In blog 1 I explained what GLP-1 is and how this hormone has evolved from a diabetes medication into a fully-fledged treatment for obesity. But one question remains central for many people: how exactly does it work? What happens in your body after you have administered that weekly injection?
In this article, as a pharmacist, I dive into the biology. Not to be complicated, but because I believe you make better decisions when you understand what is really going on. And frankly: the science behind GLP-1 is nothing short of fascinating.
⚠️ Medical disclaimer: This article is intended for general information purposes only. It does not replace medical advice or a personal pharmaceutical consultation. Always consult a doctor or pharmacist for advice on medication use.
First understanding: the hormonal hunger system
To understand what GLP-1 agonists do, we must first look at what they correct. Hunger and satiety are not simple sensations; they are complex biological processes in which dozens of hormones, nerves, and brain regions work together.
In people with obesity, this system is often disrupted. The NHG Standard on Obesity (2025) describes how hormones such as ghrelin (the hunger hormone), peptide YY (PYY), leptin, and GLP-1 become deranged in obesity. The result: a chronically increased feeling of hunger, reduced satiety after meals, and a slower resting metabolism that makes weight loss extra difficult.
This is not a lack of willpower. It is biology.
GLP-1 medication works on three fronts simultaneously
GLP-1 agonists are so-called receptor agonists: they bind to the same receptors as the natural GLP-1 hormone, but remain active for much longer. This leads to three interrelated mechanisms of action.
Front 1: The stomach — slowing down as a strategy
The first front is the stomach. Normally, a meal leaves your stomach within 2 to 4 hours. GLP-1 medication slows down that process: food remains in the stomach longer, causing you to feel full for longer after eating and reducing how quickly you feel hungry again.
Imagine turning a tap slightly. The water still flows through, but more slowly. That is also how it works with your stomach: everything still functions normally, but the pace is lower. As a result, the feeling of satiety lasts longer.
We are also honest about the downside: this same delay is exactly why some people experience nausea or bloating in the beginning. This is not a sign that something is going wrong; it is the body getting used to a new rhythm. That is why the dosage is always built up slowly over several weeks, giving your stomach time to adjust.
Front 2: The brain — the hunger center in the hypothalamus
The second front is the brain. Deep within your brain lies an area that regulates your hunger and satiety — a kind of thermostat for your appetite. GLP-1 agonists activate receptors in this area and thus directly send the signal: “you have had enough.”
What that means in practice: you already start a meal feeling fuller, you are satisfied sooner with a smaller portion, and you think about food less in between meals. Not because you are forcing yourself, but because the biological hunger signal is turned down.
Scientists from the University of Michigan published a discovery in the top journal Science in 2024 that confirms this: they found exactly which brain cells are responsible for this effect. It turns out those cells already trigger the feeling of satiety before you take the first bite. That explains something many users recognize: “I thought I was hungry, but while eating, I was already full after half the meal.”
What exactly is ‘food noise’?
Do you recognize that? You have just eaten, but you are already thinking about your next meal. You walk past a bakery and cannot ignore the smell. You are sitting at work but your thoughts constantly wander toward food — even though you are not truly hungry.
Researchers call this phenomenon ‘food noise’: a constant mental noise surrounding food that consumes a lot of energy and undermines healthy choices. It is not a character flaw. It is a brain process that occurs more frequently and more intensely in people with overweight.
What many users of GLP-1 medication notice first is not even the weight loss — it is that silence. The thoughts about food diminish. Not because they are forcing themselves, but because the medication directly intervenes in the brain areas that control these signals.
Front 3: The pancreas — smart regulation of blood sugar
The third front is blood sugar. After a meal, your blood sugar rises. GLP-1 medication helps the pancreas produce exactly enough insulin at that moment to manage that increase.
The clever part is in the timing: the medication does this only when your blood sugar is actually too high. As soon as it returns to normal, the signal stops automatically. The body therefore does not “continue” producing insulin if it is not necessary. This is a built-in safety feature that was missing in older blood sugar medications, which sometimes led to dangerously low blood sugar levels.
