The pleasures and perils of protein: appetite and aging

Why do we seek out protein-full foods when we're running on empty? And what does that preference mean for the odds of living a longer life? A brain chemical may have a lot to do with both questions! 

If you’re a human who’s really hungry, a handful of nuts, a piece of cheese or a nice juicy steak may really hit the spot. If you’re a fruit fly, a nibble of yeast will do the trick.

In a new paper in the journal eLife, U-M scientist Scott Pletcher, Ph.D., and his team demonstrate the key role that the chemical called serotonin plays in the feeding habits and life spans of fruit flies. The paper's first author is Jennifer Ro, Ph.D., now at Harvard Medical School.

Serotonin is a chemical found in the human body. It carries signals along and between nerves, a neurotransmitter. It is mainly found in the brain, bowels and blood platelets. It is thought to be especially active in constricting smooth muscles, transmitting impulses between nerve cells, regulating cyclic body processes and contributing to wellbeing.

Pletcher and his team report that it appears to play a key role in fruit flies' strong tendency to seek out protein, not sugars, when they've been deprived of food for a while. In other words, it affects the value that flies place on protein at that time, which means that it's somehow tied to how the flies figure out which foods contain protein in the first place.

Not only that, but the brain-based reward that the flies got from eating protein appeared to influence how quickly the flies aged. When that reward was blocked, the flies ate just as much food as before in their normal diets, but lived far longer; in fact, they lived nearly twice as long.

The researchers made their discovery by manipulating the genes involved in the serotonin system, as well as manipulating the flies' access to different types of food using a special chamber they developed. Called the FLIC, or Fly Liquid-food Interaction counter, this device allowed them to continuously monitor food preferences for each micro-meal and to identify how and when flies were rewarded by a protein-rich diet.

"This work builds on previous findings that the perception of food modulates aging in much the same way as dietary intake, but the brain regions and systems involved in this have been unknown," says Pletcher. "We found that the serotonin pathway is important for interpreting the composition of the food, as well as the reward that drives consumption of the food."

Protein-rich diets have previously been found to lead to shorter lifespans, he notes. "These results suggest that serotonin is directly involved in this process, though we have not yet found the mechanism," he adds.

While it's far too soon to apply their findings to our understanding of human feeding patterns or longevity, Pletcher notes that the serotonin reward system in fruit flies is very similar to that in mammals including humans.

So are many other basic systems, which is what makes fruit flies such an important species to study because one scientific team can study hundreds of generations of them.

More research in fruit flies may help us understand just why protein seems the most appealing or causes a unique sense of reward. Says Pletcher, "This paves the way for future work to understand how the brain mechanisms that allow animals to perceive and evaluate food act to control lifespan and aging."

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