Serotonin may be responsible for our love of protein.

The chemical, found mostly in the brain and digestive tract, is involved in the regulation of mood, appetite, and sleep in humans and other animals. It's also associated with feelings of happiness and is often released as a biochemical reward.

New research suggests the brain chemical may explain why humans seek out protein-rich foods when they're especially hungry.

Hungry fruit flies have feeding tendencies similar to humans. Instead of steak or cheese, when fruit flies are deprived of food they seek out protein the form of yeast.

The latest study, detailed in the journal eLife, suggests serotonin plays an important role in driving a hungry fly's preference for yeast over sugar.

The chemical reward may also influence lifespan.

When researchers blocked a single serotonin receptor, the fruit flies ate the same amount of food, but were less inclined to seek out protein when starved and lived roughly twice as long as the other fruit flies.

"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," Scott Pletcher, an associate professor of integrative physiology at the University of Michigan Medical School, explained in a news release. "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."

A dietary balance of protein and carbohydrates are important for animal health. The latest research suggests biochemical reward systems strongly influence an animal's ability to adhere to a balanced diet.

Though the human brain is much more complex than the brain of a fruit fly, researchers say their serotonin reward systems work in much the same way. Further research may help scientists understand how diet and biochemical rewards affect how humans age.

"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," said Pletcher.