Hunger and fullness: the game of hormones
Hunger is not merely a psychological phenomenon — it is a precise symphony of chemical signals. Understanding how the body swings between hunger and fullness means understanding why some foods keep you full for longer, while others leave you hunting for the counter again half an hour later.
Hunger starts changing within the first minutes of eating
Satiety signals don’t kick in only once the food has been digested. They begin as early as 15 minutes after the first bite. As soon as the stomach starts to stretch and nutrients reach the small intestine, the body sends the brain its first notifications: the window is starting to close.
In the gut, several satiety hormones are produced on contact with enzymes: cholecystokinin (CCK), enterostatin, peptide YY (PYY), glucagon-like peptide-1 (GLP-1) and gastric inhibitory peptide (GIP). They all bind to receptors in the hypothalamus — that is where the decision is made on whether you are full or not.
CCK is released mainly when fats and protein are broken down, and it slows the further digestion of food. That is why fatty, protein-rich foods are digested slowly. Free fatty acids are released more slowly than glucose — and free fatty acids are exactly what act as satiety signals. Studies on CCK and enterostatin talk about “delayed” satiety. This also explains one widespread misconception: fats and carbohydrates are not actually digested simultaneously, which makes the arguments against combining them more complicated in practice than they seem at first glance.
Peptide YY (PYY) is a relatively recently discovered hormone that, among other things, blocks the formation of the hunger hormone NPY and is released in proportion to the total caloric content of the food eaten. Protein-rich food triggers PYY release particularly strongly — which is why it is also called an “anorectic” hormone. A simple truth: it produces a feeling of fullness and ends the desire to eat.
GLP-1 and GIP belong to the incretins. They speed up insulin release even before blood sugar actually starts to rise — and, like CCK, they delay digestion and give the brain a satiety signal.
Glucose and hunger: the brain’s energy problem
Once food has been digested, a large part of it reaches the blood as glucose. The portal vein fills with glucose and stimulates receptors that signal via the vagus nerve: energy is available, that’s enough.
Glucose is the brain’s primary fuel. Its absence is one of the most powerful hunger signals there is. Despite all the negativity about carbohydrates lately, blood glucose level is probably the single most important factor in whether we feel hungry or not.
Blood glucose level stays more or less stable in the body — even during long fasting. To achieve this, the body uses glycogen stored in the liver and, when needed, also converts protein into glucose. From the body’s standpoint, supplying the brain continuously is priority number one.
Insulin is also a satiety hormone. The pancreas releases insulin so that glucose can be absorbed, and this insulin in turn stimulates cells of the central nervous system that relay the satiety signal to the hypothalamus. Insulin circulating in the blood essentially means: you’ve eaten. Between meals, free fatty acids in the blood — in addition to glucose and liver glycogen stores — also help maintain the feeling of fullness.
Which nutrient beats hunger for the longest?
The most popular method of investigating this: give test subjects foods with the same caloric content but different composition, and see who feels full for longer.
Protein has repeatedly come out as the winner. For a long time it wasn’t known why — it is now clear that protein-rich food triggers a large release of PYY, which blocks the hunger hormone. There is also a hypothesis that after a protein-rich meal gluconeogenesis takes place in the portal vein, meaning protein directly stimulates glucose receptors too.
What comes next? According to some studies, carbohydrates come out ahead of fats; according to others, the difference is minimal — individual factors play a role here. The advantage of carbohydrates is greater volume at the same caloric content: the stomach visually fills up more. Glucose is absorbed quickly, so the feeling of fullness arrives sooner. The drawback, though, is that it also fades sooner: glucose is used up faster than fatty acids, which reach the bloodstream more slowly.
Satiety from fats arrives with a delay but lasts longer. You also have to take into account the overall energy density and composition of the food: the more water and dietary fiber, the more filling the meal.
FAQ: frequently asked questions about hunger
Why does hunger come back 15 minutes after you start eating?
Often it’s an illusion — the satiety signals are still on their way. The first hormones kick in within 15 minutes, but full satiety can take longer. Eating more slowly gives the hormones time to do their work.
Is protein really the most filling nutrient?
Yes, studies have confirmed this repeatedly. A protein-rich meal triggers the release of the hormone PYY, which blocks the hunger hormone NPY and produces a longer-lasting feeling of fullness.
Why do you quickly feel hungry again after sweets?
Fast carbohydrates raise blood glucose level quickly, but the glucose is used up just as quickly. The feeling of fullness is short-lived. Adding protein or dietary fiber slows this process noticeably.
Translated from the magazine Body; Author: Martin Berkhan
Translator: Greta
Source: Hunger
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