The "hunger hormone" and how it drives appetite.
What the evidence says
- Produced primarily by stomach — "hunger hormone" signals brain to stimulate appetite1
- Rises nearly twofold before expected meal times (preprandial), falls to trough within 1 hour after eating1
- Has strong learned/conditioned component — adapts to eating schedule; when the eating window shifted to 8 AM–2 PM, mean ghrelin decreased by 32 pg/mL despite an 18-hour daily fast (p=0.006)1,2
- Follows circadian rhythm: nadir at ~0900, zenith at ~0100; pattern reciprocal to insulin and exactly in phase with leptin over 24 hours1
- Ghrelin is the only known hormone that increases food intake — it works by increasing the number of meals initiated (not their size) and elicits appetitive feeding behaviours; it also increases gastrointestinal motility while decreasing insulin secretion3
- Circulating ghrelin levels respond compensatorily to weight change: rising with weight loss and falling with weight gain — this contributes to the difficulty of maintaining weight loss3
- Ghrelin shows a strong inverse temporal association with cortisol (r = -0.79, P < 0.0001) — when cortisol peaks in the morning, ghrelin is suppressed; when cortisol drops in the afternoon, ghrelin rises, linking hunger biology directly to stress biology4
- Ghrelin released during fasting also enhances hippocampal plasticity, neurogenesis, and learning while reducing anxiety5