Quirks & Quarks

It's all about the sugar fix: Eating too much sugar causes fruit flies to eat even more

Eating too much sugar diminished the sweet sense of taste in fruit flies, causing them to overeat

Eating too much sugar diminished the sweet sense of taste in fruit flies, causing them to overeat.

Fruit flies are often used in scientific experiments. Here a fruit fly is sitting on a grape. (FREDRIK VON ERICHSEN/AFP/Getty Images)
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Eating lots of sugar makes sweet treats taste less sweet - at least it does in fruit flies.

Scientists have shown that people with obesity have a diminished sense of taste when it comes to sweet flavours and think that this may contribute to overconsumption of sugary foods. But the reason for this diminished sweet sense has been unknown.

A new study from the University of Michigan found that fruit flies also experience this diminished sweet sense and it's caused by the way too much sugar inhibits the neurons that signal sweetness. With a dulled sweet tooth, fruit flies gorged on their sugary diets and became fat fruit flies.

While fruit flies aren't humans, the enzyme responsible for this taste change is also found in our taste receptors.

"The big question is whether or not we can affect our own sensory systems in order to curb those extra desires," said Christina May, a PhD candidate at the University of Michigan, in conversation with Quirks & Quarks host Bob McDonald.

Dulled Sweet Tooth

May and her team isolated the cause of this dulled sweet tooth with experiments on mutant fruit flies.

Fruit flies on the left are fed high-sugar diets. They are seen eating more often than the control flies on the right. (Christina May)

They tested genetically obese fruit flies to see if the taste change was caused by being overweight, but the fat flies were found to have a normal sense of taste.

They also tested genetically thin flies - flies who couldn't store fat and therefore would never become obese. After being fed a high-sugar diet, they didn't gain weight but their sweet taste sense did diminish.

They concluded that it wasn't obesity itself that was causing the dulled sense of taste for sweets.

To check if the diminished sense was happening because of the taste of sweetness, the flies were fed an artificial sweetener, but they too kept their normal taste senses.

"It's not just having a lot of sweetness in your food, it's also the fact that the sweet food has increased calories as well," said May.

With a dulled sweet tooth the fruit flies ate almost twice as much as normal fruit flies and became obese.

May is now researching why fruit flies overeat sweets when they can't taste sugar as well. The flies have a neurological reward system that responds to to sweet taste and she thinks that as the taste diminishes so does that reward - so they eat more to recover some of that reward.

Sensing Sugar

Just like us, fruit flies love sugar. They will actually reach out for sugar when it comes near with their mouthpiece, called a proboscis. That's how May found that the fat fruit flies had a diminished sensitivity to sugar — they wouldn't extend their proboscis like normal fruit flies.

Graphic of a fruit fly extending its proboscis towards sugar. This test is done to test the fruit fly's sense of sweet taste. (Julia Kuhl)

The taste neurons were changing with the increase of glucose in the flies' cells from the high-sugar diet. She said an enzyme in the neuron uses glucose metabolism to change the state of the neuron which causes the decrease in sweet sense.

"We got rid of some of the enzyme in these taste neurons and that caused the flies to not overeat," said May.

By manipulating the enzymes in the neurons, the fruit flies on a high-sugar diet regained their sense of taste for sweets and stopped overeating.

Can this be done in humans?

Fruit flies and humans have a lot of things in common. We both have meal times, we both love sugar and we both have the enzyme in the taste neuron that caused the diminished sweet taste sense.

While May hopes her research will help develop therapies to treat obesity, there's still more research to be done before scientists start manipulating taste neurons in humans.

"I think anecdotally everybody is pretty familiar with how much they want to keep eating sugar even when they shouldn't, but I will say that the anatomy is a bit different," said May, explaining that a similar test could be done on mammals to further study the effects of this particular enzyme on sweet taste function.

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