When you fast, several changes occur in your stomach and digestive system:

1. Decreased gut motility: Fasting for prolonged periods can slow down gut motility, meaning food and nutrients stay in the digestive system longer before moving to the colon1.

2. Activation of the migrating motor complex (MMC): During fasting, the MMC is activated, which helps clean out undigested residual material in the gastrointestinal tract. This process acts as an intestinal house cleaner, sweeping away residual food particles and bacteria2.

3. Reduced nutrient availability: The amount of readily available nutrients in the gut decreases significantly during fasting, which can alter the gut microbiome1.

4. Changes in gut hormone levels: Fasting can affect the concentrations of gut hormones such as leptin, glucagon-like peptide-1 (GLP-1), peptide YY (PYY), and cholecystokinin (CCK)2.

5. Stimulation of autophagy: Fasting can stimulate autophagy in the gut, which helps the body recycle and remove degraded cellular components, promoting cellular renewal2.

6. Potential digestive issues: Some people may experience digestive problems such as indigestion, diarrhea, nausea, and bloating when fasting, especially if they overeat after breaking their fast4.

7. Increased production of short-chain fatty acids: During fasting, gut bacteria may ferment remaining fiber and produce short-chain fatty acids, which can support microbiome health12.

8. Potential shrinkage of stomach size: Fasting may lead to a temporary shrinking of the stomach, which can reduce appetite3.

9. Rest for the digestive system: Fasting gives the digestive system a break from constant food processing, allowing it to rest and potentially reset3.

It's important to note that while fasting can have potential benefits for gut health, it may not be suitable for everyone. Prolonged or extreme fasting can lead to negative effects, and it's always advisable to consult with a healthcare professional before starting any fasting regimen.