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Sensing Calories

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Our brains may crave calories, not just the taste of sweetness.


Transcript

The brain's appetite for calories. I'm Bob Hirshon and this is Science Update.

Artificial sweeteners may fool your taste buds, but not your brain. This according to a recent study by Duke University Medical Center and the University of Porto in Portugal. It involved genetically engineered mice that couldn't taste sweetness. Duke neurobiologist Ivan de Araujo says that these mice showed no interest in artificially sweetened water. But sugar water was a different story.

de Araujo:

They developed this robust preference for the drink that contained sucrose, even if there was no taste or flavor associated with it.

The sugar water also lit up their brains’ reward centers, just as a sweet flavor would if they could taste it. He says that if our brains also respond to pure calorie content, it may explain why people who use diet products sometimes overeat to compensate. I'm Bob Hirshon for AAAS, the science society.


Making Sense of the Research

Walk into any supermarket, and you could easily fill a cart with reduced-calorie foods, from diet sodas to low-carb tortillas. But as more and more of these products are sold, Americans keep getting heavier and heavier.

There are undoubtedly many reasons for this, some of which have nothing to do with diet food, including lack of exercise and increases in overall food portion sizes. However, some scientists have wondered if consuming reduced-calorie foods might have hidden drawbacks that can outweigh the benefits.

In reviewing this study, it's important to remember why sweet foods taste good in the first place: sugar contains calories, and we need calories to live. Throughout most of our evolutionary history, we haven't had access to shelves full of cupcakes and candy that could damage our health if we ate too much. Sugars in nature come from fruits and other nutritious foods, and since food in general wasn't always easy to come by, our bodies tell us to take it whenever we can get it. The same is true for animals, even though most animals don't have access to processed food even today.

In this experiment, the researchers genetically engineered mice so that they were “sweet-blind:” in other words, they couldn't taste sweetness at all. Normal mice strongly prefer sweetened water to plain water, whether it's sweetened with real sugar or artificial sweetener. Sweet-blind mice, on the other hand, show no preference for sweetened or unsweetened water—at least not at first. That suggests that taste is, in fact, their most obvious clue as to whether or not food has calories.

Yet over time, something interesting happened. The sweet-blind mice that could choose between water with real sugar and plain water gradually developed a preference for the calorie-packed sugar water, even though it tasted exactly the same as regular water. In contrast, the sweet-blind mice that were offered artificially sweetened water and plain water never developed a preference, no matter how long they had access to the two water sources.

Furthermore, the sugar water triggered increased dopamine activity in the sweet-blind mice's brains. Dopamine is a brain chemical associated with pleasure and reward, and normal mice's dopamine pathways also react to sugar water. On the other hand, artificial sweeteners triggered dopamine pathways in normal mice only. That suggests that normal mice were reacting to the taste and the caloric content of the water, while sweet-blind mice were responding only to the calories.

The fact that you can have one without the other suggests that both taste and calories have independent rewards. If the same is true in people, it could mean that when we eat foods with artificial sweeteners, we may not get the same kind of subconscious pleasure that we would get from a full-calorie version. That might make us more likely to seek out other high-calorie foods later. In fact, a recent study at Purdue University found that mice actually gained weight in the long run eating artificially sweetened food. The research suggests that dieters should make sure that when they switch to artificial sweeteners, they don't make up the calories elsewhere.

Now try and answer these questions:

  1. What are “sweet-blind” mice? Why were they used in this study?
  2. Why did the researchers conclude that the brain has a separate sense for calories? How many different experiments were necessary to support this hypothesis?
  3. Suppose that instead, the researchers had found that sweet-blind mice develop a preference for neither sugar water nor artificially sweetened water compared to plain water. What would that suggest?
  4. Suppose instead that the sweet-blind mice developed a preference for both sugar water and artificially sweetened water? What might those results indicate?

You may want to check out the April 25, 2008, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: the antimicrobial powers of alligator blood, getting blood from a fruit fly, mental health and blood clotting, and new insights into hardening of the arteries.


Going Further


For Educators

The BBC News article 'Diet' foods weight gain puzzle reports on the Purdue study mentioned above.

Seeing, Hearing, and Smelling the World, from the Howard Hughes Medical Institute, provides detailed information on the five human senses.


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