How fake sugar can lead to overeating

People trying to lose weight may turn to fake sugars — or foods that contain them. But these sugar substitutes have provoked controversy in the past few years. One reason: Studies have shown that they can actually boost someone’s appetite. A new study now confirms that — and points to why.

In the new study, scientists showed that eating a popular artificial sweetener made fruit flies and mice hungrier. This study is important because “nobody really knew” what happened in the brain or elsewhere to fuel that hunger, says Herbert Herzog. He works at the Garvan Institute of Medical Research in Sydney, Australia. As a neuroscientist, he studies how the brain works. In the new study, his team shows how the brain responds to a diet that contains artificial sugar. If people respond the same way — and Herzog’s team thinks they will — long term consumption of fake sugar could encourage people to overeat.

The new findings appear in the July 12 issue of Cell Metabolism.

Confusing the brain’s calorie-counter

The researchers fed a diet of natural sugar and yeast to two groups of fruit flies. One group ate nothing else. The other group also got a bonus. For five days, its diet was sweetened even more with sucralose. This is the fake sugar sold under the name Splenda.

On day 6, the second group went back to eating meals of only sugar and yeast. Throughout the day, the scientists carefully measured how much each group of flies ate. Fruit flies who had recently been eating the artificial sweetener now ate 30 percent more than the flies that had eaten the normal diet all along. However, a few days after sucralose was removed from the diet of those flies, they returned to eating normal amounts of food.

To understand how sucralose works, the researchers disabled some genes in the fruit flies. The genes they knocked out play a huge role in determining how much and what types of foods an organism eats. Afterward, they fed these flies food laced with sucralose. Flies with the disabled gene did not overeat. This showed that the fake sugar must have messed up the action of the healthy genes, causing their hosts to overeat.

The new data suggest that when these genes don’t work properly, the brain gets a bit confused. The flies’ brains have evolved to associate food that is sweet with a particular number of calories. The sweeter something is, the more calories it tends to have. But with fake sugars, the sweet taste comes with no calories. “When you take artificial sweeteners, the brain somehow gets fooled for a short period of time,” says Herzog. “But then it realizes that there are no calories coming in.” Because it can no longer trust its sweet meter to gauge how many calories are on the way, the brain starts to behave as if the animal is starving. And this urges that animal — here, a fruit fly — to grab more food.

The scientists were also curious to see if the same thing would happen in mammals. So they repeated the experiment in mice. They again fed some a diet with sucralose for a week. And compared to those not getting the fake sugar, these mice consumed more food after that artificial sugar was taken out of their food.

People’s brains and bodies are more complex than flies and mice. Still, Herzog expects this trend to occur in people as well. And that could spell trouble for potential dieters. Indeed, he notes, “Most people are not aware that they are eating artificial sweeteners in processed foods.” He thinks people need to be more aware that fake sugars may have unintended effects, including tricking the brain into wanting to eat more.

Terry Davidson works at American University in Washington D.C. This neuroscientist studies how the brain regulates what people eat. Davidson, who was not involved with this study, says the new findings add to “serious concerns about the widespread use of artificial sweeteners.” Considering the results of this and other studies, he concludes that “the best advice for most people may be to avoid starting the use of artificial sweeteners.”