Fake Sugar Can't Fake Out the Brain

May 6, 2008 — -- So you want to indulge in that sugar-coated doughnut because it tastes so sweet? You probably would want it just as much if it didn't taste sweet at all.

Medical researchers at Duke University have found that the brain has a "sixth sense" for finding calories even if the taste for sweets is taken away.

The research, published in a recent issue of the journal Neuron, is the latest in a long list of studies showing just how tricky the brain can be when it comes to shedding those extra pounds.

Even when we think we're doing right, we're often doing exactly the wrong thing, because the brain is an awful thing to waist, if you'll pardon the pun.

For example, an ongoing series of experiments at Purdue University has shown that artificial sweeteners may be so confusing to the brain that they may contribute to weight gain, not weight loss.

Researchers there found that rats given yogurt with zero-calorie saccharin gained more weight, put on more body fat and didn't make up for it by cutting back later, compared to rats given yogurt sweetened with ordinary table sugar.

The study, published in Behavioral Neuroscience, suggests that artificial sweeteners trick the brain into thinking that a truckload of calories will follow soon, thus increasing hunger and contributing to the condition known to medical professionals as "pigging out."

However, some other studies have reached very different conclusions about artificial sweeteners, so more work needs to be done on that one.

The Duke study is one of the more intriguing findings in this very fuzzy arena. Ivan de Araujo and several colleagues experimented with mice that had been robbed of the taste receptor cells that make it possible to detect the taste of sweetness.

These genetically altered "sweet-blind" mice weren't fooled by an artificial sweetener. They knew it didn't have enough calories, according to the researchers, so they selectively dined at a "sipper" that offered high-caloric sugar water while largely ignoring a sipper with a noncaloric sweetener.

The researchers concluded that it wasn't the taste of sweets that triggered the release of dopamine, which turns on the reward circuitry in the brain. It was calories, independent of taste.

The researchers completed a series of different tests, using both genetically altered and normal mice, to make sure of their findings. They found, for instance, that neurons in the food-reward part of the brain were activated by calories, not taste.

Interestingly, neither the sweet-blind mice nor the normal mice figured out instantly which sipper to visit. It took about 10 minutes for all of them to determine which sipper offered the most calories, and they spent nearly three times as much time at the high-caloric sipper as at the one with the artificial sweetener.

"We showed in this study that brain-reward dopamine systems respond to the caloric value of sucrose [the real deal] even in the absence of taste receptor signaling, palatability or changes in flavor evaluation," the scientists concluded. Noncaloric sucralose didn't cut it.

Some scientists, including cardiologist Arthur Agatston, creator of the South Beach diet, argue that eating can make you hungry. That's not to suggest that we all stop eating. Agatston says that some foods, like that sugar-coated doughnut, actually trick the brain into demanding more food, not less.

Pure sugar, and various carbs that turn to sugar once they are consumed, like white bread, tell the pancreas to produce the hormone insulin to get the sugars out of the blood and into the organs where they are needed.

If a lot of sugars are consumed, a lot of insulin is released, which reduces the blood-sugar level so quickly that "new cravings are created, requiring more quick carbohydrate fixes," Agatston claims.

His solution: limit bad carbs, like white potatoes and highly processed foods that have had their nutrients beat out of them, because those foods are absorbed quickly, causing a sudden rise in insulin, thus triggering the desire for more food. Instead, he says, dine on good carbs, like broccoli. And skip the doughnut.

Not all experts agree with Agatston, of course, except for the part about skipping the doughnut.

One reason there is so much debate on this terribly important issue of weight control is the lack of a clear understanding of how the brain reacts to various stimuli. After tons of research, for example, scientists are just now learning that the brain yearns for calories, independent of taste.

That's true among mice, at least, and it's almost certainly true for humans as well.

Lee Dye is a former science writer for the Los Angeles Times. He now lives in Juneau, Alaska.