In 2016, the Nobel Committee in Stockholm gave the Prize in Physiology or Medicine to Yoshinori Ohsumi, a cell biologist at the Tokyo Institute of Technology. He earned it for figuring out how autophagy works. The word comes from Greek: autos means 'self,' phagein means 'to eat.' Autophagy is literally 'self-eating.' It's how a cell finds its own broken parts — misfolded proteins, damaged organelles, leftover debris from normal activity — and takes them apart to reuse the pieces. Think of autophagy as the cell's janitor, quality-control inspector, and recycling plant, all in one. Fasting turns this system on. When your body stops getting food for long enough, it switches out of growth-and-build mode and into maintenance-and-repair mode. The cleanup that a normal eating schedule keeps switched off becomes the body's top priority instead. The Nobel Committee called autophagy 'a fundamental process for degrading and recycling cellular components,' one that is critical for 'cellular housekeeping, adaptation to starvation, and pathogen defense.'
Ohsumi got there through ten years of careful experiments on baker's yeast (Saccharomyces cerevisiae). Scientists had already seen autophagy happening, but nobody knew what genes ran it. Ohsumi found the first autophagy genes (called ATG genes) and mapped out how a cell builds an autophagosome — a bubble with two membranes that scoops up cellular debris — and merges it with the lysosome, the cell's digestive chamber, to break everything down. This machinery is ancient and shared: the same ATG genes in yeast do almost the same job in human cells. The medical payoff has been huge. Weak autophagy is now linked to Alzheimer's disease (the amyloid-beta and tau proteins that pile up are exactly what working autophagy would clear out), Parkinson's disease (a buildup of the protein alpha-synuclein), cancer (where autophagy's role is more complicated, cutting both ways), and type 2 diabetes (damage to the insulin-making cells in the pancreas, tied to protein buildup). On the flip side, turning autophagy on through fasting or eating less is one of the most reliable ways to extend healthy lifespan in animal studies.
Fasting moves through a fairly predictable set of stages, though the exact timing depends on your metabolic health, body composition, and how you've been eating. In the first 4–6 hours (starting from a fed state), insulin drops as your body finishes processing glucose from your last meal. The liver starts pulling from its glycogen stores — about 100 grams of glucose kept in reserve — to keep your blood sugar steady. Once that glycogen runs out, usually somewhere between 12 and 24 hours depending on how active you are and how fast your metabolism runs, your body has to switch fuel sources. The pancreas releases a hormone called glucagon, which tells the liver to start making new glucose from amino acids and glycerol (a process called gluconeogenesis) and — this is the key part — to start turning fat into ketones. Fat cells release their stored fatty acids into the bloodstream. Ketones — acetoacetate, beta-hydroxybutyrate, and acetone — become the main fuel for your brain and heart. This switch from burning sugar to burning fat and making ketones is called ketosis.
At the same time, human growth hormone (HGH) spikes during fasting — rising as much as five times higher than normal over a 24-hour fast, according to research from the Intermountain Medical Center Heart Institute. HGH is your body's main hormone for building and repairing tissue: it helps maintain muscle, burns fat, and drives cellular repair. This spike is probably an evolutionary safeguard — your body needs to stay strong enough to hunt or forage even when food isn't around. Put it together: HGH goes up, insulin goes down, glucagon goes up, and autophagy switches on. That's a completely different internal state than the one you're in after eating — and for cell health and long-term aging, it's a far better one for repair, maintenance, and detox than the constantly-fed state most people live in today.