DIETING
July 27, 2016

How Diet Drinks Make You Fat

No-calorie sweeteners can wreak metabolic disaster. They fool the tastebuds, but not the brain.

Apparently it's easier to fool the taste buds than to fool the brain. That's why drinking diet soda may end up making you eat more. At least that's what happened to the mice and flies in a new Australian study.

Artificial sweeteners can make you hungry, make you eat more and can increase belly fat, other studies, on humans, have suggested. The evidence has been showing up in studies for several years, yet there has been no good explanation for why this might happen.

When sweetness is detected, the brain expects energy to follow. When it doesn't, as happens after a drink of diet soda, the brain compensates by signaling hunger.

The researchers began their study by looking at fruit flies. After eating a diet laced with the artificial sweetener sucralose (Splenda) for more than five days, flies began to consume 30% more calories when returned to a normal diet with no sucralose.

“When we investigated why animals were eating more even though they had enough calories, we found that chronic consumption of this artificial sweetener actually increases the sweet intensity of real nutritive sugar, and this then increases the animal's overall motivation to eat more food,” said study co-author, Greg Neely, a professor on the University of Sydney Faculty of Science.

It seems that when sweetness is detected, the brain expects energy to follow. When it doesn't, as happens after a drink of diet soda, the brain compensates by signaling hunger, though the effect may take some time to kick in.

The study is the first to characterize how artificial sweeteners can stimulate appetite, with researchers identifying a complex neuronal network that responds to artificially-sweetened food by telling the animal it hasn't consumed enough energy — basically, a starvation response that makes food taste better when you are hungry.

One element of the network is neuropeptide F, a neurotransmitter found both in the brain and the body. Mammals, including humans, have a similar peptide called neuropeptide Y.

The researchers also found that artificial sweeteners promoted hyperactivity, insomnia and decreased sleep quality — behaviors consistent with a mild starvation or fasting state. Similar effects on sleep have been previously reported in human studies.

The research then continued at another Australian lab, one that performed a similar study on mice. Like the flies, mice that consumed a sucralose-sweetened diet for seven days displayed a large increase in food consumption after being switched back to a normal diet. The effect was partially mediated by neuropeptide Y.

Neuropeptide Y has many known effects. Among them are increasing food intake and storage of fat. It is inhibited by leptin, a hormone that decreases hunger.

The researchers also found that artificial sweeteners promoted hyperactivity, insomnia and decreased sleep quality — behaviors consistent with a mild starvation or fasting state.

It's not known whether artificial sweeteners will operate in people as they did in the study. Humans — even humans with horrible eating habits — eat a much more complex diet than fruit flies and mice do.

Yet the new study adds evidence to earlier research suggesting that artificial sweeteners have definite metabolic effects — they are not inert and don't offer the free ride dieters hope.

The study appears in Cell Metabolism.
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