What happens to our body when we are hungry, according to science

There hungerthe physiological one, tells us “Hey, I need new energy to make this body work!” and it is a fundamental one survival strategyso ancestral that it is regulated by multiple pathways redundantthat is, that they repeat and reinforce each other.

As confirmed in the study “Molecular Mechanisms of Appetite Regulation” published on Diabetes & Metabolism Journalwhat triggers this reaction is a sophisticated combination of nervous and hormonal stimuli that involves different actors: from the brain to the stomach; from the hunger hormone, ghrelin, to the vagus nerve. When the stomach remains empty and close to the usual meals, it is released ghrelin in the bloodstream: this reaches the hypothalamus, the hunger control centerwhich activates behaviors that lead to food seeking and reduced energy expenditure. Pressure-sensitive cells in the stomach signal when it is full, and the brain generates a feeling of satiety to avoid overeating.

The hunger control center is the hypothalamus

As the study explains “The Physiology of Hunger” – published in the magazine The New England Journal of Medicine – our organism likes balance and is in fact able to self-regulate in response to the external environment (we call it, in technical jargon, homeostatic balance). The director of this delicate balance is thehypothalamus: it makes us drink when we need liquids, it makes us sweat when it is hot outside, and when we consume more energy than we ingest, it pushes us to look for new energy, that is, food. Inside there are several interconnected nuclei, which we can imagine as specialized departments of a company. We are interested in arcuate nucleus (ARC), located in an area of ​​the brain where the blood-brain barrier is more permeable and therefore nutrients and hormones present in the bloodstream can pass more easily.

Two populations of neurons “live” inside the arcuate nucleus: one group makes you feel hungry (orexigenic), another makes you feel full (anorexigenic). The former are activated by binding to ghrelin and produce 2 neuropeptides, neuropeptide Y (NPY) and agouti-related protein (AgRP) which induce food-seeking behaviors, while at the same time reduce energy expenditure. Anorexigenic ones instead produce peptides that “switch off” hunger, pro-opiomelanocortin (POMC) and CART (cocaine-and amphetamine-regulated transcript).

What is the hunger hormone: studies on ghrelin

There ghrelin, also known as “hunger hormone“, is a peptide hormone (i.e. made from amino acids) produced and released by the ghrelin cells of the stomach. As reported by the study “Biochemistry, Ghrelin” published on National Library of Medicineghrelin toincreases before meals, anticipating the arrival of food. Activates orexigenic neuron receptors to stimulate the feeling of hunger and the search for food and inhibits anorexigenic ones. But not only that: it stimulates acid secretion in the stomach and tells the adipocytes to prepare to store fat.

Furthermore, it has receptors in it too areas related to memoryto the processing of images related to food and attention, falling within the complex circuit of reward and dopamine release. In fact, ghrelin can also be released in response to stimuli related to the pleasure of food (the so-called hedonistic hunger) and not just when we eat because we really need it.

At this point, when the stomach is empty (it takes 4 to 6 hours) the concentrations of glucose and nutrients begin to drop in the blood. Alarm: we have to eat again to recover our energy! At these signals of “raise hell”, the very thing is released ghrelinwhich travels in the blood until it reaches the Central Nervous System, precisely in the arcuate nucleus of the hypothalamus. Under fasting conditions, it is released motilinwhich stimulates the contraction of the gastric walls causing that well-known stomach rumbling.

Here it binds to specific receptors on orexigenic neurons (which stimulate hunger) “waking them up” to activate behaviors that induce the search for food. The “empty stomach” information also reaches the brain by another route. Through the vagus nervethe stomach “warns” the brain of its state: the visceral signals are first integrated into the brainstem and only subsequently they modulate the activity of the hunger centersincreasing the motivation to search for food.

The arrival of food and leptin: the satiety hormone

Just seeing or smelling our next meal, that the activity of orexigenic neurons may reducewhile anorexigenic neurons stimulate salivary secretion, the production of gastric enzymes for digestion and lubrication of the alimentary canal. I mean, we’re about to eat and the body preparesfilling us even before taking the first bite.

The problem is that if that morsel doesn’t arrive, the hunger neurons come back into activity at a gallop!

The study explains the entire process to us “The Biochemistry of Hunger Stimulating Hormone: Why Understanding This Cascade In Hypothalamus Is Beneficial”, published in the magazine Biochemistry & Physiology: when it is full, the stomach distends, activating mechanoreceptors (pressure receptors) which signal to the brain, via the vagus nerve, that there is no more space. The intestine secretes chemical messengers such as cholecystokinin (CCK), glucagon-like-peptide 1 (GLP-1) and the peptide YY (PYY) and the concentration of glucose and insulin in the blood increase. But it’s there leptinproduced by adipose tissue, the most powerful appetite suppressant hormone in the long term: activates anorexigenic neurons, thus stimulating the feeling of satiety.

Hunger as an evolutionary strategy: the link with dopamine

Dopamine, in biochemistry, means reward. As highlighted in a study from the University of Melboune, the close relationship between food, ghrelin, hunger and reward circuits may have had a fundamental role at an evolutionary level. Because we evolved with the uncertainty of food availability, when we had plenty of food available, the brain would go into mode farming: Eat as much as you can and put away for periods of famine, like a bear preparing for hibernation. Receiving a metabolic “reward” when eating was so important that it bypassed the physiological systems that tell us “ok, that’s enough, I have everything I need”. A salvation in the Stone Age… a damnation in the age of ultra-processed, high-fat foods.