The sugars we eat may influence how our brains decide whether we should put our forks down or go back for second helpings, according to a new study. The findings suggest that fructose—a sugar found in fruits as well as in sweetened products such as soda—does not trigger the brain’s satiation signal in the same way as glucose, a sugar that’s the main fuel source for most of our bodies’ cells. Fructose may instead activate pathways that increase the appeal of food.
In 2013, Kathleen A. Page of the University of Southern California and colleagues found that fructose activates the hypothalamus, a part of our brains that regulates food intake (J. Am. Med. Assoc., DOI: 10.1001/jama.2012.116975). On the other hand, glucose, which packs the same caloric punch as fructose, suppresses activity and leads to a feeling of being full.
In the new study, Page’s team looked at effects in brain areas that process rewards. They had people drink cherry-flavored water sweetened with either fructose or glucose, and then they monitored the subjects’ brain activity inside a functional magnetic resonance imaging (fMRI) machine while presenting them with pictures of food. When people drank fructose, the food images produced more activity in the orbitofrontal cortex than when they consumed glucose. Activity in this brain region has been linked to increased motivation to seek out rewards, including food or drugs.
Before allowing the subjects to leave the fMRI machine, the researchers offered them the choice of either receiving a snack that day or money a month later. People who had just consumed fructose were more likely to forgo the delayed payday in favor of food compared with those who had just received glucose (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1503358112).
One strength of the study is the combination of this decision-making test with the brain imaging data, says Peter J. Havel, a professor of nutrition at the University of California, Davis, who was not involved with the study. “It’s connecting what’s happening in the brain with what people are actually doing.”
The study, Havel says, is limited to short-term effects of fructose versus glucose. He proposes longer studies that follow how much people actually eat outside the lab after consuming the two sugars.