Hunger Hormones
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The common belief that obesity is caused by insufficient willpower collides with one central fact: hunger is a hormonally regulated drive, not a choice. The gut produces a family of hormones that signal the brain to eat or stop eating — and when obesity disrupts these signals, the eating drive becomes genuinely stronger than normal at a physiological level.
Understanding this system is why the phrase "just eat less" is not helpful medical advice.
The Hunger Hormone: Ghrelin
Ghrelin is produced primarily by the stomach and rises sharply before meals — driving the appetite that prompts you to eat. After eating, ghrelin falls. This pre-meal rise is not psychological anticipation — it is a measurable hormonal surge that creates genuine physiological hunger.
In people with obesity, two ghrelin abnormalities have been documented:
- Blunted post-meal suppression: Ghrelin falls less after eating than in lean individuals, meaning the "fed" signal is weaker
- Elevated chronic levels after weight loss: After significant weight loss, ghrelin rises and stays elevated — the body's defence against further weight reduction
This ghrelin elevation after weight loss is one of the primary reasons weight regain is so common. The body is not just allowing regain — it is actively driving it through persistently elevated hunger signals.
The Satiety Hormones: GLP-1, PYY, and CCK
GLP-1 (glucagon-like peptide-1): Released from the lower small intestine and colon during and after eating. Activates hypothalamic POMC neurons (satiety) and reduces NPY/AgRP activity (hunger). People with obesity produce less GLP-1 in response to meals than lean individuals — weaker satiety signal from the same food intake.
PYY (peptide YY): Released from the colon alongside GLP-1 in response to nutrient sensing. Directly reduces appetite by acting on the hypothalamus and brainstem. PYY levels are lower in obesity and do not rise as much after meals, contributing to the blunted post-meal satiety response.
CCK (cholecystokinin): Released from the duodenum within minutes of eating in response to fat and protein. Acts via vagal nerve fibres to signal satiety and slow gastric emptying. CCK's effect is short-lived (20 to 30 minutes), making it important for meal termination but not prolonged satiety.
Leptin: The Long-Term Energy Signal
Leptin is produced by fat cells and signals the brain about long-term energy stores. In theory, high fat mass = high leptin = suppressed appetite and increased energy expenditure. In practice, people with obesity develop leptin resistance — the hypothalamus stops responding normally to high leptin levels, eliminating the signal that should automatically moderate intake in response to excess fat.
Leptin resistance means the brain fails to receive the "enough energy stored" signal, driving eating behaviour as if in an energy-deficient state despite surplus fat stores.
The Hormonal Case for GLP-1 Intervention
In obesity, the hormonal environment is consistently biased toward eating:
- Ghrelin suppression after meals is blunted
- GLP-1 and PYY post-meal responses are reduced
- Leptin resistance removes the long-term satiety signal
- CCK response is present but very short-lived
Behavioural interventions that do not address this hormonal environment are fighting biology with psychology. METASLIM GLP-1 sublingual drops directly restore the GLP-1 signal that obesity has weakened — providing the hypothalamic satiety activation that the gut's own reduced GLP-1 response is no longer adequately delivering. This is the biological rationale for GLP-1 pathway support as a core treatment for obesity.