Limited access to mealtimes has led experimental rats to increase their appetite and food intake, according to a new study from the University of South California. The likely reason: the hunger hormone called ghrelin.

The research, published in the journal eLife, found that rats with restricted feeding schedules learned to eat more as assisted by ghrelin.

Lead author Scott Kanoski and the team probed the role of hormones and various signals and connections, potentially helping researchers in developing new weight loss therapies.

"We are looking deep into the higher order functions of the brain to unpick not just which hormones are important for controlling our impulses but exactly how the signals and connections work," Kanoski said in a press release.

Increased food intake in rats

The rat subjects, once learning of their limited access to food, increased their food consumption until it doubled. Mealtimes on some days were kept to an everyday four-hour window, followed by 20 hours without food.

Ghrelin allowed the rats to decrease their feelings of fullness, leading to gradually increased food intake.

Kanoski considered it an adaptive response to limited access to food, but mourned that it no longer applies to Western diets. "[Here] instead we need to find new ways to help us fight some of the feeding responses we have to external cues and circadian patterns," he said.

The findings revealed that ghrelin communicates with the central nervous system to regulate food intake, particularly with neurons from the hippocampus to stimulate appetite and allow large food amounts to be consumed in a limited period of time.

The neurons then "speak" to the hypothalamus to create orexin, a molecule that further promotes excess eating or hyperphagia.

Ghrelin's role in eating behavior

Previous studies found that ghrelin signals to the hippocampus to raise the food intake of rats when there is a visual signal associated with meals - in humans it could be a fast food billboard advertisement or a mere vending machine. These trigger "a cascade of signals that can be hard to resist," according to the researchers.

Apart from having circadian cues influence eating behavior more, ghrelin can increase the role of external cues in increasing food consumption.

In addition, ghrelin was discovered to increase the rate of nutrient passage through one's system, ideally slow enough to lengthen the feeling of fullness.

The team is currently studying how to decrease the effect of ghrelin through genetic suppression of its receptor in the hippocampus. This process is expected to disrupt the neurochemical signals allowing consumption of massive food quantities.

Kanoski stressed the importance of this research now that over one-third of Americans are classified as obese, with another third overweight.

"[W]e feel we have an obligation to help identify new ways to reduce the burden on society and on our healthcare systems," he said.

Photo: Juan Ramirez | Flickr

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