As the palette of artificial sweeteners has grown and manufacturers have honed the skill with which they blend them to mimic sugar taste, debate has swirled around whether these sensory stand-ins really help people consume fewer calories and avoid weight gain.
New research adds another dimension to the uncertainty: It suggests that even when artificial sweeteners fool the taste buds, they still don't fool the ultimate arbiter of our appetites -- our subconscious brains.
The latest evidence for this comes from a brain scanning study performed in the Netherlands. Paul Smeets, a neuroscientist at University Medical Center Utrecht, used a technique called functional magnetic resonance imaging (fMRI) to measure brain responses in people sipping two versions of orangeade, one containing sugar and one containing a mix of four artificial sweeteners: aspartame, acesulfame K, cyclamate and saccharin.
The mixture of artificial sweeteners was concocted to match the taste of real sugar as closely as possible. And the sugary and artificial drinks were administered on different days -- making it harder for the tasters to notice any difference between the two. Subjects often guessed wrong on which drink was which. "They didn't know," Smeets says.
Yet the fMRI scans revealed consistent differences in how their brains responded.
Both sugar and the noncaloric sweeteners activated a brain region called the amygdala, which signals sensory pleasure. But only the sugared drink turned on a cherry-sized nugget of brain tissue in a region called the caudate.
That little nugget, Smeets concluded, seemed to represent an unconscious perception of calories -- assessed quite separately from the sweet taste.
"We think the brain can distinguish, even if the people themselves cannot distinguish, between a caloric and a noncaloric sweet drink," says Smeets, who presented his results at the Human Brain Mapping meeting in San Francisco in June.
Brain-imaging experiments sometimes draw criticism for producing little more than colored spots on a brain map -- high-tech Rorschach ink blots that researchers may over-interpret according to their own biases. But another study, published earlier this year, suggests that Smeets is onto something.
Edward Chambers, an exercise physiologist at the University of Birmingham in Britain, compared the effects of sugar and artificial sweeteners on peoples' ability to do hard aerobic exercise -- and he found some striking differences.
Chambers has previously shown that the mere taste of sugar can improve endurance in athletes who have fasted for several hours. If the athletes rinse their mouth with sugared water but don't swallow any, it improves their performance in an hour-long cycling workout by a small but consistent amount. The apparent promise that sugar will soon reach the bloodstream provokes the cyclists' brains to drive their legs harder -- the same way that the promise of a paycheck in the mail motivates a cash-strapped student to go shoe shopping.
But when Chambers tried the same experiment with a mixture of the artificial sweeteners aspartame and saccharin, he saw no such effect. Tasting the artificial sweeteners didn't improve cycling speed, even though they tasted sweet.
Tasting a nonsweet sugar, maltodextrin (whose flavor was masked with aspartame and saccharin so it tasted the same), did improve cycling speed.
"The sweetness is the conscious perception of the substance," Chambers says. "But there also appears to be this unconscious nutrient-sensing occurring" in the mouth. Athletes couldn't consciously distinguish the sugared and nonsugared drinks, but their brains picked up the difference.
Chambers also did fMRI scans of his athletes' brains. He found that the sugared mouth rinses strongly activated two reward centers in the brain, the anterior cingulate cortex and caudate, whereas the artificial sweeteners activated these brain areas only weakly -- similar to Smeets' results.
These studies heighten lingering questions about the usefulness of artificial sweeteners. Although some studies over the years have found that artificially sweetened foods and drinks reduce calorie consumption and weight gain, others suggest they might not.
"We have some idea that in the short term there's calorie savings" with artificial sweeteners, says Adam Drewnowski, director of the Center for Obesity Research at the University of Washington in Seattle. "But then people say that what you do today at lunch may not translate into saving pounds of body weight at the end of four weeks. So we need more studies."