As more research accumulates about the negative health effects of soda, the beverage industry has increasingly challenged the scientific validity of studies in this area, on the grounds that they do not offer convincing evidence.
That is what happened in August, when a national study found that 5-years-olds who drank at least one serving of soda per day were more likely to have behavioral problems than those who didn’t drink any soda. Researchers from Columbia University’s Mailman School of Public Health, the University of Vermont, and Harvard University found that close to half of the 3,000 children studied drank soda. Those who drank four servings a day were two times more likely to destroy others’ possessions, get into fights, and physically attack people than those who did not drink any soda.
The study received widespread media coverage, provoking the American Beverage Association (ABA), the industry group that represents soft drink companies, to issue a response contesting the science behind the study:
“It is a leap to suggest that drinking soda causes these or any other behavioral issue. The science does not support that conclusion.”
“The authors themselves note that their study ‘is not able to identify the nature of the association between soft drinks and problem behaviors,’” the ABA further added.
But the researchers say this does not detract from the study’s value.
“This is just the first step,” says Dr. David Hemenway, professor of health policy at Harvard University School of Public Health and senior author of the study.
The authors of the study admit that the link between soda and behavior is far less established than the link between soda and other health issues like obesity and diabetes. So far, there have only been a few studies into the association, although all have shown strong connections.
Another study from 2012 by Dr. Hemenway and colleagues of 2,900 students in Boston public high schools found that drinking soda was “strongly and significantly” associated with carrying a weapon and perpetrating violence against siblings, peers, and dates.
A 2006 study by researchers at the University of Oslo found that among 7,300 Norwegian high school students, those who drank four or more glasses of soda per day were more likely to have mental health problems, such as hyperactivity; “feeling panicky, anxious, dizzy, tense, sleepless, sad, worthless, and hopeless; finding fault within the self; and finding everything a burden” than those who drank less.
And earlier this year, a study by the U.S. Centers for Disease Control and Prevention reported that among a group of 16,200 American high school students, those who drank soda daily were more likely to be involved in problem behavior, like not wearing a seatbelt, or even victimization—being bullied, threatened, or injured on school property, and getting physically hurt by their boyfriend or girlfriend.
The data from the study of 5-year-olds came from the Fragile Families and Child Wellbeing Study, a large survey that asks parents—three quarters of them unmarried—about their parenting attitudes and behaviors.
This included questions about how much soda their children drank and what kind of problem behaviors they engaged in. Because the data about soda was cross-sectional, meaning the mothers were only surveyed once and not over a period of time, it is not clear whether drinking soda preceded behavioral problems. Indeed, it may be the other way around.
“There is the possibility that really aggressive kids just want more soda, and the parents are just more willing to give it to them because they don’t want to deal with the aggressive behavior,” says Dr. Shakira Suglia, an assistant professor of epidemiology at Columbia University’s Mailman School of Public Health and lead author of the study.
The researchers attempted to control for other factors that might be indicative of less engaged parenting, such as kids’ juice and candy consumption, the mother’s history of depression, parental violence, and paternal incarceration history.
Even after controlling for those factors and others like gender and whether the parents were on welfare, the link between soda consumption and bad behavior remained. In fact, there was what is known as a dose-response relation: with each serving of soda a child had during the day, the frequency of bad behavior increased. Children who consumed four or more sodas each day were more than twice as likely to act out.
“The beverage industry has a tough row to hoe because they don’t want to say it’s bad parenting—that bad parents let their kids drink soft drinks. They don’t want to say that, I don’t think,” says Dr. Hemenway.
One nutrition expert took issue with the amount of coverage the study received.
“This is a minimally studied area with no consensus and minimal research,” says Dr. Barry Popkin, professor of nutrition at the University of North Carolina Gillings School of Global Public Health. “We would need solid randomized controlled trials with behavioral outcomes,” to show a causal association between soda and behavior.
In a randomized controlled trial, a group of children who drink soda would be compared over time to a “control group” of children who do not drink soda. Soda consumption would be manipulated by researchers. And importantly, factors like parental engagement would be balanced, on average, between the groups. In this study, drinking soda would be the “independent variable,” that would in theory help researchers get closer to understanding whether it causes problem behavior.
In contrast, the Columbia/Harvard/Vermont study is “observational,” meaning the researchers did not have control over the groups they compare and were only measuring soda and behavior as a relationship of correlation and not cause and effect.
“Randomized controlled trials are very hard to do. They are incredibly expensive and only look at one or two little things,” says Dr. Hemenway, adding that such a study of soda drinking and children probably would not get approved by an ethics committee.
He points out that observational studies in the mid-twentieth century showing that cigarette smokers were more likely to have lung cancer than non-smokers were also met with skepticism, even though it is now accepted that cigarette smoking is the number one risk factor for lung cancer.
Similar to the soft drink industry’s frequent dismissal of epidemiologic research for failing to show how soda causes health problems, tobacco companies claimed that evidence was lacking for “scientific conclusions regarding the cause and effect relationship between environment factors, including smoking, and lung cancer.”
“Even there they didn’t know the cause and effect. There had to be so many studies before people would believe it,” he says. “If you keep seeing the same thing, that’s a pretty big effect.”
Drs. Hemenway and Suglia believe that because of the strength of the relationship between drinking soda and violence, their study opens the door for investigating which ingredients might be responsible for behavior changes in kids.
“I think it should be looked at more. I think there’s definitely a diet and brain connection. So there has to be a diet and behavior connection, particularly for little kids, but it hasn’t been examined,” says Dr. Suglia. “There’s the old wives tale that you give kids sugar and they get hyper. Scientifically, the data’s not very good.”
“We all get emails now from people who say that it’s the red dye. I had a long conversation with the scientist that said it’s the high fructose corn syrup. Some people think it’s the caffeine,” said Dr. Hemenway.
Despite the lack of certainty here, they both say there is enough information and nutritional understanding that recommends against children drinking soda.
“Clearly these kids who are drinking a lot of soda seem to be very different than the kids who aren’t,” says Dr. Hemenway.
Edited by Dana March
Photo Credit: Liz Kasameyer via Flickr