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Interviewee: Albino Oliveira-Maia,
Scientists have known for a long time that sweet taste activates pleasure pathways in the brain. So, does consuming diet drinks really satisfy you? No, says Duke University neuroscientist Albino Oliveira-Maia; your brain can sense the missing calories.
“Satisfaction that we take from eating and from food depends on a second pathway other than the taste,” says Oliveira-Maia, “This pathway is a result of the caloric content, of the calories that are in the food that we are eating.”
He gave sugar water or water sweetened with sucralose (the non-caloric sweetener Splenda®) to specially-bred mice that could not taste sweetness. They used probes that recorded brain activity and matched that to special “lickometer” reading that recorded mouse licks from liquid dispensers. As reported in the journal Neuron, the researchers saw a response in the reward centers of animals that got sugar, but not in those fed the “diet” water.
The graphs pictured represent signals from the nucleus accumbens, a region in the brain associated with pleasure and reward. In the upper left is an overlay of many signals from one probe. The plot at the right is a 3D map of the isolated signals (yellow) and the activity of other probes (grey.)
Oliveira-Maia says that “the dopamine system, which is involved in all kinds of reward, it’s involved in taste, people have known for many years now that it’s involved in addiction. It’s also involved in how calories are impacting the brain reward system.”
He says sweetness is a quick way for our brains to judge the value of a food, but once it makes it to our stomach, another set of signals tells us the calorie content. “If we think of it from an evolutionary point of view, that’s what we’re looking for when we’re eating, we’re looking for calories.”
What does Splenda® have to say?
McNeil Nutritionals, the makers of Splenda®, released a statement in response to the Duke University study. They said, “It’s important for living things to have mechanisms to detect calories. This programming helps assure the consumption of foods that provide sustenance and energy for survival.”
They also pointed to a 2007 study they funded that showed kids who switched to their non-caloric sweetener and increased their exercise level did a better job of maintaining a healthy weight than the control group.
Dieting makes you fat?
A study from Purdue University published in Behavioral Neuroscience in February 2008, showed that rats fed artificially-sweetened yogurt ate more calories and gained more weight (and fat) than rats fed yogurt sweetened with real sugar. And, later on, they ate more of a new, calorie-rich sweet food they were offered, presumably because on some level they forgot that sweet things can satisfy them and provide calories.
One theory the researchers are working with is that eating artificially sweetened foods disrupts the “sweet taste = calories” association that we expect from our food. They think that could throw off our natural moderation of eating and weight gain.
See a note on the Splenda® website addressing the issue.
What about humans?
A study published in the journal NeuroImage in February of 2008 showed a difference between MRI scans of people who drank sugar water versus those who drank sucralose-sweetened water.
Only the sugar group showed activity in a specific portion of the midbrain that is associated with pleasantness. An interesting twist is that consciously, the subjects did not recognize the difference, but on another level, their brain knew when they drank the real sugar.
Researchers continue to ask if it will one day be possible to help people eat fewer calories and still feel satisfied. That, says Oliveira-Maia, is the million-dollar question.
The research was published in Neuron, March 2008. It was funded by NIH and Philip Morris. Dr. Oliveira-Maia is also supported by a grant from the Government of Portugal. Also referenced in the article: Pediatrics, October 2007; Behavioral Neuroscience, February 2008.
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