Feeling peeved at your partner? You may want to check your blood sugar. A new study suggests that low levels of glucose in the blood may increase anger and aggression between spouses. The researchers say their findings suggest a connection between glucose and self-control, but other experts disagree about the study’s implications.
Glucose is a source of fuel for the body, and its levels in the blood rise and fall throughout the day, as the body metabolizes meals that include carbohydrates. Researchers have suspected since the 1960s that low glucose or swings in glucose may play a role in human aggression. In two 2010 studies, psychologist Brad Bushman of Ohio State University, Columbus, attempted to figure out just what that role is, first by measuring vengefulness among people with symptoms of type 2 diabetes  (a disease in which the body can’t regulate glucose levels properly), and then by providing sweetened drinks to strangers competing on a computerized task . Both studies suggested that higher glucose levels can make strangers less likely to treat each other aggressively.
Bushman wondered about the relationship between glucose levels and aggression among romantic couples. So he and colleagues at the University of Kentucky and the University of North Carolina recruited 107 married couples and equipped them with blood glucose meters, voodoo dolls, and 51 pins to record their glucose and anger levels over time.
For 21 days, the couples used the meters to measure their glucose levels each morning before breakfast and each evening before bed. They also assessed how angry they were at their spouse at the end of each day, by recording how many of the 51 pins they stuck into their voodoo dolls just before bed when their partner wasn’t looking. After 21 days, the couples were invited into the lab. There, they played a computer game that allowed them to blast their spouse with an unpleasant noise—a mixture of fingernails scratching a chalkboard, ambulance sirens, and dentist drills—as loudly and for as long as he or she wanted, as a proxy for their willingness to act aggressively and make their partner suffer.
At home and in the lab, spouses with lower evening glucose levels showed more anger and aggression toward their partners , as measured by the voodoo dolls and their willingness to blast them with noise , the team reports online today in the Proceedings of the National Academy of Sciences. Those in the lowest 25th percentile of evening glucose levels stuck an average of twice as many pins in their voodoo dolls as those in the upper 25th percentile, and they selected louder and longer noise blasts for their spouse.
Bushman argues that “glucose provides the energy the brain needs to exercise self-control,” and when glucose levels are low, aggression is more likely.
“This is a very impressive study,” says psychologist Roy Baumeister of Florida State University, Tallahassee, who was not involved in the work. “Being able to control ourselves and our impulses is one of the most important elements of the human psyche, so understanding self-control and what fuels it is really quite important.”
But psychologist David Benton of Swansea University in the United Kingdom, who studies dietary influences on the brain and behavior, is less impressed with the findings, which he says are “not particularly surprising.”
“Based on previous better conducted studies it seems that low blood glucose can be one of many factors that predispose to aggressive behavior,” Benton writes in an e-mail. He points out that alcohol consumption can both lower glucose levels and lead to more aggressive behavior, but the study did not collect information on what the couples ate or drank. Without more information, “taking a single measure of dynamic response, at different times in different people, will tell us little,” he writes.
Still, Bushman says that at the very least, “if you know you’re going to talk to your partner about something that can cause conflict, it may not be a bad idea to down that sugary drink first.”