Have scientists found a way to 'switch off' protein hunger?

We all know what it's like to shed those extra kilos, only to find ourselves gorging on burgers once the hunger pangs strike. But now, there could be hope for dieters after scientists identified the brain cells that control protein-craving.

The researchers led by Johns Hopkins University's Mark Wu identified the brain cells in fruit flies that regulate protein hunger and were able to control those cells, affecting what the animals ate.

To study protein hunger, the team starved flies of yeast (the animal's protein source) for one week. Afterwards, they discovered that the flies ate more yeast and less sugar than flies that ate a control diet.

"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 researcher Janet He. "A better understanding of the basic mechanisms that regulate the consumption of different nutrients may help to provide clues to addressing the obesity epidemic."

Using novel genetic tools, the team identified a specific circuit, a set of brain cells that communicate with one another, which controls protein-seeking behaviour. When the circuit was stimulated, flies ate more yeast than normal. In contrast, when the researchers turned off the circuit, the flies ate less yeast. The cells in the circuit were more active, which was demonstrated by increased firing activity, when the flies were starved of yeast. Turning the circuit on or off did not affect the animals' general hunger or thirst.

The researchers also discovered that the circuit serves dual functions in regulating feeding. Following protein starvation, this circuit promotes protein feeding and also simultaneously suppresses sugar intake. One branch of the circuit is activated when the flies are deprived of protein, leading them to seek more protein and increase their consumption of that nutrient. Meanwhile, the other branch of the circuit acts to reduce interest in eating sugar when the flies are protein-starved.

When Wu and his colleagues closely examined the brain circuit during protein starvation, they discovered structural changes in the brain cells, but only in the branch controlling protein hunger. Some of the changes included elongated processes and an increased number of active zones, which are the sites where brain cells release chemicals to communicate with their neighbours. The part of the circuit regulating sugar intake did not show any changes.

These findings suggest that the structural changes in the protein branch of the circuit are responsible for the long-lasting increase in protein-seeking behaviour.

"Adult flies usually have a sweet tooth, but when they are starved of protein, the brain makes it a priority to find this nutrient. Once they finally get some protein, the blockade on sugar feeding lifts but the flies still continue to be interested in eating protein," said Wu. "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."

They are now trying to identify specific molecules involved in regulating protein hunger and plan to examine protein-specific hunger in mammals. They noted that this research may eventually have implications for obesity research, because the amount of protein in the diet significantly affects caloric intake.

The study is published in Science.