Teenagers who drink lots of soft drinks get into more fights and carry more weapons than their peers who drink less, found a new study.
And while the study couldn’t determine whether soft drinks actually cause violence, the findings add to a growing – yet still controversial – body of research on the effects of nutrition on behavior.
“We were surprised at how large the effect was,” said David Hemenway, director of the Harvard School of Public Health’s Injury Control Research Center in Boston.
“It was maintained even when we controlled for alcohol and tobacco and family stuff like eating dinners together,” he said. “There was a very strong, stable relationship between more soft drinks that people said they drank and more fights with things like pushing and shoving.”
There has long been interest in how diet affects behavior, not just among scientists, but also among legal experts. In a notorious 1979 San Francisco murder trial, lawyers blamed the killer’s actions on his recent switch from a health-food diet to one filled with Coca-Cola and other junk food.
Their argument worked. Instead of a homicide ruling, the defendant was convicted of a lesser offense of voluntary manslaughter. The legal strategy became known as the “Twinkie Defense,” and the precedent raised a number of questions that persist, despite years of research on the subject.
A 2006 study in Norway found that teens who drank lots of soft drinks suffered from worse mental health compared to those who drank fewer. And a study published earlier this year found higher levels of antisocial behavior in American college students who drank the most soda.
For the latest study, Hemenway and colleague Sara Solnick surveyed more than 1,800 students in Boston public schools. During 40-minute sessions that covered a range of topics, kids answered questions about how much non-diet soda they had gulped down in the past seven days, whether they had been violent towards others, and if they had carried around a knife or gun in the previous year.
Nearly 30 percent of respondents reported consuming more than five cans of soda each week, the researchers reported in Injury Prevention. Heavy soda drinkers didn’t seem to get less sleep than anyone else, but they were more likely to have indulged in alcohol and tobacco over the previous month.
Even when the researchers controlled for alcohol, tobacco, BMI and other details, though, they found a steady rise in violence alongside higher levels of soda drinking. Twenty-three percent of teens who drank fewer than one soft drink a week reported carrying a weapon, for example, compared to 43 percent who drank five or more cans a week. And violence towards peers rose from 35 percent in the low-consumption group to 59 percent in the heaviest soda-drinkers.
It’s far too soon to claim that soda causes violence, and the new study only shows a correlation, said Bernard Gesch, who researchers diet and behavior at the University of Oxford in the United Kingdom.
Still, there is growing evidence that sugar might deserve at least some of the blame. Some research has shown that willpower is directly related to how the brain metabolizes glucose, Gesch said. There is also good evidence that people who perpetrate violence tend to have abnormalities in that glucose-digesting process. And consuming massive amounts of sugar could set those people up to commit violent acts.
In his own research, Gesch found a 26 percent drop in discipline-worthy offenses among prisoners who were given nutritional supplements, while behavior stayed the same in a group that was randomly assigned to take placebo pills. A similar study by Dutch researchers showed a 48 percent drop in bad acts with supplementation.
“The evidence that links diet with heart disease could easily fill a warehouse,” he added. “The evidence that links diet and brain function could fill two or three filing cabinets. If the brain is the crown jewels of our species, why is it receiving so little research?”