In that trial, led by Dr. David Ludwig, director of the New Balance Foundation Obesity Prevention Center at Boston Children’s Hospital and a professor of pediatrics at Harvard Medical School, researchers included 224 overweight or obese teens. For one year, half the group received home deliveries of bottled water and diet drinks and were encouraged to avoid sugar-sweetened beverages. The other teens received $50 gift cards to the local supermarket to purchase whatever they wanted.
Before the intervention began, all of the teens reported drinking at least one sugar-sweetened beverage a day, but after one year, those getting zero-calorie home deliveries had almost completely switched to downing only sugar-free beverages. They had also gained significantly less weight than those who didn’t receive the deliveries: 3.5 lb. (1.6 kg) vs. 7.7 lb. (3.5 kg) respectively. The control group also ended up consuming fewer sugary beverages by the end of the study, likely because of an overall increase in aggressive public-health messaging about the importance of eating well and exercising.
“These results show that sugary beverages can have an effect on body weight quickly,” says Ludwig, “perhaps more so than any other single food product. We know of no other study where you eliminated one specific category of food and then show a changed body weight at one year.”
(MORE: Are Cesarean Sections Contributing to Childhood Obesity?)
The trend did not persist, however, when the home deliveries ceased. The researchers continued to observe the teens for additional year during which time none received deliveries of no-calorie drinks. Not surprisingly, these teens began to drink more sugar-sweetened beverages and their weight started to creep back up.
That suggests that changing children’s eating and drinking habits isn’t simply a matter of educating them about nutrition and healthy foods. It also requires changing their environment, so that healthier alternatives become both accessible and convenient. “Children and adolescents will readily change their beverage habits if other products are available,” says Ludwig. “As long as we maintain environments of sugar-sweetened beverages where they are ubiquitous, and heavily marketed, it shouldn’t surprise us that they are drinking a lot of them. But if we create an environment that makes alternatives easy and convenient, they will drink those instead.”
That is essentially the idea driving New York City’s controversial ban on large-sized sugary beverages. If jumbo-sized cups aren’t available, then the thinking goes, residents might not consume as many sugary drinks as they currently do.
(MORE: Should Parents Lose Custody of Their Extremely Obese Kids?)
The beverage industry maintains, however, that sugary drinks are only one part of the obesity problem — and that’s certainly true. In a written statement responding to the new studies, the American Beverage Association said, “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 fact remains: sugar-sweetened beverages are not driving obesity.” Still, the new results make a strong case that reducing overconsumption of sodas and sugary drinks is a good place to start.
The group further contends that the average American gets about 7% of daily calories from sugar-sweetened beverages, and that consumption of these drinks already dropped by 20% between 2001 and ’10. However, youngsters tend to drink more than adults: some studies show that kids get about 15% of their daily calories in liquid form, largely from sodas and 100% fruit juices.
Public-health experts say more should be done to reduce the amount of liquid calories people consume. The new findings suggest that despite criticism about becoming a nanny state, New York City may be on to something with its big-soda ban; experts eagerly await follow-up studies to see if the new policy will have any effect on residents’ waistlines.
The results could reinvigorate efforts to control soda consumption with soda taxes or other restrictions on where the beverages can be sold. “I think we have passed a watershed here,” says Katan about the strength of the data connecting sugar-sweetened beverages and obesity. “The next step will be to see what we can do about it.”
MORE: Obesity in Kids: Three Lifestyle Changes That Help
Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.