While a good amount of work has been done examining the neurochemistry of cravings, addiction and hunger signaling, less research has been conducted into nutrient-specific hunger. Adding to that burgeoning body of knowledge, researchers have figured out how protein cravings work in the brains of fruit flies. What's more, they were able to switch them on and off.

In the study, researchers at Johns Hopkins University in Baltimore eliminated yeast from the diets of fruit flies for a week. When the yeast – which had been the flies' only source of protein – was reintroduced, the bugs ate more of it than sugar. What's more, the researchers discovered that there were physical changes in brain cells related to the protein starvation, which led to a permanent increase in protein-seeking behavior.

"Flies have been a great model system for brain research so we can learn a lot about how our own brain circuits work by peeking inside the heads of flies," said Janet He, program director at the National Institute of Neurological Disorders and Stroke, which funded the study. "A better understanding of the basic mechanisms that regulate the consumption of different nutrients may help to provide clues to addressing the obesity epidemic."

Beyond observing the flies' feeding behavior, the researchers were also able to identify a brain circuit involved with protein-seeking behavior. When that circuit was stimulated, the flies ate more yeast; when it was turned off, they ate less. In neither case did the circuit affect overall hunger.

The green areas indicated the protein-specific craving pathways in the brain(Credit: Qili Liu, Ph.D., Johns Hopkins University)

Interestingly, it was also discovered that the brain circuit had another function. When it was active after protein starvation, it not only increased protein feeding, but also suppressed sugar consumption.

"Adult flies usually have a sweet tooth, but when they are starved of protein, the brain makes it a priority to find this nutrient," said Dr. Wu. Once they finally get some protein, the blockade on sugar feeding lifts but the flies still continue to be interested in eating protein. In this way, the circuit we identified promotes a single-minded focus on eating protein when the animal is protein-starved, but also allows for more flexible eating patterns with a continued preference for protein, when the need for protein is less."

Wu and his team say the research could one day help fight obesity in humans, as regulating the boosting the amount of protein consumed while lowering total calories from sugar could have a dramatic affect on weight.

The study has been published in the journal Science.

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