Cellular Housekeeping at the Molecular Level
Autophagy—literally 'self-eating'—is a conserved cellular process where the cell digests and recycles its own components. Fasting triggers autophagy through two interconnected nutrient-sensing pathways: inhibition of mechanistic target of rapamycin (mTOR) and activation of AMP-activated protein kinase (AMPK), which phosphorylates and activates ULK1 (unc-51-like kinase 1), the master autophagy initiator.
Once activated, ULK1 nucleates autophagosome formation through recruitment of Beclin-1 and the VPS34 complex. These cup-shaped structures engulf cytoplasmic contents and fuse with lysosomes where hydrolytic enzymes catalyse degradation. LC3 inserts into autophagosome membranes and serves as the primary autophagy marker.
In intestinal epithelial cells, autophagy serves specialised functions maintaining barrier integrity. Paneth cells, specialised in secreting antimicrobial peptides, depend critically on autophagy. Paneth cell autophagy defects, induced by ATG16L1 mutations (frequent in Crohn's disease patients), lead to impaired antimicrobial peptide secretion and dysbiosis. Similarly, goblet cells secreting protective mucus rely on autophagy; deficiency impairs mucus layer thickness and epithelial permeability.
Xenophagy—selective autophagy of intracellular bacteria—represents a direct antimicrobial mechanism. Bacteria invading the epithelium trigger recruitment of autophagy machinery to bacterial surfaces; LC3 conjugation marks bacteria for lysosomal degradation. Impaired xenophagy permits intracellular bacterial survival, driving chronic inflammation. IRGM, another Crohn's disease susceptibility gene, regulates xenophagy; loss-of-function variants impair bacterial elimination.
Fasting duration thresholds determine autophagy intensity. In mice, measurable autophagy begins after 12-16 hours of food deprivation; maximal activation requires 24-48 hours. In humans, 24-hour fasting induces substantial autophagy. Shorter fasts (16-18 hours) produce measurable but partial autophagy activation.