Additionally, the medication ensures that the liver produces less extra sugar in between. The result: more stable blood sugar throughout the day, with fewer peaks and troughs. And that has a pleasant side effect: fewer energy dips and fewer sudden feelings of hunger after such a dip.
The gut-brain axis: an underestimated communication network
Hunger feels like something that happens in your head. But your gut has much more influence on that than most people think. The gut and brain are constantly in contact with each other via a kind of highway of nerves and hormones that send signals back and forth.
GLP-1 medication makes clever use of that connection. After the injection, the active substance reaches not only the stomach and pancreas through the blood, but also specific brain areas that are directly accessible to substances from the bloodstream. Thus, one injection per week works in several places at once: stomach, brain, and blood sugar.
That is precisely why the effects are so broad. It is not a remedy that presses a single button — it influences an entire network. And that explains why the results in practice go further than was initially expected.
Why ‘just eating less’ is so difficult: the hormonal context
This is perhaps the most undervalued insight in modern obesity science. The NHG describes in the revised standard (2025) how multiple hormonal systems are structurally disrupted in people with obesity:
- Ghrelin, the hunger hormone, remains elevated — even after weight loss. The body ‘remembers’ the higher weight and actively pushes for recovery.
- Leptin, the satiety signal from fat cells, works less effectively due to leptin resistance — the signal is present, but the brain no longer responds to it properly.
- Resting metabolism slows down during weight loss. In people with obesity, this effect is stronger than average, meaning the energy intake required to maintain weight becomes lower as one slims down — a biological mechanism that sabotages dieting.
GLP-1 agonists address a part of this disruption: they increase the satiety signal, indirectly inhibit the hunger hormone through brain action, and thus help rebalance the system. They are not a miracle cure, but a pharmacological correction of a biologically disrupted system.
Summary: three key mechanisms in your body
For an overview — this is what GLP-1 agonists do in your body:
- Stomach: delayed gastric emptying → longer feeling of fullness, reduced appetite
- Brain: inhibition of the hunger center in the hypothalamus, reduction of food noise via the reward system
- Pancreas: glucose-dependent insulin stimulation, inhibition of glucagon release → more stable blood sugar
The combined effect is more than the sum of its parts: less hunger, earlier satiety, fewer obsessive thoughts about food, and a more stable blood sugar. That is why the clinical results are so impressive.
Next blog: comparing the various GLP-1 drugs
Now that you know how GLP-1 agonists work, the logical next question arises: which medications actually exist, and what are the differences? In blog 3, we compare Ozempic, Wegovy, Mounjaro, and Saxenda — from active ingredient to dosage and indication.
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Sources:
- NHG-Standaard Obesitas (versie 2.0, herzien). Nederlands Huisartsen Genootschap. Oktober 2025.
Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-Like Peptide-1. Cell Metabolism. 2018;27(4):740-756. DOI: 10.1016/j.cmet.2018.03.001
Müller TD, Finan B, Bloom SR, et al. Glucagon-like peptide 1 (GLP-1). Molecular Metabolism. 2019;30:72-130. DOI: 10.1016/j.molmet.2019.09.010
Nauck MA, Quast DR, Wefers J, Meier JJ. Clinical Consequences of Delayed Gastric Emptying With GLP-1 Receptor Agonists and Tirzepatide. Journal of Clinical Endocrinology & Metabolism. 2024;110(1):1-15. DOI: 10.1210/clinem/dgae232
Kim KS, Park JS, Hwang E, et al. GLP-1 increases preingestive satiation via hypothalamic circuits in mice and humans. Science. 2024;385(6707):438-446. DOI: 10.1126/science.adj2537
Hayashi D, Edwards C, Emond JA, et al. What is food noise? A conceptual model of food cue reactivity. Nutrients. 2023;15(22):4809. DOI: 10.3390/nu15224809
Drucker DJ. The expanding landscape of GLP-1 medicines. Nature Medicine. 2025. DOI: 10.1038/s41591-025-04124-5
Aanvullend: Multidisciplinaire Richtlijn Overgewicht en Obesitas bij Volwassenen. Kennisinstituut FMS. 2023. — Farmacotherapeutisch Kompas, indicatietekst Obesitas, Zorginstituut Nederland. farmacotherapeutischkompas.nl
