The Hormones of Hunger and Satiety

Weight loss is a much-discussed topic in scientific literature today.  This is for good reason: according to Dr. Jen Beshel, approximately one-third of the global population is overweight or obese.  While trendy diets may dominate the market, hunger and satiety are ultimately controlled by the hormones ghrelin and leptin.

Dr. Beshel discusses the effects of leptin at length in her research and in her talk in the classroom.  Leptin is associated with the satiety response, and obese mice in lab studies were found deficient in it, despite it being produced in fat cells.  Beshel’s research in her paper A Leptin Analog Locally Produced in the Brain Acts via a Conserved Neural Circuit to Modulate Obesity-Linked Behaviors in Drosophila examines this phenomenon in fruit flies, or Drosophila.  Fruit flies possess chemical agents, unpaired 1 and unpaired 2, produced respectively in the brain and in fat tissue, which have similar functions in flies to leptin in mammals.  Knocking out unpaired 1 in fruit flies, the leptin analog produced in the brain, caused them to behave as though they were starved, even when they had just been fed. Flies without unpaired 1 ate more, were more attracted to the smell of food, and, perhaps most notably, had higher body mass than wild type flies.  The difference in weight was moderate when flies were fed a neutral diet, but it increased significantly when they were fed a high-fat diet intended to simulate the diet of a modern person.

In contrast, ghrelin is a hormone that is produced in the stomach and causes the feeling of hunger.  A study reported by Scientific American “Hunger Hormone” Ghrelin Aids Overindulgence suggests that its effects on humans is comparable to those of a lack of unpaired 1 in fruit flies.  Scientific American reports a study in which people were conditioned to “associate random images with the smell of food.”  Some subjects were given ghrelin, and some were given a placebo. When subjects’ brains were scanned later, the reward centers of ghrelin subjects’ brains were activated when they looked at conditioned images.  Further, the subjects given ghrelin rated the images associated with food smells more pleasant than the placebo subjects did.

In spite of the rather disheartening research, though, Dr. Beshel suggests that everything is not fixed in place.  The increased weight of flies in her lab could be reversed with a supplement of human leptin. While it is best to make healthy decisions for oneself, genetic elements predisposing people to obesity may be subverted as science continues to lead us to a healthier future.

Works cited

Beshel, J., Dubnau, J., Zhong, Y., (2017). A Leptin Analog Locally Produced in the Brain Acts via a Conserved Neural Circuit to Modulate Obesity-Linked Behaviors in Drosophila.  Cell Metabolism. 25, 208-217.

Hopkin, K., (2018).  “Hunger Hormone” Ghrelin Aids Overindulgence.  Scientific American, Scientific American https://www.scientificamerican.com/podcast/episode/hunger-hormone-ghrelin-aids-overindulgence/