Identified a hormone that would hinder weight loss. It is the growth hormone (GH, i.e. Growth Hormone), which the scientific world has known for decades, but which has only recently been understood to have had an impact on weight loss.
What is growth hormone?
Growth hormone is crucial in bone development and organ and tissue growth. Researchers at the University of São Paulo in Brazil have discovered, however, that they also play another role, so far completely unknown: it is responsible for conserving energy during weight loss.
It is a physiological mechanism, very important especially for animals that live in nature: in case of difficulty finding food, in fact, the organism protects them by preserving their energy supplies as much as possible.
But what, for animals, is a defense tool, can become an obstacle for those who want to lose weight or do not want to take it back after following a diet.
The role of hormones in weight loss
The scientific world was already aware of the role of a hormone, leptin, in the body’s response to weight loss. Fat cells produce leptin, which inhibits the sense of hunger. When we lose weight, leptin levels in the blood decrease and this leads us to feel more hungry. Some people develop leptin resistance: this means that they no longer respond to the stimuli of this hormone and experience hunger much more often.
As Professor José Donato Junior, who led the research, explains, “leptin was considered the main hormone responsible for energy conservation when we are hungry”.
At least until now. Indeed, the new study suggests that growth hormone (GH) also plays an essential role.
How GH receptors work
“The hormone receptors GH – explained José Donato Junior – are present in large quantities in muscles and tissues, in the liver and in the organs involved in the growth metabolism. But we also found it in the brain: this is something new”.
And, while leptin levels decrease if caloric intake decreases, GH levels increase.
The researchers identified an abundant presence of GH receptors in the hypothalamus, which has, among its functions that of regulating energy homeostasis, that is the balance between calories expended and calories consumed.
In the hypothalamus, a small group of neurons produces peptides, related to the Agouti protein (AgRP): when these neurons release the AgRP, the appetite increases, leading the body to retain its energy reserves even more firmly.
The study, published in the journal Nature Communications, allowed us to demonstrate that the HG hormone receptors in the hypothalamus have the ability to activate these neurons, triggering the release of AgRP.
The study was conducted on a genetically modified strain of mice, i.e. deprived of the AgRP specific GH receptor (AgRP GHR KO mice).
In a series of experiments, researchers deprived mice of food and evaluated their energy expenditure.
The control mice, which had the GH receptor, responded to reduced food by reducing their energy expenditure.
In AgRP GHR KO mice, however, the decline in energy expenditure was much less pronounced. As a result, these mice lost more weight during the study. Loss of high energy density fat accounted for most of this weight loss, but there was also a loss of lean mass, which included muscles, bones, organs, tendons and fluids.
In another experiment, the researchers gave non-private mice of the GH receptor a drug called Pegvisomant, which blocks GH receptors.
Once again, after depriving them of food, the energy expenditure of these mice decreased, but significantly less than they did in mice that had not received Pegvisomant.
Conclusions: new scenarios for the treatment of obesity
“GH – concluded Donato – is therefore not only involved in the metabolism of growth but also influences the metabolic responses that lead the body to conserve energy when we are hungry or on a diet “.
In other words, we found that weight loss triggers an increase in GH levels of the hypothalamus, which activates AgRP neurons, making it more difficult to lose weight and intensifying the sense of hunger. Exactly the same function performed by leptin.
The authors concluded that the GH hormone does not seem to play a significant role in the energy balance when animals have the opportunity to feed themselves adequately. “Instead, it signals an energy deficit to the brain, triggering neuroendocrine responses to conserve the body’s energy stores.”
The authors hypothesize that this could be the reason why weight loss interventions based exclusively on leptin are ineffective: they act, in essence, only on a part of the mechanism.
This opens up interesting perspectives: researchers are in fact convinced that the compounds that act on GH receptors could represent a promising approach to facilitate weight loss and improve the effectiveness of treatments for obesity.