Your Brain on Fructose

Your Brain on Fructose Photo Credit: Lauri Andler (Phantom) [GFDL or CC-BY-SA-3.0], via Wikimedia Commons

A brain imaging study supports growing evidence linking fructose to weight gain.


Fructose and the brain. I’m Bob Hirshon and this is Science Update.

Fructose, a sugar used in many processed foods, may fail to fill us up and even make us hungrier. This according to a brain imaging study by University of Southern California endocrinologist Kathleen Page and her colleagues. Page’s team found that after people drank a glucose beverage, parts of the brain associated with food-seeking became less active. But fructose didn’t have that effect.

In addition to that we saw that fructose did not cause feelings of fullness, whereas the participants reported an increase in feelings of fullness after the glucose drink.

The study adds to a growing body of evidence linking fructose consumption to overeating and obesity. Page notes that our bodies may be tuned to respond mainly to glucose, since that’s the dominant sugar in our bloodstream. Furthermore, back when humans got fructose mostly from fruits and vegetables, wanting more of the same wouldn’t have been a bad thing. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

Look at almost any processed food package, and there's a good chance you'll find “high-fructose corn syrup,” “corn sweeteners,” or just plain “fructose” in the ingredients. While fructose is a naturally occurring sugar that lends sweetness to fruits, root vegetables, and honey, it's also become a widely used commercial sweetener, especially in the United States, where it's replaced table sugar in many products due to its low cost. 

The increasing use of fructose in foods has roughly paralleled the rise of obesity. The same can be said of many other trends, and a primary tenet of science is “correlation is not causation” (in other words, just because two things tend to happen together doesn't mean one causes the other—like picnic cancellations and people carrying umbrellas.) And if fructose is making us fat, it may not be doing so any differently than other kinds of sugar, or extra calories, for that matter.

Still, there's a growing contingent of scientists who worry that fructose may promote obesity differently and more intensely than common table sugar. One hypothesis, which has some support, is that fructose doesn't make you feel full in the same way that glucose does. This study adds credibility to that hypothesis.

In this study, published in the Journal of the American Medical Association, the researchers used functional magnetic resonance imaging (fMRI) to observe changes in brain activity before and after participants drank a sugary drink. Some participants drank a glucose beverage, while others drank a fructose beverage with the same caloric content. The fMRI images reveal how much blood flows to different parts of the brain—an indicator of which parts are working harder at that particular time.

They found that the glucose drink suppressed activity in parts of the brain like the hypothalamus,  which regulate appetite and feelings of motivation and reward. The decreased activity suggests that people were feeling satisfied and no longer driven to seek food. The fructose drink did not produce that decrease in activity. Moreover, the subjects themselves reported feeling more full after consuming the glucose drink than the fructose drink. Finally, consuming fructose was associated with lower levels of insulin, a hormone that tells the brain when you've had enough to eat.

The results suggest that our bodies may not respond to fructose in the same way that we respond to glucose: we may be “tuned” to feel satisfied only after consuming glucose. This makes sense, if you consider that glucose is the primary sugar that circulates in our bloodstream. What's more, throughout most of human history, when we weren't scarfing down processed, fructose-sweetened cookies, crackers, and soda, the fructose we did consume came from fresh fruits and vegetables. And being driven to eat fruit wouldn't have had nearly the same impact on our weight. In fact, at times when fresh fruit was harder to come by, it may have helped us nourish ourselves by consuming as much as we could when it was available. 

Now try and answer these questions:

  1. What are fructose and glucose? Why are we consuming more and more fructose lately?
  2. What was the hypothesis these researchers explored?
  3. What did the brain images reveal? Why was this significant?
  4. What other evidence helped back up the conclusions from the brain images?
You may want to check out the January 4, 2013, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: Why we might be better tree climbers than we think, what ancient wells can tell us about Neolithic technology, plants that sense pests, and exploring the growing link between fructose consumption and obesity.
Learn more about the science behind hunger and weight gain by exploring the Science Updates Artificial SweetenersSensing Calories, and Time-Restricted Eating.

Going Further

For Educators

Learn more about the science behind hunger and weight gain by exploring the Science Update lessons Artificial SweetenersSensing Calories, and Time-Restricted Eating.

To learn about fMRI and other forms of brain imaging, see the Science NetLinks lesson Watch Your Thoughts! Diagnostic Imaging and the Brain.

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